THI* DOOK II A PART lOFTHE LIBRARY OF H BARBARA STATE COUEG 3010 MINERALS AND HOW TO STUDY THEM DANA'S SERIES OF MINERALOGIES System of Mineralogy Sixth edition, entirely rewritten. With Appendices I and II, completing the work to 1909, 1333 pages and more than 1400 illustrations; and with the Third Appendix, by William E. Ford, Professor of Mineralogy in Sheffield Scientific School of Yale University, completing the work to 1915, 87 pages. Complete volume. 6f by 10, half leather. A Text-book of Mineralogy With an extended Treatise on Crystallography and Physical Mineralogy. By Edward Salisbury Dana, Late Professor Emeritus of Physics and Curator of Mineralogy, Yale Uni- versity. Fourth edition, rewritten and enlarged, by William E. Ford (1932). 851 pages, 6 by 9, 1089 figures. Cloth. Minerals, and How to Study Them A book for beginners in Mineralogy. By the late E. S. Dana. Second Edition. 380 pages, 5 by 7^, 319 figures. Cloth. Manual of Mineralogy For the Student of Elementary Mineralogy, the Mining Engineer, the Geologist, the Prospector, the Collector, etc. Fourteenth edition, entirely revised and rewritten, by Wil- liam E. Ford. 476 pages, 5 by 7j, 360 figures, and 10 plates. Cloth, or flexible binding. QUARTZ CRYSTAL-NORTH CAROLINA Two fifths natural size. MINERALS, AND HOW TO STUDY THEM A BOOK FOR BEGINNERS IN MINERALOGY. THE LATE EDWARD SALISBURY JDAN A YALE UNIVERSITY, Atw HAVB.N, Author of A Text-book of Mineralogy, Sixth Edition of Dana's System of Mineralogy, etc. TTCMb more tban 300 Illustrations. SECOND REVISED EDITION TOTAL ISSUE, TWENTY-SEVEN THOUSAND NEW YORK JOHN WILEY & SONS, INC. LONDON: CHAPMAN & HALL, LIMITED COPYKIGHT, 1895, BY EDWARD S. DANA AU Rights Reserved This book or any part thereof must not be reproduced in any form without the written permission of the publisher* PRINTED IN u. e. A. 12/35 PRESS OF BRAUNWORTH 8c CO.. INC. BOOK MANUFACTURERS BROOKt-YN. NEW YORK QE. to PEEFACE. THE author has occupied some hours, which could not be devoted to more serious labor, in preparing this little book, in the hope that it might serve to encourage those who have a desire to learn, about minerals, and also to in- crease the number of those whose tastes may lead them in this direction. He shares with most teachers at the pres- ent time the conviction that the cultivation of the powers of observation is a most essential element in the education of young people of both sexes; he believes, further, that no subject is better fitted to accomplish this object and at the same time to excite active interest than that of Miner- alogy. The attempt has been made to present the whole subject in a clear, simple, and, so far as possible, a read- able form without too much detail and at the same time without cheapening the science. As the understanding of the different parts of the subject requires some preliminary knowledge of physics and of chemistry, a little elementary matter in these departments has been introduced. Much attention has been given to the illustrations, most of which have been made expressly for this book; others (reduced in size) are taken from the sixth edition of the System of Mineralogy (1892) ; several have been borrowed iii iv PREFACE. from Tschermak's Mineralogy, and one from a work by Baumhauer. The correct representation of real crystals and of the actual specimens from the cabinet is a difficult matter, and in this the author has been so fortunate as to secure the services of the skillful wood-engraver Mr. TV. F. Hopson of New Haven. Any suggestions which would tend to give this volume greater accuracy or usefulness will be always gratefully received. NEW HAVEN, July 1, 1895. TABLE OF CONTENTS. CHAPTER PAGH I. MINERALS AND MINERALOGY: INTRODUCTORY REMARKS.. 1 II. SOME PRELIMINARY HINTS AS TO How TO STUDY MIN- ERALS 8 Suggestions about making a Collection 11 III. THE FORMS OF CRYSTALS AND KINDS OF STRUCTURE 14 The General Characters of Crystals 14 The Systems of Crystallization 21 I. Isometric System 22 II. Tetragonal System 31 III. Hexagonal System 36 Rhombohedral System 39 IV. Orthorhombic System 41 V. Monoclinic System 44 VI. Triclinic System. ... 45 Irregularities of Crystals 48 Distorted Crystals 48 Pseudomorphs 55 Groupings or Aggregations of Crystals . 56 Twin Crystals 57 Parallel Grouping 60 Irregular Grouping 62 Structure in General 63 IV. THE OTHER PHYSICAL CHARACTERS OF CRYSTALS 70 1. Characters depending upon Cohesion 70 Cleavage 70 Fracture 73 Hardness and Tenacity 74 v Tl TABLE OF CONTENTS. CHAPTER PAOK 2. Specific Gravity or Relative Density 79 3. Characters depending upon Light 88 Luster 88 Color 90 Transparency 92 Other Optical Characters 93 4. Characters depending upon Heat 95 5. Characters depending upon Magnetism 96 6. Characters depending upon Electricity 96 7. Taste and Odor 97 V. THE CHEMICAL CHARACTERS OF MINERALS 'J9 The Chemical Elements 100 The Chemical Formula, etc 104 Kinds of Chemical Compounds among Minerals 109 Percentage Composition 116 Classification 118 VI. THE USE OF THE BLOWPIPE 121 1. General Description of Apparatus 121 2. How to Use the Blowpipe 127 3. Examination in the Forceps 130 4. Use of the Platinum Wire 186 5. Use of Charcoal 140 6. Use of the Closed and Open Tubes 147 7. Chemical Examination by Acids and other Reagents. 153 VII. DESCRIPTION OF MINERAL SPECIES 158 VIII. THE DETERMINATION OF MINERALS 339 APPENDIX 365 GENERAL INDEX 369 IXDEX TO MINERAL SPECIES 878 MINERALS, AND HOW TO STUDY THEM, CHAPTER I. MINERALS AND MINERALOGY. WE are to learn about minerals and how to study them; but, before we can fairly begin, we must understand clearly what substances we may call minerals, and what specimens have a right to a place in the collection that every one who wishes to become a mineralogist must make. We all know, in the first place, that minerals are the materials out of which the earth is built, and we often hear that division of nature to which they belong called the Mineral Kingdom, in distinction from the Animal and Vegetable Kingdoms, which embrace the animals and plants which live and grow upon the earth's surface. And here it is important to realize how little we can know by actual contact and direct observation about this earth, though we live upon it. It is possible, indeed, to measure its size and shape, to find out its density as a whole, to study its surface features and the changes which they have undergone; but of the materials of which it is made we can know little beyond those which form the surface upon which we walk. The miner digs down a 2 MINERALS, AND HOW TO STUDY THEM. little distance, and the artesian-well borer goes down still deeper, and we may have a chance to examine the spec- imens that their work brings up; or perhaps we can go down with the miner and see them in place. But th> deepest mines descend to less than three quarters of a mile, and though this seems deep to one who is let down a shaft in a bucket, it is but a little way compared with the whole distance to the earth's center, which would require a journey of nearly 4000 miles. Even the deepest artesian-well bor- ings hardly go down to the depth of one mile. Our knowledge, to be sure, is increased a little by the fact that we find now on the surface of the earth rocks made, as we have reason to believe, of materials brought up in a molten condition from great depths below. This is true of the lava thrown out by a volcano, and of such igneous rocks, for example, as form the Palisades along the Hudson River; and these occurrences give us some idea as to what kinds of matter there are, and in what condition, far below the surface. Further, we are able to weigh the entire earth, too, and find what its density is; and as this is nearly twice as great as that of the rocks on the surface, it gives a suggestion as to the heavy nature of the mineral material that must make up the interior. Thus the mineralogist is limited to the study of the lit- tle part of the crust of earth which he can reach with his hammer; and he cannot extend his collection much be- yond this, unless indeed he takes in some of those rare visitors from outer space called meteorites which once in a while tumble down to the earth, usually with a bright light and loud explosion. MINERALS AND MINERALOGY. 3 Now what does this study show of the hard rocky mate- rial of which the earth, so far as we can examine it, is made up; for example, of the sand of the seashore, the granite, the trap, the slate and marble of the hills ? We find, in the first place, that it in general consists of different kinds of substances, each one having certain peculiarities or characters of its own, by which it can always be recognized; and it is to each of these individual kinds that the name MINERAL is given. Thus, more particularly, the sand of the seashore can be separated without much difficulty into various sorts of grains, each kind alike in chemical substance, as the chemist can prove in the laboratory, and with certain characters of hardness, density, luster, and color of its own, which enable us after a little practice to distinguish the different kinds with comparative ease. Most of the grains are alike clear and glassy, hard enough to scratch glass, and as we learn to know them better we call them quartz. There are also black grains; some of these are heavy and jump to a magnet, and often they are sorted out by the waves into little rifts on the white sand ; these are called magnetite, or magnetic iron. There are other black grains, too, which the magnet does not attract, perhaps some red, glassy ones which are fragments of garnets, and, it may be, still others, depending upon where the sand comes from, and what kind of rock has been ground up by nature's mill and sorted out by the water to make the sand. If a piece of granite is taken, here too it is possible to distinguish several kinds of mineral substances, though it 4 MINERALS, AND HOW TO STUDT THEM. is not quite so easy to separate them. There are hard glassy grains with irregular surface, which, like the greater part of the sand-grains, are quartz. There are white or yellow or pale flesh-red fragments, also hard, though not so hard as the others, but which are sure to show one or two smooth surfaces of fracture: these are feldspar. Then there is the mica, more easily recognized still, which is either nearly white and silvery, or black (and sometimes both kinds), and which with a touch of the knife separates into very thin scales or leaves. Besides these there may be a little coal-black tourmaline, some bright red garnets, and other kinds which we shall learn later. If a cavity or open space in the granite can be found, here it is often possible to find the same kinds of substances, only larger and more distinct and very likely in the regular form which are called crystals. If, instead of a coarse-grained rock like granite, we ex- amine a fine compact one such as the trap-rock of the Palisades on the Hudson, it very probably appears all alike to the eye; but if we crush some of it to powder, the magnet will pick out some magnetic iron, as from the seashore sand. Or the skillful mineralogist may make a slice thin enough to be transparent, so that he can study it under the micro- scope, and then recognize a variety of different minerals. In seams and cavities in these rocks other sorts are often found, not like those in the solid rock. Sometimes we find a rock, like the white marble of Vermont, which the examination of the chemist shows to be alJ of the same chemical substance, and which has throughout the same characters of hardness, density, color, MINERALS AND MINERALOGY. 5 and so on; then it is said to be a mineral itself, and not, like most rocks, a mixture of a variety of different min- erals. These different kinds of substances, then, which make up the rocky material of the earth so far as we can study it, and into which we can separate the seashore sand, the granite, and most other rocks, are called MIN- ERALS. Each one has, first of all, a definite chemical composition, wherever it is found. Moreover, if in the form of a crystal, it has a shape of its own, too, by which it may be distinguished; it has also certain characters of hardness and density, luster, color, transparency, and others. And because to it belong all these different char- acters, which distinguish it from other kinds, it is called a MINERAL SPECIES. It is the work of the mineralogist to study these min- erals; to learn all the different kinds; what the characters of each are; how they are classified and how distinguished from each other; how they occur in nature; and some- thing about their practical uses. All the knowledge which the many mineralogists have learned, after long years of patient observation and study, both in the field and the laboratory, has been arranged in systematic form and makes up the Science of Mineralogy. The question as to what particular minerals go together to make the different kinds of rocks, how these are formed, and what changes of position or of character they have experienced all these and other similar questions are re- ferred to the geologist. The science of the geologist, or geology, is much broader than mineralogy: it treats of 6 MINERALS, AND HOW TO STUDY THEM. the history of the earth and all the changes it has gone through; the different kinds of rocks; the way the moun- tains have been built up from them; the growth and de- velopment of different kinds of life from the earliest times down to the present. It was stated at the beginning of the chapter that min- erals belong to the MINERAL KINGDOM; but it is important to remember that all substances mineral in nature are not necessarily called minerals. The mineralogist, for example, usually excludes from his cabinet many mineral substances, such as the pearl of the oyster-shell and the shell itself, the lime of the bones of animals, and the opal-like form of silica secreted by the growth of plants, as the tabasheer found in the joints of the bamboo. In general mineral substances such as these, which are formed immediately by the processes of animal or vegetable life, are not called minerals. Further, the mineralogist does not, as a rule, admit among minerals gases like the oxygen and nitrogen which make up the air; and of the liquids he includes only the metal mercury, and perhaps also water. The many beautiful kinds of salts made by the chemist are also not called minerals. The rock salt or sodium chloride which is mined, sometimes in fine clear cubi- cal blocks, is the same sodium chloride which, as the table-salt of every-day life, is so commonly used. But the table-salt obtained from evaporating sea - water or the brines of salt-wells, or from the solution of crude rock salt, though when manufactured it may be formed in crys- MINERALS AND MINERALOGY. tals as fine as those found in the rocks, is not called a mineral, because not made by nature alone. So, too, the fine crystals of blue vitriol, or copper sulphate, made by the chemist, do not find a place in a mineral cabinet, though the much less fine specimens of the same material found in some of the Arizona mines do. In the same way, the crystals of the metals and of many interesting compounds formed in the metallurgical process of making iron or lead or zinc are called furnace-products, and not minerals. These substances, however, are all very inter- esting, and their study is a very important help to pure mineralogy. In recent years the chemist has busied him- self in imitating, so far as he can, the possible processes of nature, and thus making "artificial minerals" Recently the diamond has been formed in minute crystals, also small but fine clear rubies, and so, too, quartz, feldspar, mica, and many common species. It must be acknowledged, however, that the specimens thus obtained in the laboratory are in most cases very minute and much less beautiful than those of nature; for the chemist in the laboratory has only a limited time for his experiments, and often must use violent means, as the great heat of a furnace. while nature works slowly and gently MINERALS, AND HOW TO STUDY THEM. CHAPTER II. SOME PRELIMINARY HINTS AS TO HOW TO STUDY MINERALS. A MINERAL, we have seen, is a substance formed by nature alone, a solid with one or two exceptions, and one having as a rule a definite form of its own and certain characters of hardness, density, luster, color, and still others, and, most important of all, a definite chemical com- position. The first group of characters, having to do with the form and structure and so on, are called PHYSICAL CHARACTERS, while those depending directly upon the composition are called CHEMICAL CHARACTERS. All of these will be described in some detail in subsequent chap- ters, but it is necessary first to gain a little knowledge as to how to study minerals, where the object is to learn as much as possible about each and to distinguish one kind from another. The mineralogist must first of all use his eyes and other unaided senses in studying minerals; in other words, he must gain all the information he can about minerals by looking at them and handling them. If he learns to do this wisely, he will be surprised to find how keen his senses become and how much he can find out. But as he gains in experience he will see that this only carries him to a certain point, and he should always recognize the im- HINTS ON THE STUDY OF MINERALS. 9 portance of confirming the conclusions reached by his eye and hand by more positive tests. Often, even in the case of the commonest species, the appearance may lead one who depends upon it alone quite astray. The old saying that " all is not gold that glitters," and the names ap- plied to certain common minerals of "fool's gold," "false galena," and others like them, express the result of experi- ence that the senses unassisted may readily be deceived. The trained eye of the mineralogist will show him, first of all, the form of the mineral, as to whether it has the regular geometrical shape of a crystal or not, or is simply granular, fibrous, and so on. It will show him whether it has the natural, easy, smooth fracture of many crystal- lized substances, called cleavage, or only the fracture of ordinary kinds. It will tell him, too, what peculiarities of luster the surface of a mineral presents, depending upon the way in which it reflects light, whether metallic, glassy, greasy, or silky, and so on; also what the color is, whether it is transparent or opaque, and many other points. The touch will show whether the " feel " is greasy, as is true of a few very soft minerals, or harsh, as are the majority. Again, a mass in the hand will often be recog- nized at once as heavy or light as compared with familiar substances of the same appearance. Thus the common minerals quartz, feldspar, calcite, have nearly the same density, and one can easily become so accustomed to them that a piece of gypsum seems light and one of barite (heavy spar) seems heavy. So a piece of the metal alu- minium seems very light because we instinctively 10 MINERALS, AND HOW TO STUDY THEM. it with the other much denser metals which we are ac- customed to handle. The taste may sometimes tell, for instance, that rock salt is in hand, while the odor is occasionally a useful character, as the clayey odor of some minerals when breathed upon. But it requires some education and experience before the senses are so on the alert that all the characters noted are perceived at once and rightly estimated; this is what every one should strive for; and one of the great benefits to be derived from the study of mineralogy is that it culti> vates and stimulates the powers of observation. When the senses alone stop, simple tests to aid them come in. A touch upon the smooth surface of a mineral with the point of a knife serves to show whether it is relatively soft or hard. The color of the powder ob- tained by scratching with the knife or upon a plate of rough porcelain or ground glass, called the streak, is some- times quite different from that of the surface, and in such cases this is a very important character. Then come more careful tests: the determination of the relative density or specific gravity; the use of the blow- pipe, giving the comparative degree of fusibility; and a number of simple chemical trials, to show the presence of sulphur or arsenic, silver, lead or iron, barium or stron- tium. Then follow still other tests, till we come to the re- fined methods of the trained mineralogist with his beauti- ful goniometer for measuring angles, the microscope and optical instruments, the accurate chemical analysis and other means by which most of nature's secrets may be HINTS ON THE STUDY OF MINERALS. 11 learned and the characters of each mineral thoroughly studied. SOME SUGGESTIONS ABOUT MAKING A COLLECTION OF MINERALS. A very important matter in the study of minerals is the student's own collection; for every one who desires to really learn mineralogy must have a collection of his own to examine and experiment upon. It is very desirable that the school or college should have a larger cabinet for refer- ence and study, but this does not take the place of tfhe in- dividual collection, which will be studied, arranged, labeled, and handled over and over again till every specimen is per- fectly familiar. Further, the student should obtain his specimens as far as he can by collecting for himself. No matter if he lives in a region that does not seem at first to afford very much, he can certainly find something that is worth keeping until he obtains better; and occasionally he will have the opportunity to make trips to some of the noted localities, where he can find more and a great variety. There is nothing more delightfully instructive and health-giving than to spend a day in the open air, with a good hammer in hand, a bag for the specimens, and plenty of soft paper (and perhaps some cotton) to wrap them up in. The hammer should be of hard steel that will not chip on the edges; it may weigh from a pound to a pound and a half, and the face should be square or slightly oblong and the edges sharp, while the back has the form of a 12 MINERALS, AND HOW TO STUDY THEM. wedge, as seen in Figure 1 (one-fourth natural size). A cold-chisel or two, for working into cracks or crevices, will often be found useful; also a small light hammer with a sharp edge for trimming specimens. This will often do no damage, when a blow from a heavy hammer would shatter the specimen and destroy it. Do not break the crystals out of the rock, as a rule. A detached crystal of garnet is interesting when quite perfect, but in general the crystal is most interesting and instructive when in its own home. The seller of min- erals soon discovers this, and it is un- fortunately not an uncommon trick at some localities for instance in the Alps for the local collector working for his daily bread to exercise his ingenuity in mounting a loose crystal J4 natural size. . . . , . , . in a mass of rock in which it never belonged, thus to increase the value of the specimen and deceive the unwary purchaser. The student is not advised to spend a great deal of money in buying specimens, particularly at any one time. Still it is less easy to collect personally now than it was years ago, and many students may not have opportunity to do the traveling that it requires: and even here the reward is often small, unless at a quarry or mine where work is being carried on all the time. HINTS ON THE STUDY OF MINERALS. 13 Hence a little money is by no means thrown away if judiciously expended from time to time, for it will serve to buy a few small characteristic specimens of the common species and pure fragments for blowpipe tests. Fine specimens, especially of the rarer species, are now very expensive, but sufficiently good ones of the minerals it is important for the student to know well may be obtained, for very little money.* It is better to collect small specimens rather than large, as far as possible, such as will go in a little paper tray 2 inches square, or 2 by 3 inches, or at most 3x3. These trays are inexpensive and are very useful for the arrange- ment and preservation of a cabinet. If the specimens are placed loose in a drawer, it can hardly be opened a few times without throwing them into confusion, and sooner or later they will be badly injured. The sizes mentioned are the most useful, though 3x4 inches might well be added. A depth of half an inch is sufficient for the tray, but the drawers, if possible, should not be less than 2 or 3 inches deep. All the specimens in a collection should be care- fully labeled, particularly as regards the locality. *A list of the most important minerals is given at the close of the book, and those most useful for trial with the blowpipe are there indicated. 14 MINERALS, AND HOW TO STUDY THEM. CHAPTER III. THE FOKMS OF CKYSTALS AND KINDS OF STRUCTURE. THE principal characters of minerals, by which one species is distinguished from another, have been briefly alluded to in the preceding chapter. It is now necessary to study some of these characters more fully. First the PHYSICAL CHARACTERS will be considered. These include the form and structure, the cleavage, frac- ture, hardness, tenacity, elasticity; also the density; fur- ther, the color, luster, degree of transparency, and some few others. The present chapter is limited to a discussion of the crystalline form and the structure in general. THE GENERAL CHARACTERS OF CRYSTALS. If we examine the specimens of the different mineral species in a good cabinet, we see that many of them occur commonly in regular solid forms with smooth faces, which forms, as we study the subject further, we find to be char- acteristic of each individual species. These regular forms are called crystals. The cubes of fluorite (Fig. 2) or galena, the six-sided prisms of quartz (Fig. 3), the twelve- sided, twenty-four-sided, or even more complex forms of garnets, are common examples of crystals. Further, even when a specimen does not show this regular external form, there is usually a definite crystalline structure, which may THE FORMS OF CRYSTALS AND KINDS OF STRUCTURE. 15 be shown in the easy fracture called cleavage, as that of calcite, or which may be indicated in other ways, as we shall soon learn. How is this regularity of form and structure to be explained ? First, we will speak of crystals. The physicist, as the result of his studies into the struc- ture of different kinds of matter, has concluded that every te, Group of Cubic Crystals. Quartz Crystal. body is built up of minute particles called molecules, which are much too small to be perceived even by the strongest microscope. In a solid body, as a lump of iron, ice, sulphur^ he thinks of these molecules as bound to- gether by a strong force of attraction, called cohesion, so that it requires a hard blow or great pressure to change its shape. In a liquid body he thinks of them as free to move or roll over each other, so that the liquid takes at once the shape of the vessel in which it is contained, whatever that may be. In a gas, he believes that the molecules are sepa- 16 MINERALS, AND HOW TO STUDY THEM. rate from each other, a long distance in fact compared with their size, and that they are darting about very rip- idly, colliding against each other and any confining surface. The result of this is that the gas at once fills entirely a vessel into which it is introduced and presses against its sides; the pressure being simply the result of the bombard- ment of these little rifle-balls. The pressure of the external air, for example, is shown by the collapse of the cheeks when the air within the mouth is drawn away. The relation between these minute particles or molecules thus explains the condition of a body, as solid, liquid, or gaseous; for example, the distinction between ice, water, and steam. But more than this: When a liquid turns into a solid because the temperature falls, as when water freezes, or liquid sulphur or molten iron hardens on cooling, the force of cohesion comes into play to bind the particles together into a rigid mass. So, also, when by slow evaporation from a jolution, as of salt or alum in water, the dissolving liquid is removed, the substance in solution also passes back into the solid form under the action of this same force of cohe- sion. Thus the solid is formed from the liquid by the action of the forces acting between these little particles. Further, if the molecules are all of one kind, as in a given chemical substance, and if there are no hindering causes, these molecules will build themselves up after some regular pattern and the external result is the geometrical form, which is called a crystal. It is somewhat as if the mole- cules were little building-stones, built up into a solid structure by forces acting between them and causing them THE FORMS OF CRYSTALS AND KINDS OF STRUCTURE. 17 to arrange themselves after a definite manner when they are free to do so. This regular building of the molecules, which, as has just been shown, may take place from a liquid, happens also, even more perfectly, when a solid is formed direct from a gas. Water vapor in the air, if cooled down suf- Sciently, is formed into the solid snow, and the little snow- crystals, that fall silently through the atmosphere and which we may catch on our coat-sleeve on a cold winter day, are often of wonderful regularity and beauty of form. The figure (4) gives some of the many forms of snow-crystals drawn by Scoresby in a visit to the Arctic 4. Snow -crystals. many years ago. So too, as will be described later, if a mineral containing arsenic is heated in a glass tube open at both ends, the arsenic driven off, uniting with the oxygen of the air, forms the vapor of oxide of arsenic; this is condensed a little higher in the tube, where it is cooler, and there deposited in minute spangling octahedral crys- tals (Fig. 6, p. 23), which are sometimes quite large and very perfect in form. It is not always easy to make good crystals, whether starting from a liquid or from a gas. This is true in part because we cannot give the time required for the perfect p/ocess, in part because there are other hindering con- 18 MINERALS, AND HOW TO STUDY THEM. ditions. But sometimes we can succeed well, and the growth of an octahedral crystal of alum in a strong solution can be watched from day to day and a large and fine crystal may be the reward of our skill and patience. In nature's laboratory the conditions are more favorable, particularly because there is never any limit of time, and the many beautiful and complex crystals of minerals with brilliant faces show the result. Even here, however, the building process often cannot go on freely, and imperfect crystals, or perhaps a mass with only a confused crystal- line structure and without distinct external form, may be all that is produced. The quartz, feldspar, and mica in the rock called granite have usually formed together in such a way that neither one has had an opportunity to build itself up into perfect crystals, and yet the student who understands the optical study of thin sections of a rock in polarized light can prove that each grain, formless though it may be externally, has all the internal molecular structure of the crystal. In a cavity in the granite we are not surprised to find crystals of quartz and feldspar, perhaps also of mica, as the cavity here means that each has had an opportunity to exercise its tendency to build itself regularly with something of the freedom which a perfect crystal requires. Another familiar example of crystallization is given by the ice covering a pond, which is as truly crystalline in structure as the perfect snow-crystal; but here there are no crystals, and it is easy to understand why. The slow dis- section of the mass, however, under the melting action of the sun reveals something of the regularity in the molec THE FORMS OF CRYSTALS AND KINDS OF STRUCTURE. 19 ular building, and the same thing is proved by an exami- nation in polarized light. Sometimes in the freezing of a little pool of water on a sidewalk the formation of the slender crystalline ribs of ice may be watched as they shoot out, forming a framework which may soon lose its distinctness as the entire surface is frozen over. We learn, then, that a crystal is the regular solid form which a chemical substance takes when it passes into the solid state from that of either a liquid or a gas, if under such conditions that the molecules are quite free to ar- range themselves according to the direction of the at- tractive forces acting between them. The crystal is, therefore, the outward expression of the structure in the arrangement of these molecules, and itt form is for this reason the most important of all the physi- cal characters of a given species and the one which in gen- eral most definitely distinguishes it from others. It is interesting to note that a small crystal is just as per- fect and complete an individual as a similar one of great size; there is among the crystals of a given species no such connection between size, on the one hand, and age and maturity, on the other, that belongs to the individuals of a species in the animal and vegetable kingdoms. Some crystals are so minute as to be almost microscopic; others may be of enormous size, as the gigantic quartz crystals occasionally found in the Alps, or the equally large beryl crystals from New Hampshire. A cave opened a few years ago at Macomb, New York, contained 15 tons of great cubic crystals of fluorite; another cave in Wayne County, Utah, contained a great number of enormous crystals of MINERALS, AND HOW TO STUDY THEM. gypsum, some of them three feet or more in length. But the very small crystals and the like ones of enormous size are not essentially different except in this comparatively unimportant respect of magnitude. And yet there are many interesting points of resem- blance between crystals and living plants. Crystals grow as well as plants, and under favorable conditions so rapidly that the increase in size may be watched not only from day to day, but from hour to hour, or even from minute to minute. The complex forms that are built up especially in such cases of rapid growth are often wonder- fully plantlike in aspect. This is true, as every one has noticed, of the delicate frost-figures which form so quickly upon a window-pane or a paving-stone in winter; also, in other more permanent cases, the arborescent or dendritic forms of native gold or silver or copper are good examples of the same fact. The terms used in describing them are indeed given because of their resemblance to forms of vegetation. Furthermore, as a wounded plant tends to heal itself when, for example, a branch has been broken off, or as a slip or graft tends to develop a full individual; so, too, a broken crystal may be more or less healed, but in the last case the material which repairs the injury must be supplied from an outside source. Thus the silica to mend a broken quartz crystal must come from a foreign solution, and the crystal itself only directs the way in which the molecules of the solution are laid down; it is interesting, however, that the growth takes place more readily on a surface of fracture than on a natural crystalline face. In this way the grains of quartz in a sandstone, formless be- THE FORMS OF CRYSTALS AND KINDS OF STRUCTURE. 21 cause only fractured fragments, often tend to build them- selves up into complete crystals. Still again, although a crystal never has an old age in the sense that this is true of a plant or an animal, it is nevertheless a fact that many crystals tend to change or decay as time goes on, if subjected, for example, to the corroding effects of some foreign solution. Even the beautiful gems, such as the sapphire, emerald, topaz, garnet, hard and comparatively insoluble as they are, have this liability to undergo what is called chemical decomposition, with the loss of all their beauty and a total change of chemical substance. This is spoken of again in a later part of the chapter, where pseud omorphs are de- scribed, but it is worth noting here because somewhat analogous to the change that old age brings to a living organism. THE SYSTEMS OF CRYSTALLIZATION. The forms of crystals are so varied and the difficulties in studying them minutely so great that we shall only attempt here to learn some of their simplest kinds. In the first place, it is important to understand that it can be shown that all crystals belong to one of six classes, or systems, which are named as follows: I, ISOMETRIC; II, TETRAGONAL; III, HEXAGONAL; IV, ORTHORHOMBIC; V, MONOCLINIC; VI, TRICLINIC. The characters of each system and the relations between them will be briefly mentioned after the chief forms in each have been described. 22 MINERALS, AND HOW TO STUDY THEM. I. Isometric System. The principal forms of the Isometric System are the cube, octahedron, dodecahedron, the two trisoctahedrons, the tetrahexahedron, and the hexoctahedron. Cube. The cube has six equal faces, each one of which is a square, and the angle between any two faces is a right angle, or 90. It is shown in Fig. 5. Galena and fluorite often occur in cubes. Octahedron. A regular octahedron (Fig. 6) has eight like faces,* each a triangle with equal sides and three equal angles (each 60); the angle between any two adja- cent faces is 109 28'. Magnetite is often in octahedrons. Dodecahedron. The rhombic dodecahedron (Fig. 7) has twelve equal faces,* each of which is a rhomb with plane angles of 60 and 120, while the angle between two adja- cent faces is 120^ 7 TEis is a common form with garnet. These forms may occur together on the same crystal. Thus crystals of galena often show the cube and octa- hedron together. Fig. 8 is generally described as a cube modified by an octahedron, and Fig. 9 as an octahedron modified by the cube. If a cube is cut out of a block and the solid angles sliced away carefully, the new surface making equal angles with the three cubic faces, the result is to give finally an octahedron. It is seen that the octa- hedral faces are little triangles on the solid angles of the * Octahedron is named from the Greek otcroi, eight, and eSpa, face, or the eightfaced solid. Dodecahedron is similarly named from SooSetca, twelve, and t=8 pa, fact. THE FORMS OF CRYSTALS AND KINDS OF STRUCTURE. 23 cube and equally inclined to the three cubic faces. On the other hand, the cubic faces are small squares on the six solid angles of the octahedron. The angle between two adjacent faces of a cube and an octahedron is 125 16'. Figures 10, 11 show the cube and dodecahedron -to- gether, and Fig. 12 the octahedron and dodecahedron. Both the cube and octahedron have twelve similar edges, and these are cut off equally, or truncated, by the twelve faces of the dodecahedron. In Fig. 13 we have again a form (not shaded) resulting from the combination of the faces of the cube (a), octahedron (o), and dodecahedron ((/). The angle between adjacent faces of the cube and dodecahedron is 135; b<>- tween those of the octahedron and dodecahedron it is 144 44'. Trapezohedron. A trapezohedron has twenty-four equal faces, each a four-sided figure or tranezoid. It is shown MINERALS, AND HOW TO STUDY THEM. in Fig. 14, which is a common form with garnet. There may be a large number of different trapezohedrons, all having the same general form but differing in the angles between the faces. A similar remark may be made about each of the other type-forms of this system yet to be de- scribed. It requires much more study than is possible for the beginner to leani how these forms are mathematically distinguished from one another. Figures 16 to 19 show combinations of the trapezohe^ 16. 17. dron (n or m) wuli the cube (a), octahedron (o), and dodecahedron (d). The last two are common forms with garnet. The trapezohedron is also called a tetragonal trisocta- hedron because its form suggests an octahedron in which three faces take the place of a single octahedral face, each of them being a four-sided figure or tetragon. THE FORMS OF CRYSTALS AND KINDS OF STRUCTURE. 25 There is also another trisoctahedron, called a trigonal trisoctahedron, shown in Fig. 15, which has, again, twenty- four faces, three of these also corresponding to an octahedral face; but each is a three- sided figure (trigon) or an isosceles triangle. This form does not often occur alone, but may be seen on complex crystals of galena. Fig. 20 shows a figure of galena with the cube (a), octahedron (o), dodeca- hedron (d)', also two different trigonal trisoctahedrons, lettered p and u. Tetrahexaliedron. A tetrahexahedron (Fig. 21) is a twenty-four-faced solid,* each face an isosceles triangle and four together having the same position as the face of a cube. Fig. 22 shows a combination of the cube and a 21. 22. tetrahexahedron ; the latter is said to bevel the edges of the cube because the two planes are equally inclined to the two adjacent cubic faces. Hexoctahedron. A hexoctahedron (Fig. 23) is a forty- eight-faced solid; each face is a scalene triangle, and six faces have the same general position as a face of an octa- hedron, f * Named from rerpa, four, e, six, and edpa. face. { Named from e?, six,oKroJ, eight, aud ed pa, fact. 26 MINERALS, AND HOW TO STUDY THEM. Fig. 24 shows a combination of the cube (a) with the hexoctahedron; it is a common form with fluorite. Fig. 25 is a common garnet form, the dodecahedron (d) with the hexoctahedron, the latter beveling the edges of the former. Many of the figures thus far given and some of those which follow are shaded, so as to appear solid to the stu- dent learning about crystals for the first time. It is ob- 24. vious, however, that when the form is complex that the shading is impossible, and for the experienced crystallog- rapher it is quite unnecessary; hence these more complex figures are shown in line only. Through the rest of the book these line figures will be freely used. The student will soon find that they appear as solid to him as the others.* All the forms that have been mentioned belong to what is called the ISOMETRIC SYSTEM, in which the crystallog- rapher refers the planes to three equal axes at right angles * It will be a great help to the student if he has a few models to handle. These are made in great perfection in wood and are not very expensive. The student may also try to cut them for himself out of soft wood or plaster-of-paris; even a potato can be employed for temporary use. THE FORMS OF CRYSTALS AND KINDS OF STRUCTURE. 27 to each other. The position of the axes passing through the centers of the crystals is shown in Figs. 26, 27, 28. It will be seen to be true of all these, as it is of all their combinations, that the arrangement of the faces is the same about each one of the six cubic faces, or in other words about the ends of the three axes. Another way of stating this is to say that all these isometric crystals have three equal planes of symmetry at right angles to each other.* These three equal planes of symmetry are planes parallel to the cubic faces and have a corresponding position in the other simple crystals or com- binations of them. Each plane of symmetry divides the ideal crystal into two symmetrical halves, and here the three sets of halves made by the three planes parallel to each pair of cubic faces are identical; hence the planes of symmetry are said to be equal. The axes are the lines in which these three planes of symmetry intersect each other. * A plane of symmetry is a plane which divides the solid into equal halves such that if one half is placed against a mirror the re- flection completes the form. This is one form of the geometrical definition applying to an ideal crystal ; it will be explained later (p 50) how this must be broadened to cover the crystal lographic symmetry of actual crystals. 28 MINERALS, AND HOW TO STUDY THEM. A cube, as well as the other isometric forms mentioned, has also six other planes of symmetry passing diagonally through the opposite edges, and hence parallel to each pair of the dodecahedral faces. 29. 30. Fig. 29, of cuprite, and Fig. 30, of the rare species micro- lite, both drawn on a larger scale, are added to show some rather complex combinations of isometric forms. In Fig. 29 the cube (a) and the dodecahedron (d) predomi- nate; the faces of the octahedron (o) are small; n and /? are faces of two different trapezohedrons. In Fig. 30 the octahedron (0) predominates and then the cube (a), while the dodecahedron (d) is subordinate; the faces m belong to a trapezohedron, and p to a trigonal trisoctahedron. There are also several other forms belonging to the Isometric System, but which are described as half-forms, or forms in which only half of the faces in the correspond- ing whole form are present. To them the rules of sym- metry do not apply, but their faces are also referred to three equal axes at right angles to each other. The most important of these half-forms are the tetrahedron and pyritohedron. THE FORMS OF CRYSTALS AND KINDS OF STRUCTURE. 29 Tetrahedron. The tetrahedron (Fig. 31) is a form with four equal triangular faces, each of them an equilateral triangle. It is considered in Crystallography as the half- form of the octahedron, since half the faces of the octa- hedron, taken every other one, will if extended form a 31. tetrahedron. Perhaps a study of Fig. 32 will make this clearer. The angle between two adjacent faces of a te- trahedron is 70 32'. Fig. 33 shows a combination of a cube (a) with the four faces of a tetrahedron (0). It is seen that the planes are present only on the alternate angles of the cube. In 35. Fig. 34, a combination of a cube and a tetrahedron, the latter predominates. Fig. 35 shows a combination of the tetrahedron before figured (o) with another similar form (lettered o,) made up of the four remaining faces of the octahedron. It might be asked why this form cannot be regarded as an octahedron in which four faces are ac 30 MINERALS, AND HOW TO STUDY THEM. cidentally larger (compare remarks on p. 49) than the others; but this is impossible, for it can be proved, per- haps at once by difference of luster, that the eight faces are not all alike, but only four and four. This, however, is a somewhat difficult subject for a beginner. Pyritohedron. The pyritohedron (Fig. 36) is a twelve- sided solid, or dodecahedron, each face of which is a penta- gon, but not here as with the pentagonal dodecahedron of geometry a regular pen- tagon. In crystallography the name dodecahedron is usually given only to the rhombic dodecahedron described above (Fig. 7), and this form, the pyritohedron, takes its name from the species pyrite or iron pyrites, because of common occurrence with it. The pyritohedron is the half-form of the tetrahexahe- dron. If in combination with the cube, the solids in Figs. 37, 38 result; Fig. 39 is a combination of an octahedron and pyritohedron. There are also other half- forms in the Isometric System, thus of the two trisoctahe- drons and the hexoctahedron; but they are not very com mon and will not be described here. THE FORMS OF CRYSTALS AND KINDS OF STRUCTURE. 31 II. Tetragonal System. The chief forms of the Tetragonal System are the two square prisms and pyramids and the eight-sided prism and double eight-sided pyramid. 40. 41. IJ Square Prism and Pyramid. One of the square prisms is shown in Fig. 40 and the square pyramid corresponding to it in Fig. 41, while Fig. 42 is a combination of the two forms. The square prism has, like the cube, angles of 90 be- tween the faces, but it differs from the cube because the four vertical faces are not like the two end faces, or basal planes as they are called. This is often shown in a crystal by the difference in the smoothness of the two kinds of faces; or there may be easy fracture, or cleavage, parallel to one set of planes and not to the other. The square pyramid looks somewhat like a regular octa- hedron, but here the faces are isosceles triangles (not equilateral) and the angle between two faces over a hori- zontal edge differs from that over one of the vertical edges in fact, either angle is characteristic of a given MINERALS, AND HOW TO STUDY THEM. species and differs from one species to another. There may be a great many square pyramids of the same type as this but differing in their angles and consequently flatter or sharper at the extremity. Fig. 43 shows an acute square pyramid, p, while Fig. 44 44, represents another crystal of the same species (octahe- drite) in which this pyramid p is present but with it three others, z, i, v, each flatter or more obtuse at the summit than the others. There is also another square prism and another square 45. 46. pyramid diagonal to the set just described; they are shown in Figs. 45 and 46. Fig. 47 shows these two forms together. Taken alone these two forms cannot be dis- tinguished from the other two shown in Figs. 40, 41, but THE FORMS OF CRYSTALS AND KINDS OF STRUCTURE. 33 49. if they occur together thfc distinction is obvious. Thus Fig. 48 shows the two prisms on the same crystal, the faces of one truncating the edges of the other. Figs. 49 and 50 show the same two prisms in skeleton lines with the axes represented inside; it is seen that in the prism (Fig. 49) first mentioned often called the unit prism the horizontal axes join the middle points of the opposite edges, while in Fig. 50 they join the centers of the oppo- site faces. The latter form is often called the diametral prism, because the faces are parallel to the axes or diam- eters. In Figs. 51 and 52 the two pyramids are again shown, 51. and here the position of the axes should also be noted. Fig. 51 is often called a unit pyramid or one of the unit series; Fig. 52 a pyramid of the diametral series. In Figs. 53, 54 the combinations of each square prism with the pyramid of the diagonal set are shown. Fig. 53 resembles a cube modified by an octahedron (Fig. 8, p. 23), but it differs from it in that the faces lettered p, while they make equal angles with the two adjacent faces a, make 34 MINERALS, AND HOW TO STUDY THEM. different angles with the basal plane or base c. The same statement could be made in regard to the form of Fig. 54. 55 There is also an eight-sided prism made up of eight like faces, and it is shown on the Jjf^ complex crystal represented in Fig 59; its />' ^^ faces are lettered h. Further, there is also a jjjgllB' double eight-sided pyramid, as shown in Fig. \10 55. This is often called a zirconoid, because common with the species zircon. In Figs. 56, 57, 58, representations of crystals of zircon, the faces x (in part lettered) belong to a zirconoid or 57. 88. double eight-sided pyramid. The same is true of the faces lettered z in the figures 59, 60. Fig. 59 represents a complex crystal of wernerite, and Fig. 60 is a map of the top of the same crystal, or a basal section, as it is called. Note the prism and pyramid of the unit series, m and r; the prism and pyramid of the diametral series, a, e; also the eight-sided prism h and the double eight-sided pyramid z already referred to. All of the forms that have been described belong to THE FORMS OF CRYSTALS AND KINDS OF STRUCTURE. 35 what is called the TETRAGONAL SYSTEM, in which the planes are referred to three axes at right angles, of which the two in the horizontal plane are equal, the third or ver- tical axis is longer or shorter. It will be seen by examining the figures, especially Figs. 58, 59, 60, that the grouping of faces is the same about the faces lettered a, but different from them about the face c, the basal plane. It will be seen, too, that about c the faces of the same kind are all arranged in fours or eights. In other words, all these tetragonal crystals have a pair 60. a ^\m' of equal planes of symmetry parallel to the faces a and at right angles to each other. There is also another pair at right angles to each other parallel to the faces m. All these four planes meet in a common vertical line, which is called the vertical axis. There is, finally, a fifth plane of symmetry parallel to the top and bottom of the crystal, or the basal plane c, and hence at right angles to this vertical axis, but it differs from either of the other pairs. In the Tetragonal System there are also some half- forms but only one will be specially described. 36 MINERALS, AND HOW TO STUDY THEM. Sphenoid. The sphenoid (Fig. 61) is a four- faced solid 61. looking like a tetrahedron, but differing from it since the faces are isosceles (not equilat- eral) triangles. It is described as the half- form of the square pyramid shown in Figs. 41 and 51. III. Hexagonal System. The chief forms of the Hexagonal System are the two six-sided prisms, the two double six-sided pyramids, and the twelve-sided prism and double twelve-sided pyramid. These will be briefly described first, and then the char- acteristic forms of the Rhombohedral part of the Hexago- nal System will be mentioned. Hexagonal Prism and Pyramid. The hexagonal prism and pyramid are shown in Figs. 62 and 63, while Fig. 64 gives a combination of the two. The angles of the hexago- nal prism are exactly 120, and the terminal face or basal plane is a regular hexagon. The faces of the hexagonal 62. - " " . , , pyramid are isosceles triangles, differing in angle accord- ing to whether the pyramid is more obtuse or acute, THE FORMS OF CRYSTALS AND KINDS OF STRUCTURE. 37 These angles, and the others that depend upon them, are characteristic for a given species. There is also another hexagonal prism and pyramid di- agonal to the others and looking much like them. These two sets correspond to the two square prisms and pyra- 65. ;m raids of the Tetragonal System. Compare Fig. 65 of the unit prism and Fig. 66 of the second or diagonal prism and note the position of the axes shown in each; also the position of the axes in Fig. 67 of the unit hexagonal pyra- mid. Fig. 68 shows the combination of the unit hexago- nal prism and pyramid with the basal plane. Fig. 69, of a crystal of beryl, shows a combination of the unit prism and pyramid, m and p\ the diagonal prism and pyramid, a and s; also the basal plane c. There is also a prism bounded by twelve similar faces, and a double pyramid bounded above and below by twelve triangular faces. This double twelve-sided pyramid is often called a berylloid, because common with crystals of beryl. Two berylloids are shown in Figs. 70, 71; the faces are lettered (in part) n and v respectively. Fig. 71 is an enlarged map, or basal section, of the top of a crystal much like that of Fig. 70. Note, also, on Fig. 71 the hex- 38 MINERALS, AND HOW TO STUDY THEM. agonal prism, m, and pyramids, u and p, of the unit series; the prism, a, and pyramid, s, of the diagonal series. These hexagonal forms belong to the HEXAGONAL SYS- 69. 70. 71. TEM, where the planes are referred to the four axes shown in Figs. 65, 66, 67, three in a horizontal plane equal and cutting each other at angles of 60, and the fourth vertical axis either longer or shorter. It will be seen that the faces are arranged in the same way about each face a and hence about each end of the horizontal or lateral axes, but differently about the basal face c, that is, at the extremity of the vertical axes. The faces are arranged about the face c in sixes or in twelves instead of fours and eights as in the Tetragonal System. All the hexagonal forms have three equal planes of sym- metry making angles of 60 with each other parallel to the faces m; also another set of three, diagonal to the others and parallel to the faces a; and a seventh plane parallel to the top or base of the crystal. There are several half-forms in the Hexagonal System, THE FORMS OF CRYSTALS AND KINDS OF STRUCTURE. 39 but the only ones that will be described here are those of the Rhombohedral System. The RHOMBOHEDRAL SYSTEM is generally treated as a branch of the Hexagonal System. In the forms belonging to it the faces are in threes about the extremities of the vertical axis c, and there are only three vertical planes of symmetry making angles of 60 with each other inter- secting in this axis. The important forms are the rhom- bohedron and the scalenohedron. Rhombohedi'on. The rhombohedron is a six-sided solid, each face of which is a rhomb; it is shown in Figs, 72, 73, 74. There may be a great many rhombohedrons, as shown 72. 73. here, differing in angle and consequently more or less obtuse or acute. The rhombohedron looks somewhat like a cube if the cube is placed with the line joining two opposite angles vertical; in fact, the cube comes between the obtuse and acute rhombohedrons, having an angle of just 90. The rhombohedron may be regarded as a half-form of the hexagonal pyramid, but this subject is a rather diffi- cult one and cannot be followed up here. Scalenohedron. The scalenohedron (Fig. 75) is a twelve- sided solid, looking a little like a double six-sided pyramid, 40 MINERALS, AND HOW TO STUDY THEM. but the faces are scalene triangles and the edge is zigzag, up and down, like that of a rhombohedron, instead of hori- 75. zontal as in the pyramid. Moreover the angles i between the faces over the edges which meet ^NL in the vertex are only alike every other one JB Ik in other words, there are two sets of three |A each, those of one set more obtuse than those sjjm of the other. ' a The two hexagonal prisms before described and the hexagonal pyramid of the diagonal series also belong to the Rhombohedral System. The number of species crystallizing in the rhombohe- dral division of the Hexagonal System is very large, and some of them, as, for example, calcite, are very highly complex. In the figures of calcite given here, 76 to 80, 76. the faces r,f, e belong to different rhombohedrons; v to a scalenohedron ; m is the unit hexagonal prism ; c the basal plane. Fig. 81 represents a more complex crystal, also of cal- cite, and Fig. 82 gives a basal projection of it. Here there are several rhombohedrons, r, e, 0,/J the scalenohedrons THE FORMS OF CRYSTALS AND KINDS OF STRUCTURE. 41 v and t; the prism m. These figures show well the sym- metry about three planes meeting in angles of 60. Fig. 83 shows a crystal of hematite; u and r are faces of two 81. rhombohedrons, and n faces of the hexagonal pyramid of the diagonal series. IV. Orthorhombic System. The characteristic forms of the Orthorhombic System are the rhombic prism and pyramid; there are also other forms called domes. Rhombic Prism. Fig. 84 shows what is called a rhombic prism, the terminal face of which (formed by the baaal 84. 85. plane c) is a rhomb, not a square as in the square prism, which it somewhat resembles. The angle between two faces over one vertical edge is obtuse, or greater than 90, 42 MINERALS, AND HOW TO STUDY THEH. the other acute and just as much less than 90. For in- stance, if the angle of the front edge is 100, the angle of the side edge would be 80. There may be a great many rhombic prisms on the crystals of the same species, differing in the angles of their two edges. Rhombic Pyramid. The rhombic pyramid is shown in Fig. 85; its cross-section, like that of the prism, is a rhomb, and its edges belong in three sets, with different angles for each. These angles are characteristic for the crystals of a given species. There may be a great variety of rhom- bic pyramids, differing in their angles, and each corre- sponding to a given rhombic prism. Fig. 86 shows a combination form belonging to this system which looks like a cube a little, and resembles it 86. in that the angles between the faces are 90, but differs because the faces instead of being all alike belong in three sets of two each. It is for this reason that they are lettered a, b, c. Of these c is called the base, and a and b are called pinacoids. This form also resembles the second or diametral prism of the tetragonal system (Figs. 45 and 50); but in that form the four verti- cal faces (a) were all alike, while here, as has been stated, they are only alike two and two. Domes. The forms shown in Figs. 87, 88 are called domes, from the Latin for house (domus), because when they meet above they make a horizontal edge like a hip- roof. These domes are often and very conveniently called horizontal prisms; that of Fig. 87 is also called a macro- dome, because the faces (I) are parallel to the longer THE FORMS OF CRYSTALS AND KINDS OF STRUCTURE. 43 lateral axis (see Fig. 84); those (e, d) of Fig. 88 are similarly called brachydomes, because the faces are paral- lel to the shorter axis. Figs. 159-161, of sulphur, on p. 171, show some simple orthorhombic crystals; the faces p and s belong to rhom- 87. bic pyramids, and n is a dome. Fig. 89 shows a more complex crystal, and Fig. 90 is a basal section of the same. Here the faces lettered e, f belong to two rhombic pyra- mids; d, li, k to domes; a, b are pinacoids, and c is the All these rhombic forms belong in the ORTHORHOMBIC 90. SYSTEM, 'n which the planes are referred to three unequal axes at right angles to each other, as shown in Figs. 84, 85, 86. It will be seen that the grouping of faces about the ex- tremities of each of the axes differs from that about the 44 MINERALS, AND HOW TO STUDY THEM. others, but there are three planes of symmetry parallel to the faces a, b, c, which, however, are all unlike. V. Monoclinic System. Oblique Rhombic Prism. Fig. 91 shows an ohlique rhombic prism in which the end plane is rhombic in form, but it is oblique to the faces of the prism, instead of being at right angles as described in the Orthorhombic System. This oblique prism and a variety of other forms of the same group belong to the MONOULINIC SYSTEM, in which the planes are referred to three unequal axes, two of the angles between which are right angles, while the third (in front) is obtuse. In Figs. 91 and 92 the forms resemble Figs. 84 and 86 somewhat, but the top or basal plane c is inclined to the front edge of m (Fig. 91), and to the plane a (Fig. 92), not at right angles to them; the faces , b are also called pinacoids, while m is the prism. The monoclinic forms are too difficult to be described fully here, but it is not hard to learn what is most essential about them. Thus in Figs. 93 to 95 (of pyroxene) it is seen that the faces are in pairs alike on either side of a middle plane parallel to the face lettered b. In other THE FOEMS OF CRYSTALS AND KINDS OF STRUCTURE. 45 words, this middle plane parallel to b is a single plane of symmetry, the only one existing. The forms are named much as in the Orthorhombic System: m is a prism; u, v,o are pyramids (half -pyramids since each face occurs only four times); a, b are pinacoids, and c is the basal plane. Planes on the alternately obtuse and acute edges between cand a (Fig. 92) are called orthodomes; those on the four similar edges between c and b are clinodomes. Fig. 96 shows a basal projec- tion of a more complex mono- clinic crystal; here the sym- metry parallel to the faces b is clearly exhibited. Fig. 97, of datolite, shows another com- plex monoclinic crystal; here g, m x) are clinodomes, also u, x, orthodomes ; n, e, X, F, q are all pyramids. VI. Triclinic System. Finally there is one other group of forms, which belong to the TRICLINIC SYSTEM. In this system the planes are referred to three unequal axes all oblique to each other. Here all the intersections are oblique and the like faces 46 MINERALS, AND HOW TO STUDY THEM. are in pairs only on opposite sides of a crystal; there is no plane of symmetry at all. Fig. 98, of axinite, and Fig. 99, of albite feldspar, show two triclinic crystals. Here it is seen that the like planes are in sets of two each, one in front, the other behind, repre- sented in dotted lines. In Fig. 99 there is some resem- blance to a monoclinic crystal, but the angle between the faces b and c is not 90 as it must be there; and moreover the angles bin, bM are different, as are also the angles bo, bp. Hence m and M are different planes, and also o and j9, The subject of triclinic crystals will not be carried further, because of its great difficulty. When we come to examine a large number of crystals of different species, we are at first discouraged by the almost infinite variety which we find. But as we gain in experi- ence, much of this difficulty gradually disappears and we become able to classify a large part of them into one of the six groups or systems which have been mentioned. To learn all about crystals, however, is a difficult matter, and it will be best for the student to master the other characters of minerals, and to become well acquainted with the com- mon species, before taking up a larger book to study the subject of crystallography in detail. THE FORMS OF CRYSTALS AND KINDS OF STRUCTURE. 47 MEASUREMENT OF ANGLES. It is a help in studying minerals to be able to measure the angles between the faces of a crystal, for in this way it is possible to tell a square prism from a rhombic prism, a cube from a rhombohedron, and so on. The simplest method, when the crystal is large enough, is to use the in- strument shown in Fig. 100, called a contact goniometer. This is best made of brass with a circle or semi-circle, 100. divided accurately into degrees and half-degrees, and pro- vided with two arms moving on a pivot at the center. In Fig. 100 these arms are fixed at the center o, and sup- ported by om, but they can slide forward and back so as to accommodate them to crystals of different size. The crys- tal is placed between the jaws a and c, in such a position that the two faces whose angle is to be measured are ex- actly in contact with them, and the edge between these faces is at right angles to them. The angle (measured from the zero, 0, at Jc] is then read off from the upper edge of d, for the lines from d and k pass precisely through the center o and are parallel to the edges at a and c. 48 MINERALS, AND HOW TO STUDY THEM. A very fair substitute for an expensive goniometer can be made from a cheap protractor, by first cutting out two 101. ^ arms of thin wood shaped like the steel ones of Fig. 101, and then putting through a brass pivot on which they can turn.* It is not necessary nor desir- able that these arms should be permanently attached to the protractor. One pair of edges (the inner edges to the right in the figure) must be exactly in line with the center or pivot; between them the angle is read off when the arms are placed on the protractor, the pin then passing through its center and one edge through its zero. The other pair of edges (to the left) must be parallel to those first mentioned, so as to give the same angle; the two faces of the crystal whose angle is to be measured are placed be- tween them, as before explained. For measuring the angles between the faces of very small crystals an instrument called a reflecting goniometer is used. This is described in larger works. It is some- what expensive, and its use requires both skill and ex- perience. It demands, moreover, polished faces if good results are to be obtained. IRREGULARITIES OF CRYSTALS. Distorted Crystals. Most of the crystals of minerals would give a very poor impression of nature's workmanship * One of the small brass screws used to fasten together sheets of manuscript can be employed. THE FORMS OF CRYSTALS AND KINDS OF STRUCTURE. 49 to one who expected always to see them exactly like carefully-made models, or like the figures given on the preceding pages. The cubes of galena which we find are often flattened or drawn out. An octahedron (Fig. 102) may be flattened so as to look like Fig. 103; a dodeca- hedron (Fig. 104) may take the forms shown in Fig. 105 or 106. And so of other crystals. But it is not really to oe supposed that the forms are badly made, like a bad model; 102. 103. on the contrary, the size of the like faces on a crystal may vary, and so the shape of the solid as a whole, but the angles 104. 105. 106. between them remain the same. Moreover, when we study a crystal more carefully, we find that what is really essential is not the size or shape of each face, but the way in which the little molecules of which the whole is built up are arranged. For example, in a cube the essential point is the fact that the structure is the same in the direction of the three cubic faces. It follows from this that in the cube not only are the angles between two adjacent faces 50 MINERALS, AND HOW TO STUDY THEM always 90, but the six cubic faces are all similar; and therefore if there is the easy fracture, called cleavage, par- allel to one cubic face, there will be the same cleavage also parallel to the others. But the actual size of the faces is a matter of no importance. In fact, in one species the cubes are sometimes lengthened so as to be like fine hairs. Similar remarks can be made in regard to the distorted octahedron and dodecahedron figured above, and indeed about any distorted form. The symmetry in the molecu- lar structure, and hence the angles between the faces, re- main unchanged, although the symmetry of the external geometrical form is not that of the ideal crystal. Another good example of what is possible in a distorted crystal can be explained by referring to Fig. 107, a cube 107. with octahedral faces on its solid angles. Now instead of this, the ideal form, it is common to find in natural crystals no two of the triangular octahedral faces of the same size; some of them may even be ab- sent; while the cubic faces vary widely also. But such a crystal is not essentially different from Fig. 107, for every octahedral face is identical with each of the others if it is equally inclined to the three adjacent cubic faces, that is, to the three crystallographic axes, even if the faces all differ in size. In other words, it is here, as always, the position of the face, as showing the kind of molecular structure, not its size, which is essential. In the same way a cube may in nature look like a square prism, for the angles between the faces are all right angles in both cases, and so the goniometer will not tell the differ- THE FORMS OF CRYSTALS AND KINDS OF STRUCTURE. 51 ence between them, as has been already explained; but the molecular structure of the two is not to be confounded. In the square prism there is the same arrangement in the transverse directions, but a different one up and down; hence the square top of the crystal is not like the four similar oblong vertical faces, and we may find cleavage parallel to one set and not to the other. From the fact that so much variation is possible in the size of the like faces on a crystal, and hence in the shape of the whole, it is evident that the practical study of natural crystals is much more difficult than the study of the mod- els which give the ideal forms. This is especially true because most crystals are so implanted on the rock, or em- bedded in it, that only part of the form has been devel- oped. Thus quartz crystals are often attached at one extremity, while only the other end Jias had a chance to grow freely. Or the crystals may be implanted upon a surface of rock so that only a series of minute faces and angles are visible. In such cases the study of the form is really a difficult matter requiring much skill and experi- ence, and the beginner should not be discouraged because he cannot tell at once what the form of a crystal really is. Even here, however, some conclusion can often be drawn from the shape of the faces: thus, if regular triangles, they probably belong to an octahedron; or if rhombs, to a rhombic dodecahedron; and so on. Besides the crystals that have been just spoken of, which, while they look at first irregular, are really perfect in the matter of the position of the faces and of the angles between them, there are others which are really deformed, MINERALS, AND HOW TO STUDY THEM. 109. Some peculiar conditions attending the growth of the crystal, or perhaps some force which has acted upon it since it was formed, has resulted in bending or twisting it 108. out of its normal shape, so that it may differ widely in angle from the regular form. Thus the faces may be curved, as with the barrel-shaped crystals of pyro- morphite, or like the peculiar convex faces common on crystals of the diamond; or the whole crystal may be bent, as is seen sometimes in the case of crystals of quartz, or of stibnite, or of some kinds of chlorite (Fig. 108). Or again, aside from this curving and twisting, a crystal may have had its shape more or less changed by some forco exerted in the rock since it was made; it may even have been broken and again cemented together, so that many irregu- larities may result. Fig. 109 shows a crystal of beryl which has been broken into many pieces, these slightly displaced from each other, and the whole cemented together by quartz. Other irregularities of crystals besides that mentioned also occur. The faces of crystals, instead of being perfectly smooth, are often rough, perhaps because made up of a multitude of crystal points. Or they may be covered with fine lines, or striations, as those on the cubic faces of pyrite (see Fig. 183 on p. 213), which are ex- plained by the successive combination of another face (the THE FORMS OF CRYSTALS AND KINDS OF STRUCTURE. 53 pyritohedron) in narrow lines with the cubic face. In Fig. 110 the fine lines represent striations on a dodecahedral face due to the pres- ence also of the octahedron. This oscil- latory combination, as it is called, may go so far as to make the crystal nearly round, like some prismatic crystals of tourmaline. Striations are also sometimes due to twinning, as is common with the triclinic feldspars 111. ( see P- 59 > also Fi S- 2 ~ 3 > P- 289 )- -Fig- m shows an octahedral crystal of magnetite with twinning lamellae appearing as stria- tions on an octahedral face. Again, the faces may have a multitude of little elevations or depressions, the latter like the pits spoken of on p. 65 as produced by etching; in fact their presence can some- times be explained as due to etching by nature. The same cause the action of some partial solvent after the formation of the crystal often explains the rough faces alluded to. Connected with the subject of minute eleva- tions on the face of a crystal is the replacing of the face by one or more others varying a very little from it in angular position. The very low pyramids seen on some cubes of English fluorite are good examples of this (see Fig. 211, p. 245). Such planes are often called vicinal planes (from the Latin vicinus, neighbor- ing). Crystals which have formed rapidly may have only a more or less regular skeleton shape, like the crystal of salt 54 MINERALS, AND HOW TO STUDY THEM. represented in Fig. 112. The salt-crystals sometimes show 112. distinctly one face only with the depression in the center, so that they are called hopper-shaped crys- tals. The cavernous crystals of pyromorphite and vanadinite give other examples. Crystals, often for the same reason, enclose foreign substances, sometimes in the form of liquids, as the quartz crystals that contain water, occasionally with a movable bubble of air. Or the liquid may be carbon dioxide, then often with a bubble of the same substance in the form of gas. In such cases the crystal must have been formed under great pressure, sufficient to keep the gas in the liquid form. Fragments of such crystals heated in the gas-flame fly to pieces with great violence, because of the expansion of the gas formed from the liquid by heat. Crystals, too, frequently contain foreign solid substances of many kinds; quartz crystals thus enclose clay, particles of carbon, etc. The famous groups of large calcite crys- tals from Fontainebleau contain some sixty per cent of sand; it is most remarkable that the 113. force of crystallization was powerful enough to marshal into place the cal- cite molecules under such circum- stances. Sometimes the impurities' are regularly arranged in the crystal, and then a curious effect is obtained in a cross-section cut and polished. Fig. 113, of garnet enclosing quartz, shows this well THE FORMS OF CRYSTALS AND KINDS OF STRUCTURE. 55 Another interesting example is afforded by the variety of andalusite called made or chiastolite. Sections obtained at different points in a long crystal, when polished, may show the forms given in Fig. 114. Pseudomorplis. A peculiar class of false forms, in which the crystalline shape does not belong to the chemical sub- stance, must be briefly described here. These are called pseudomorphs. Pseudomorph* means false form, and the name is ap- plied to a specimen having the form characteristic of one species and the chemical composition of another. This seemingly difficult contradiction is easily explained. Most chemical compounds are liable to undergo a change or alteration when subjected to certain conditions, as moist- ure, the action of alkaline waters or acid vapors. Thus oxide of copper, the mineral cuprite (Cu a O), is lather easily changed chemically to the carbonate of copper, called malachite (CuC0 3 .Cu(OH) 2 ); the calcium sulphate, anhydrite (CaSOJ, assumes water and changes to the hydrated sulphate, gypsum (CaS0 4 + 2H 2 0); the sulphide of iron, pyrite (FeS a ), changes to the hydrated sesquioxide, limonite (2Fe,O s .3H 2 0). Now, in these and similar cases, if the original mineral was in crystals the external form is usually preserved, * From rpfvdijS, false, and juop(pt},form. 56 MINERALS, AND HOW TO STUDY THEM. often perfectly, while the chemical nature and the molec- ular structure have changed. Hence we describe the false forms mentioned as pseudomorphs of malachite after cu- prite, so too gypsum after anhydrite, limonite after pyrite. Other examples are pseudomorphs of chlorite after garnet, pyromorphite after galena, kaolin after feldspar. In a few rare cases, where the same chemical com- pound occurs in nature in two distinct crystalline forms, each with its own molecular structure, a change may take place in the structure of one of these minerals without alteration of the chemical substance. Thus the rare min- eral brookite (titanium dioxide, Ti0 2 ) may be changed to rutile (also TiO a ). Such pseudomorphs have the special name of paramorphs. The cases where the original substance has entirely dis- appeared and some other has come in to take its place are also called pseudomorphs. Thus we occasionally find quartz in the form of calcite, or of fluorite, or of barite, that is, pseudomorph after one of these; also tin-stone in the form of orthoclase feldspar; native copper in the form of aragon- ite. Even fossil wood may be said to be a pseudomorph of quartz or opal after the original wood, the structure of which it sometimes preserves with wonderful perfection. GROUPINGS OR AGGREGATIONS OF CRYSTALS. Some crystals occur isolated and alone, and then the form is usually developed on all sides, and with something of the regularity which the ideal model shows. Thus we find perfect garnets in mica schist or granite, and gypsum THE FORMS OF CRYSTALS AND KINDS OF STRUCTURE. 57 crystals in clay. But it is still more common to find crystals grouped together either irregularly, as in the majority of cases, or perhaps in parallel position, or again in the peculiar way called twinning; the last mentioned will be described first. Twin Crystals. The most interesting and important case of the grouping of crystals, or parts of crystals, is shown in those curious complex forms called TWINS. Fig. 115 represents a twinned octahedron, and Fig. 116 two twinned cubes. In the first case the growth of the 116. crystal as a whole has been such that one half has been developed in reversed position to the other, as if it had been revolved around through half the circumference or 180, and this about an axis (called the twinning axis) at right angles to two opposite octahedral faces.* This is de- scribed as a contact-twin. In Fig. 116 the two cubes interpenetrate each other, but each one is in such a position as if it too had been revolved 180 around an axis running through two diagonal angles (the same octahedral axis as in the first case). This is called a penetration-twin. * Such a line joining the middle points of two opposite octahedral faces is called au octahedral axis. 58 MINERALS, AND HOW TO STUDY THEM. These two cases illustrate all that is most essential about a twin. In every case the two crystals, or parts of crystals, are in such a position that one seems to have been turned around 180 with reference to the other, and this usually about an axis at right angles to some simple crys- talline plane, which is then called the t winning-plane. Fig. 1 17 shows a twin crystal, of the contact type, of cassiterite or tin-stone; Fig. 118 one of columbite, which 118. 122. also illustrates the point that the difference in direction of the striations of the two halves shows that the crystal is twinned. Fig. 119 again is a penetration-twin of stauro- lite, also called cross-stone. Figs. 120, 121, 122 show what are called repeated twins. These last are often very regular in form, the complex or twinned crystal being made up of perhaps three, five, six, or even eight parts of crystals or complete crystals, symmetrically arranged so as THE FORMS OF CRYSTALS AND KINDS OF STRUCTURE. 59 to resemble a star in many cases. Fig. 123, of rutile, shows another kind of repeated twinning. Still again the twinning may be repeated in the form of thin parallel layers, each in reversed position 133. to the next. This is called polysynthetic twin- ning and is best illustrated by a piece of a triclinic feldspar showing fine lines or stria- tions on a surface of .basal cleavage; these lines are simply the edges of the thin succes- sive parallel plates. By holding the specimen so as to catch the reflection of light from a distant window and turning it through a very small angle it may be seen that first one set of edges reflect together and then the other. This is illustrated by a figure of albite on p. 289. Fig. Ill on p. 53 shows polysynthetic twinning lamellae in a crys- tal of magnetite. It will be understood that the revolution spoken of has not actually taken place in ordinary cases; the rule is simply given in this form so as to show best the mathe- matical relations in position of the two parts. Still it is most interesting to note that in a few cases it is possible to cause the molecules of part of a crystal to change their 124 position so as to produce twinning arti- ficially, as by pressure in the proper direction. Thus Fig. 124 represents a cleavage ^ piece of calcite placed with its obtuse edge on a firm surface and then pressed by a knife (not too sharp) at a. Steady uniform pressure ^rves to reverse the position of the molecules in the part \ 80 MINERALS, AND HOW TO STUDY THEM. lying to the right so that the whole is pushed to the side and assumes a twinned position with reference to the rest. If skillfully done, no change in the transparency of this part takes place and the new surface gee is per- fectly smooth. In nature pressure may have produced twinning after the formation of the crystal; it is then called secondary twinning. The twinning layers or la- mellae observed in most cleavage masses of otherwise clear calcite are often to be explained in this way. This may also be true in some cases of the similar layers common on large crystals of pyroxene and which cause a separation or " parting" parallel to the basal plane, appearing much like the easy fracture called cleavage. It is evident that, since the crystals on a single specimen of a species may be grouped in a great variety of ways, it is not always easy to decide whether a given case is a twin or not; this often becomes a matter requiring careful study, exact measurement of angles, and calculation, per- haps also of optical study. For example, it is common to find quartz crystals crossing each other at a great variety of angles, but real twins like Fig. 125 are rare. The two parts of a true twin are usually symmetrical with reference to the twinning plane, and there is always the reversal of one half in the way described. Parallel Grouping. One very com- mon case of the grouping of crystals, which the beginner is apt to confound with twinning, is where the crystals or parts of crystals are parallel to each other, so that the axes 125. THE FORMS OF CRYSTALS AND KINDS OF STRUCTURE. 61 of all have the same directions, and are not inclined as in most twins. This is illustrated by a pile of cubes with faces parallel and having re-entrant angles between them. The crystals of many species are at times arranged in this way, but in every case it will be found that if the complex group is held so as to reflect the light from a window, the faces in adjoining crystals which reflect at the same time are always similar faces. An octahedron of fluorite, built up of a multitude of little cubes in parallel position, is a not very rare example of this. Fig. 126 shows a complex crystal of analcite, formed of a number of single crystals all parallel; 126. 127. 128. it is hence not a twin. The large and complex crystal of quartz which forms the frontispiece illustrates well this parallel grouping in the subordinate parts. Parallel grouping is most interesting when the result is to build up a compound form with branching and rebranching parts like the limbs of a shrub or tree, and hence giving rise to a kind of structure called arborescent or dendritic; here all the crystals or parts of 129. crystals have their axes in the same direction. This is shown in Fig. 127, of native copper. Fig. 128 is similar, though here there is also some twinning, but not distinctly shown on so small a scale. 62 MINERALS, AND HOW TO STUDY THEM. In Fig. 129 the little plates of hematite are grouped together so as to form a large crystal, but with such varia- tion in their position that the top has the shape of a rosette. Such a crystal is called by the Germans an Eisenrose, or iron-rose. Another curious and interesting case is where a number of crystals are implanted upon the surface of another which has obviously so influenced their growth that they are in parallel groups, and in a definite position relative to it. Many cases of this have been noted, as, for example, the rutile crystals on a tabular crystal of hematite, as shown in Fig. 130. Fig. 131 is a related case; it consists now of rutile alone, and has been described as a pseudomorph (see p. 55) of rutile after hematite. In the natural healing of 130. 131. the broken surface of a crystal, as of quartz (alluded to on p. 20), it follows, almost as a matter of course, that the new molecular growths should be in directions parallel with the old ones. The common method of grouping of crystals, however, is quite irregular, and it is only exceptionally that twins or parallel groupings are noted. And here a few terms often used in describing specimens must be explained; THE FORMS OF CRYSTALS AND KINDS OF STRUCTURE. 63 they are given to describe the way the crystals or crystal- line parts are aggregated. If the crystals are very small and closely clustered, giv- ing a rough surface a little like coarse sandpaper, it is said to be drusy. Or the crystals may cross each other more or less regularly like the meshes of a net, and this form is said to be reticulated. When the crystals or crys- talline fibers making up a mass are very slender, some- thing like needles, the structure is said to be acicular; if they are fine like hairs, and perhaps found in a tangled wad, it is said to be capillary; if fine and soft like moss, mossy. STRUCTURE IN GENERAL. Minerals are not always in distinct crystals, like those of garnet or quartz, or even in aggregates of crystals. On the contrary, many of the specimens in a mineral cabinet show no crystalline faces at all, and then they are simply called masxive. There are, however, important distinctions of structure between massive minerals. First of all, the distinction between crystalline and amor- phous must be well understood. A piece of clear quartz, or rock crystal as it is often called, is said to be crystal- line; a piece of glass which very likely the eye alone could not distinguish from it is amorphous or formless. For the mass of quartz, though it has no definite external form, but is bounded only by irregular fracture surfaces, is just as truly crystalline in structure as a perfect crystal; in fact it may be itself a fragment of a large crystal. This is true because the essential idea about a crystallized mineral 64 MINERALS, AND HOW TO STUDY THEM. 132. is that there should be the regular arrangement of the molecules out of which it is built up. It is not always easy, often impossible, with the eye alone to decide whether this regularity of structure exists. It is shown by the cleavage, as will be explained in a later part of this chap- ter; but when there is no cleavage, it is usually by optical examination in what is called polarized light that this can be most easily proved. For example, the bright colors given by a thin fragment of a quartz crystal in polarized light shows at once, to one who understands the subject of optics, that it is crystalline in structure. In the glass, on the other hand, the molecules have no definite arrangement at all, and hence no action on po- larized light. It is somewhat like the difference between a pyramid built of blocks of stone laid in regular rows or after a more intricate pattern, and one in which the blocks are tumbled in without order only here we should need mortar to cement the whole together, while na- ture's molecules, in either case, are bound together by the force of cohesion. It is interesting to note here one of the ways which the skillful mineralogist has of finding out how the molecules of a crystal or crystalline mass are arranged : and this may be a very important matter, for next to the chemical substance this molecular structure is the most essential character of a mineral. He does this by what he calls etch- ing, that is, by allowing some liquid (or sometimes a gas), which has the power of dissolving the substance examined, THE FORMS OF CRYSTALS AND KINDS OF STRUCTURE. 65 co act upon a smooth surface for a short time. Then it is removed, the surface washed off and examined under the microscope. Often a multitude of little cavities or pits are found on the surface whose shape shows clearly how the molecules are built up. It is very much as if the stones of the pyramid spoken of were so smooth and closely fitted that no joints were visible, and the mason sbould go to work and pull out a number till he could see what the pattern was. Fig. 132 shows the etching-figures on the faces of a crystal of quartz; the variation in their form reveals the complex structure which is described further on p. 275. To continue the discussion of the structure of massive minerals. In general it can be said that such specimens are crystalline in structure even when there is no distinct evidence of it to the eye, for the amorphous glasslike condition is the exception, not the rule. There are, however, cases in which the structure seems to be inter- mediate between the distinct crystalline and the amor- phous, and this is called crypto-crystalline, as is true of many of the massive kinds of quartz. Among the speci- mens of massive crystalline minerals there are a great variety of different kinds of structure, and it is necessary to become acquainted with the terms used in describing these. It has been stated that the easy fracture in certain directions, called cleavage, is the surest and easiest way of showing that a massive mineral is crystalline in struc- ture. Such a mass is said to be cleavable, which means capable of being cleaved, or showing cleavage. The kinds 66 MINERALS, AND HOW TO STUDY THFM. of cleavage are spoken of more fully on pp. 70-72. This cleavage proves that the mass is crystallized, and also often reveals what sort of structure it has. The cleavage may be in the same direction in all parts of the mass, as if it were a piece of one large crystal; or, more commonly, the cleavage directions may be the same in one little spot only that is, for each individual grain. In the second case the mass is obviously crystalline and the structure is said to be cleavable and granular. Such a mass of galena is really made up of a multitude of little grains, each one of which has its own directions of cleavage and presents to the eye its own edges. If the individual grains are large, the structure is said to be coarse-granular} and if small, it is fine-granular. From the latter we pass to the closely compact kinds in which the structure is some- times not obvious to the eye at all; they may then be said to be impalpable. But this extreme is rare; for example, a piece of white marble, even if it is so fine-grained that the particles cannot be seen by the eye, usually sparkles in a strong light from the reflection of the mul- titude of minute cleavage-faces. This granular structure may belong also to other min- erals which do not have the distinct evidence of crystalline structure in the cleavage, and then the grains may often be more or less easily separated from one another; some- times, as in granular kinds of pyroxene, they are found to be imperfect crystals. Again, if the mass seems to be made up of layers, whether separable or not, it is called lamellar. If it is in thin leaves or plates, which can be separated from one THE FORMS OF CRYSTALS AND KINDS OF STRUCTURE. 67 another, it is called foliated, as with graphite. It is further called micaceous when the separation takes place as readily as in a piece of mica. If the mass is made up of little columns, it is called columnar. When there are distinct fibers, it is said to be fibrous; asbestus is said to be fine-fibrous, and with it the fibers are easy to pull apart, or separable, but this is not always the case. There are many intermediate kinds between the fine-fibrous and coarse-columnar. If the fibers, or little columns, or leaves all go out from a center like the spokes of a wheel, the structure is said to be radiated, as wavellite (Fig. 133), or perhaps stellate when it is star-shaped, as gypsum (Fig. 134). If the layers are arranged in parallel position about one or more centers, the structure is said to be concentric; as malachite (Fig. 135). All these terms, granular, foliated, lamellar, columnar, fibrous, radiated, etc., ordinarily refer to the structure of the mass when it is more or less distinctly crystalline. When the external surface of the mineral is in the form of rather large rounded prominences, it is called mammillary, as malachite (Fig. 136); if the prominences are smaller, somewhat resembling a bunch of grapes, it is called botryoidal, as prehnite, smithsonite, and chalcedony (Fig. 137). If the surface is made up of little spheres or globules, it is called globular, as hyalite or prehnite (Fig. 138). If the form is that of a kidney, it is called reni- form, as hematite (Fig. 139). It will be understood that there is no sharp line dividing these different cases. Sometimes the mineral takes the form of a delicate 68 MINERALS, AND HOW TO STUDY THEM. 133. 134. tes. 133 to 140 varieties of structure: 133, Wavellite, radiated; 134, Gypsum, stellate ; 135, Malachite, concentric; 130, Malachite, mammilla,,/; 137 Chall , , cedony. botryoidal ; 138, Prehnite, globular ; 139, Hematite, reniform 140 , stalactitic. THE FORMS OF CRYSTALS AND KINDS OF STRUCTURE. 69 branching coral, and it is then called coralloidal, as cer- tain varieties of aragonite (which see). If made up of 141. Concretions from Clay-beds. forms like small stalactites, it is said to be stalactitic, as limonite (Fig. 140). If the material has clustered about a center like those curious forms of impure calcium carbonate called con- cretions common in the clay (Fig. 141), it is named concretionary. Dendrites, or dendritic forms, are those which have more or less the shape of a branching shrub or tree, as the forms of manga- nese oxide (Fig. 142) often seen on surfaces of smooth limestone or inclosed in moss- agates. Some dendritic forms are made up of little crystals grouped together in parallel position as remarked on p. 61. 70 MINERALS, AND HOW TO STUDY THEM. CHAPTER IV. THE OTHER PHYSICAL CHARACTERS OF MINERALS. BESIDES the external form of minerals shown in th crystals there are also a series of other physical characters, based (1) upon the molecular force of cohesion, (2) upon density, and (3) upon the action of light; also other characters of less frequent use, depending upon heat, elec- tricity, magnetism ; finally, a few minerals have distinctive characters of taste and odor. 1. CHARACTERS DEPENDING UPON COHESION. The characters depending upon the molecular force of cohesion will be described first. These include the cleav- age, fracture, hardness, tenacity, and elasticity. Cleavage. The forms of crystals, as has been repeatedly stated, de- pend upon their molecular structure; but this internal structure reveals itself also by the cleavage, or the natural easy fracture which yields more or less smooth faces in cer- tain definite directions. A cleavage surface marks a direc- tion in which the force binding the molecules together is relatively weak. Thus galena is said to have cubic cleavage because its molecules separate readily in a direction parallel THE OTHER PHYSICAL CHARACTERS OF MINERALS. 71 to each pair of parallel cubic faces; in other words, when fractured it breaks into a multitude of little cubes (Fig. 143). Fluorite, or fluor-spar, has octahedral cleavage, since the separation of the molecules is 143. easy parallel to each pair of octa- hedral faces; hence from a single mass we may with care break out an octahedron. This cleavage oc- tahedron, by the way, is readily distinguished by careful examina- tion from a real octahedral crys- Cubic cieavage-Gaiena. tal, because the faces are uneven and splintery, not uni- form like the normal faces of a crystal. Even when the faces of a crystal are rough and uneven, they are quite dif- ferent from the surfaces formed by cleavage, however per- fect. The octahedral cleavage of fluorite is also seen in the case of a cubic crystal, since its solid angles may be easily broken off, giving a form like Fig. 107 on p. 50, which we have learned is a cubic modified, or with its angles replaced, by the planes of an octahedron. Again, from a piece of sphalerite or zinc blende a dodecahedron may sometimes be broken out because of its perfect dodecahedral cleavage; or if this is difficult because the fragment having like cleavage directions is too small, the skillful observer can yet prove that the cleavage-faces have the position of dode- cahedral planes and make angles of 60 and 120 with each other. Further, a piece of calcite breaks readily into a number of rhombohedrons, all of the same angle, and it is hence said to have perfect rhombohedral cleavage (Fig. 144). 72 MINERALS, AND HOW TO STUDY THEM. In the same way we find that amphibole has prismatic cleavage; mica has highly perfect basal cleavage or cleav- age parallel to the end or basal plane, yielding excessively 144. thin sheets; topaz has also basal cleavage. Gypsum has perfect cleavage parallel to the side plane of the crys- tal, yielding plates almost as thin as those of mica; these plates show two other cleav- i Cieavage-Caiche. ages on the edges, but dif- ferent in character from each other, as is more fully described under the description of this species. Feldspar shows two cleavages, both nearly perfect but one a little more so than the other, and these make an angle of 90 or nearly 90 with each other. All these are cases of perfect cleavage, and it will be at once seen how important a character the cleavage is. But the cleavage is not always perfect, as in the exam- ples given; sometimes it is obtained with difficulty, or the surfaces yielded may be only partially smooth; in such cases it is said to be imperfect, or interrupted, or difficult. Occasionally cleavage exists in cases where it is so hard to obtain that it is ordinarily not noted at all. Thus crystal- lized quartz usually shows only a conchoidal fracture, and the absence of cleavage is a character which at once dis- tinguishes it from the feldspar with which it is often associated ; yet a crystal of quartz which after being heated has been plunged into cold water often shows cleavage parallel to the pyramidal planes, perhaps also parallel to THE OTHER PHYSICAL CHARACTERS OF MINERALS. 73 the prism. Cleavage-rifts in these directions are some- times seen in natural crystals. It is a help to remember that, even if a crystal does not actually show broken surfaces, the cleavage is often clearly indicated by a fine pearly luster on the face of the crystal parallel to it. This is seen on the basal plane of apophyl- lite, on the side plane of stilbite, and in many other cases. This pearly luster is due to the presence of cleavage rifts, though the crystal has not actually parted, just as a pile of thin glass sheets shows a pearly luster on top because of the repeated reflections. Similarly the cleavage rifts can often be seen in a transparent crystal; a flat clear crystal of barite or of celestite (Fig. 145) often shows the prismatic cleav- age in two directions making an angle of about 104 with each other. A massive specimen of a mineral may show cleavage as a multitude of little smooth faces changing position with that of the grains to which they belong; if these grains are very small, the cleavage may appear only as a fine spangling of the surface, as was mentioned on p. 66. Fracture. The nature of the surface given by fracture, when not the smooth surface of cleavage, is often an important char- acter to note, especially in distinguishing the varieties of a 74 MINERALS, AND HOW TO STUDY THEM. mineral species. Thus glass, as well as quartz and many mineral species, shows a shell-like fracture surface which is called conchoidal (Fig. 146), or, if less distinct, small conchoidal, or subconchoidal. 146. Conchoidal Fracture Obsidian or Volcanic Glass. More commonly the fracture is simply said to be uneven, the surface is rough and irregular. Occasionally it is hackly, like a piece of fractured iron. Earthy and splin- tery are other terms sometimes used and easily under- stood. Hardness and Tenacity. By hardness the mineralogist understands the degree of resistance which the smooth surface of a mineral offers to a point or edge tending to scratch it. A diamond easily makes a scratch on a smooth topaz crystal; the topaz scratches a quartz crystal, while the quartz scratches a glass surface, and the glass in turn scratches one of cal- cite. This means that each substance named is harder THE OTHER PHYSICAL CHARACTERS OF MINERALS. 75 than that which it scratches, or, in other words, softer than the one by which it is scratched.* Mineralogists have found it convenient to select a num- ber of minerals for the comparison of hardness, and they are designated by the numbers from 1 to 10, as given in the following list. Crystallized varieties are to be taken in each case, that is, a crystal with even surfaces or a smooth cleavage fragment. 1. Talc. 6. Orthoclase. 2. Gypsum. 7. Quartz. 3. Calcite. 8. Topaz. 4. Fluorite. 9. Corundum. 5. Apatite. 10. Diamond. When it is said that the hardness of a mineral is 4, this means that it is scratched as easily as fluorite, for example by a mineral which follows in the list, while it will itself scratch all of the minerals which precede. If the hard- ness of a mineral is given as 5.5, this means that it is a little harder than apatite and a little less hard than ortho- clase. The student should practise with the minerals in this scale up to topaz or corundum, and then with them experi- ment upon some other known minerals, until he learns just what degree of hardness each of the numbers corresponds to, especially those up to 7. He ought soon to become so proficient as to be able to determine the lower grades of * In general a faint scratch can be made on the surface of a crys- tal by the edge of another of the same species; this is readily proved with quartz. 76 MINERALS, AND HOW TO STUDY THEM. hardness by his knife without the use of the reference species at all. He will find at once the following general distinctiona between minerals of the several grades: No. 1 : has a soft, greasy feel in the hand, like talc and graphite. No. 2: can be scratched easily by the finger-nail, as a cleavage-piece of gypsum. No. 3 : can be easily cut by the knife, but is not scratched by the nail. No. 4: is scratched by the knife without difficulty, but not easily cut like calcite. No. 5 : is a little hard to scratch with a knife. No. 6: is hardly touched if at all by the knife, but it will scratch ordinary glass. No. 7: scratches glass easily, but is scratched by topaz and a few other minerals mentioned in the list given in Chapter VIII. The minerals which are as hard as or harder than quartz are few and include all the highly-prized gems. Some further notes on hardness are given in connection with the list referred to. The beginner will need a word of advice in regard to testing for hardness. In the first place treat a mineral, especially a crystal, with as much consideration as possible. A scratch on a piece of plate-glass, like a daub with a paint- brush on a white wall, is a little thing, but it may have a sadly disfiguring effect and make a pane worth when new a thousand dollars unsalable except to be cut up. So a scratch on a crystal disfigures it and destroys its value in large measure in the eyes of one who is a true mineral- THE OTHER PHYSICAL CHARACTERS OF MINERALS. 77 ogist. Hence make as minute a scratch as possible, not longer than this , and if possible put the scratch, not on the most prominent surface, but where it will show least. Treat the crystal, then, as if it had feelings to be hurt by the cut, and never scratch its smooth surface wantonly, nor if it is possible to obtain the desired information with- out it. On the other hand, it is necessary to be sure and distin- guish between a real scratch on a smooth surface and the crushing of a rough surface by the knife-edge; a very hard mineral may often be scratched in this way. The danger of making a mistake of this kind is made less if, besides the useful knife-point, the mineral be rubbed on a piece of glass; better have a piece at hand (not disfigure a window-pane). Only do not make the opposite mistake and call a white ridge left by a soft mineral on the glass, which can be easily rubbed off, a scratch. Once more, it is necessary to remember that minerals are often altered, as the chemist says; that is, they may have undergone some chemical change, particularly on the surface, which has rendered that soft when the original mineral was really hard. Thus it is often easy to make a scratch on a crystal of corundum because of a little sur- face change, while the mass within is very hard. If the mineral is used to scratch with, the danger of a mistake here is lessened. There are also some other characters depending upon the force of cohesion acting between the molecules of a mineral. These include the following, which are some- times grouped under the general head of TENACITY: 78 MINERALS, AND HOW TO STUDY THEM. Malleable: capable of being flattened out under the blow of a hammer without breaking or crumbling into fragments. This is conspicuously true of gold and silver, and makes it possible to beat out gold into leaves of extreme thinness. The property of malleability belongs only to the native metals and, in an inferior degree, to a few compounds of silver. Ductile: capable of being changed in shape by press- ure, especially of being drawn out into the form of wire. This is true of gold, also still more of silver and platinum. It is a property which belongs in a high degree only to the native metals among minerals. Sectile : capable of being cut by a knife like cold wax, so that a shaving may be turned up with care, and yet the mineral breaks with a blow, and is not properly malleable, sometimes not at all so. Cerargyrite is eminently sectile. Gypsum and a number of soft minerals are imperfectly sectile. No sharp line separates the minerals which show these characters and the truly brittle minerals. Flexible : the mineral bends easily, and stays bent after the pressure is removed. This is shown in talc. Brittle : separating into fragments with a blow or with a cut by a knife. This is true, in varying degrees, of nearly all minerals. The elasticity is another physical character based upon cohesion. A mineral is said to be Mastic when it is capable of being bent or pulled out of shape and then returning to its original form when re- lieved, as a plate of mica. Oil the other hand, a cleavage- plate of chlorite is inelastic. THE OTHER PHYSICAL CHARACTERS OF MINERALS. 79 2. SPECIFIC GRAVITY OR RELATIVE DENSITY. It has been shown that the eye-examination of a mineral tells the observer something about its form, if crystallized, and of its structure in other cases; it tells whether it has cleavage or not, and with what kind of surface it breaks; a simple trial also shows how hard it is. At the same tim -, as it rests in the hand, it should be noted whether it seems distinctly heavy or light as compared with some common substances of similar appearance. In this way a first suggestion is obtained, not exact and, as we shall see, not always correct as to the density of the mineral. It is necessary at the outset to have a pretty clear notion as to what a difference in density means. Suppose two doors of just the same size, and both swung carefully on hinges so that they move almost without friction, but one of wood, the other of iron. Every one knows that the force required to push the iron door will be much greater than that which the other requires. Again, a kick against a wooden ball resting on the ground encounters more resist- ance than against a ball of paper, but less than one of stone. These experiments show that in a piece of iron there is more matter to move, it has a greater mass for the same bulk than the wood. In other words, it has a greater density. This is generally expressed by saying that the weight of the iron is greater than of the wood of the same size, and this is true because the attraction of the earth, which gives an iron ball its weight, is in proportion to the mass or, for different bodies of the same size, to the density. 80 MINERALS, AND HOW TO STUDY THEM. We may say, consequently, that the relation of the den- sities of different bodies is given by the weights of blocks having the same bulk. Suppose we could cut out like blocks of aluminium and iron and weigh them, the weights would be not far from the ratio of 1 to 3, and this would be the relation in density. Now to make this comparison for all bodies, it is evidently important, in the first place, to choose some one of them as the standard, and make the comparison with this. The standard substance adopted is pure water, and, if great accuracy is required, this must be taken at the tem- perature 39.2 F. (4 Centigrade), where it has its maxi- mum density; for water, if growing cooler or warmer than 39.2 F., expands a little and grows less dense. The den- sity of minerals is then compared with water, and this density is called the specific gravity. Consequently the specific gravity of a mineral may be stated to be the weight of a fragment divided by that of an equal volume of water. The specific gravity of sulphur is 2, of corundum 4, of pyrite 5, etc., these numbers meaning that they are re- spectively 2 times, 4 times, and 5 times as dense as water, or, in other words, a given bulk a cubic foot, for example weighs 2, 4, and 5 times more than the same bulk a cubic foot of water. In order to find the specific gravity, it is not practicable to compare at once the weights of equal volumes, simply because (though it is easy to weigh, for example, a piece of calcite) it is not possible to get its volume with sufficient accuracy. Hence it is necessary to make use of a well- known principle in hydrostatics, that when a body is im- THE OTHER PHYSICAL CHARACTERS OF MINERALS. 81 mersed in a liquid it is buoyed up by the liquid, and to such an extent that it weighs, for example in water, less than it does in the air by the weight of the water it dis- places. Hence, if we find first the weight of the fragment on the pan of a delicate balance, and then its weight in the water, it being suspended from the pan by a fine thread, and subtract the two weights, the difference is the weight of the equal voh;me of water. For example, the weight of a little quartz crystal is 3.455 grams in the air; in the water it is 2.156; the loss of weight, or weight of a volume of water exactly equal to it, is therefore 1.299; hence the specific gravity is 3.455 3.455 3.455 - 2.156 1.299 In the description of species the specific gravity is often expressed by the initial letter G; thus for a quartz crystal G = 2.66. A spring-balance* like that of Jolly makes the operation very easy. This consists of a delicate brass spring, one end of which is attached to the top of a vertical scale, and from the other hang two pans, the lower one of which is im- mersed in water. The small fragment whose specific gravity is to be determined is placed in the upper pan, and the amount that the spring is stretched noted by the num- ber (JV,) coinciding with the reflection of an index in a strip of mirror upon which the scale is graduated ; then in the lower pan, and the scale number (N 9 ) again noted; the number (n) when the pans are empty is also ob- * This is figured on page 69 of Dana's Manual of Mineralogy, 1887. MINERALS, AND HOW TO STUDY THEM. served. The specific gravity is then given by the ex- pression In this case we do not have the actual weights given, but numbers which are proportional to them. A simple balance for de- termining the specific grav- ity, which also does away with the necessity of using definite weights, is shown in 3 Fig. 147 (one sixth natural I size), and can easily be cou- | structed * with a little care. | After the frame is made >, with the two uprights, a g thin piece of wood is cut in the form of the steelyard | beam, ale. This should be g graduated into inchesf and | tenths of inches from b, the axis, to the end c; the dis- tance from 5 to a, where the pans are supported, being made 4 inches. A fine wire passed through at b answers as the axis, and the pans are * The author is indebted to Prof. Penfield for the opportunity to figure and describe this balance. f The metric scale may be conveniently used instead. THE OTHER PHYSICAL CHARACTERS OF MINERALS. 83 also held by wires, best of platinum; the lower pan is im- mersed in water. A small piece of lead permanently placed between b and a serves to counterpoise the long arm, and a little rider is first moved to some point, as d, where it serves to make be exactly horizontal, as shown by the mark on the upright near the end c. A number of weights (as from to If grams) are further needed; these may be easily made of soft copper wire bent with a hook at one end, or they may be little glass tubes containing 1, 2, 3 or more shot and with a wire hook fused in at one end (see those at g). The fragment* experimented upon is first placed in the pan e, a suitable counterpoise chosen, and the number (JV,) on the scale where it must be placed to make be again horizontal recorded; it is then transferred to the pan /, immersed in the water, and the scale number again noted (N 9 ), the same counterpoise being employed. The first number then divided by the difference of the two numbers gives the specific gravity: For example, a small pyrite crystal is placed in the pan e> and a counterpoise chosen which balances the beam when placed 14.45 inches from b; the crystal is then transferred * A fragment or crystal as heavy as 7 grams may be employed with the weights mentioned; but it should be understood that it is not necessary to know the actual weight either of the fragment or of the counterpoise; thus if the actual weight of the counterpoise is C, and of the mineral in air and in water Wi and TF respectively, then, using the numbers Ni and N* as above, we shall have TF, X 4 = G X N- and TT.X4 = (7x^i; that is, ~ = ~. 84 MINERALS, AND HOW TO STUDY THEM. to the pan /, which requires that this counterpoise should be moved back to 11.55 inches. The specific gravity is thus found to be 14.45 14.45 14.45 11.55 2.9 = 4.98. It is evident that the accurate determination of the ipecific gravity is a somewhat difficult matter, requiring a ood deal of care, but, as was suggested at the "beginning, the hand can often give very valuable information in this direction after a little training. We are accustomed to handle fragments of rocks, as of granite (specific gravity about 2.7), of marble (specific gravity = 2.7), and of other like substances, and we know about what to expect as to the weight of them; hence if we pick up a piece of barite or heavy spar, perhaps thinking it is marble, which it may resemble so closely that the unaided eye cannot distinguish them, we notice or ought to notice at once that it is un- expectedly heavy, for its specific gravity is 4.5. So a piece of corundum, we say, feels heavy because it has a specific gravity as high as 4. This last, by the way, is an interesting case, because corundum is the oxide of the very light metal aluminium, and its relatively high density is connected with its great hardness; in other words, it is evident that its molecules must be much crowded together. In the same way we note that the substance carbon forms the hard and relatively heavy * mineral diamond (specific gravity = 3.5) * In these statements, as in some similar cases, the word heavy ia used instead of dense, that is, of high specific gravity, and also the word light to express the opposite character; the meaning will b clear even if these terms are not quite scientifically employed. THE OTHER PHYSICAL CHARACTERS OF MINERALS. 85 and the soft and light mineral graphite (specific gravity = 2). In other words, the density depends not only upon the kind of molecules (all lead compounds, for example, have necessarily a high specific gravity), but also upon the way they are built up together. Again, if we take up a metallic mineral, we compare it, perhaps without conscious thought, with other common metallic substances. Hence a piece of aluminium seems very light, because the specific gravity is only 2.5, about one third that of iron and less than one quarter that of silver. On the other hand, a fragment of galena seems heavy because its specific gravity is 7.5, or nearly equal to that of metallic iron. When we come to study minerals we find that they divide themselves as follows: I. Minerals of Unmetallic Luster* These may be roughly subdivided into three classes : 1. Minerals of relatively low density; the specific gravity not higher than 2.5. Examples are as follows (here G. = specific gravity): G. G. Borax 1.7 Stilbite 2.2 Sulphur 2.05 Gypsum 2. 3 Halite 2.1 Apophyllite 2.4 The zeolites (as stilbite above) mostly fall between G. = 2.0 and G. = 2.3. 2. Minerals of average density: specific gravity 2.6 to 3. Common examples are : * This matter of luster is more definitely spoken of on a later page of this chapter. 86 MINERALS, AND HOW TO STUDY THEM. G. G. Quartz.. 2.66 Feldspar 2.6-2.75 Beryl 2.7 Talc 2.8 Calcite 2.7 Muscovite 2.8 The scapolites (G. = 2.5-2.8) also belong in this group. The common minerals tourmaline (G. = 3.0-3.2), apatite (G.= 3.2), vesuvianite (G.= 3.4), amphibole (G.= 2.9-3.4), pyroxene (G.= 3.2-3.6), epidote (G.= 3.25-3.5) fall between this and the following group. Some varieties of garnet also belong here; others have a specific gravity up to 4.3. 3. Minerals of high density : specific gravity 3.5 or above; a fragment seems heavy in the hand; if the specific gravity is above 4.5, it seems very heavy. G. G. Topaz 3.5 Witherite 4.3 Diamond 3.52 Barite 4.5 Staurolite 3.7 Zircon 4.7 Strontianite 3.7 Scheelite 6.0 Celestite 3.96 Cassiterite 7.0 Corundum 4.0 Wolframite 7.5 Eutile 4.2 Cinnabar 8.0 The compounds of lead also belong here, the commonest of which are: cerussite, the carbonate; anglesite, the sulphate; and pyromorphite, the phosphate. They all have a specific gravity between 6 and 7. Compounds of iron (as siderite, G. = 3.8), of copper (as cuprite, G. = 6.0), of silver (as cerargyrite, G. = 5.5), and of the other heavy metals have also high specific gravities. THE OTHER PHYSICAL CHARACTERS OF MINERALS. 87 II. Minerals with Metallic Luster. The average is about 5, the specific gravity of pyrite and hematite. If the density is much lower than 4, the mineral seems light in the hand, as graphite (G. = 2). If the density is as high as 7, or above, they seem notably heavy, as galena, 7.5. Crystallized uraninite, which has a submetallic luster, has the remarkably high specific gravity of 9.7 when unal- tered. The variation is wider in the case of the metallic minerals than with those of un metallic luster, as vill be seen by comparing the following densities of the common metals :* Aluminium 2.5 Arsenic 5.7 Antimony 6.7 Zinc 7.1 Tin 7.3 Iron 7.8 Copper 8.9 Bismuth 9.8 Silver 10.6 Lead 11.4 Mercury (liquid) 13.6 Gold 19.3 Platinum (pure) 21.5 The quick judgment which comes with practice is al- ways of value, but it should be applied with discretion, for the mineralogist must be continually on his guard lest he be misled. In the first place, the size of the mass is an important factor, for a big lump of quartz seems heavy, of course, though its specific gravity is not relatively high. Also we * It is interesting to add the following, although these metals do not exist as such in nature, and are only interesting to the chemist : Lithium 0.59 Sodium 0.97 Potassium 0.86 Magnesium 1.8 88 MINERALS, AND HOW TO STUDY THEM. may get a wrong impression in handling a specimen if the mineral we are interested in only forms a small part of it; a little galena in a large mass of quartz will not make it heavy. Also, if the mineral is open and porous, and is made up of interlacing fibers, like some specimens of ce- russite, it may appear light, even if the specific gravity is actually high, because the eye is deceived by the appear- ance of bulk, while the solid mass present is not great. Some further suggestions on this subject are given in the closing chapter of this book. 3. CHARACTERS DEPENDING UPON LIGHT. Of the characters which are observed by the eye several have not been mentioned in detail as yet, to which perhaps the attention may be first attracted. These are those which depend upon the reflection or absorption of the light: (1) the luster or the appearance of the surface in- dependent of the color, due to the way the light is reflected; (2) the color; and (3) the degree of trans- parency. Luster. The difference in luster is not in all cases easy to de- scribe, but the eye notes it at once, and after a little train- ing seldom makes a mistake. The kinds of luster distinguished are as follows: Metallic : the luster of a metallic surface as of steel, lead, tin, copper, gold, etc. This is not always easy to distinguish, and the rule is an important one that the luster is not called metallic unless the mineral is quite opaque, so that no light THE OTHER PHYSICAL CHARACTERS OF MINERALS. 89 passes through even very thin edges. The luster of some minerals, as columbite, is said to be submetallic when it lacks the full luster of the metals. In a few cases a min- eral has varieties with metallic and others with uninetallic luster; this is true of hematite. Vitreous, or glassy luster: that of a piece of broken glass. This is the luster of most quartz and of a large part of non-metallic minerals. Adamantine, or the luster of the diamond: this is the brilliant, almost oily, luster shown by some very hard min- erals, as diamond, corundum, etc.; also of some others, having heavy molecules, as the carbonate (cerussite) and the sulphate (anglesite) of lead. All of these refract the light strongly, or have a high refractive index. Metallic-adamantine is a term used to describe a variety of the adamantine luster verging upon metallic, as seen in some dark-colored varieties of cerussite. Resinous or waxy : the luster of a piece of rosin, as that of most kinds of sphalerite or zinc blende; near this, but often quite distinct, is greasy luster, shown by some speci- mens of milky quartz. Pearly, or the luster of mother-of-pearl: this is common where a mineral has very perfect cleavage and hence has partially separated into thin plates. Thus the basal or top plane of crystals of apophyllite shows pearly luster. Silky, the luster of a skein of silk or a piece of satin : this is characteristic of some minerals with fibrous struc- tures, as the variety of calcite (or of gypsum) called satin spar; also of most asbestus. The luster of minerals is also described according to the 90 MINERALS, AND HOW TO STUDY THEM. brightness of the surface; it is called splendent in freshly fractured galena, but dull in jasper; while again it may be glistening or glimmering according to the nature of the surface. These terms explain themselves. Color. To understand what the color of a mineral means a little knowledge of optics is required. In the first place, W3 must recall that ordinary light can be separated, as by a glass prism, into the ribbon of colors ranging from the red to the blue and violet which is called the spectrum. This is shown in the rainbow, where the place of the prism is taken by the raindrops. All these different colors together give to the eye the effect of white light. If now this white light, as the ordinary sunlight, be passed through a piece of red glass or reflected from a surface of red paint, part of the colors are stopped or absorbed by the glass or paint and the rest give together the effect of red which the eye notes. Similarly a piece of malachite appears green for the same reason that the grass does, because the surface absorbs a part of the light, and the remainder which reaches the eye gives to it the effect of green. The variation in color is very wide and includes all kinds from white to black, running through many shades of red, yellow, green, and b'lue. Most of the terms used in describing the color are so familiar that they explain themselves. Thus we speak of azure-blue, cherry-red, and so on. Some examples of color among common minerals are given in Chapter VIII. THE OTHER PHYSICAL CHARACTERS OF MINERALS. 91 Now a few words as to the importance of color and the extent to which it may vary with the same species. As we go on to study the different mineral species we learn that the varieties of those with metallic luster do not vary much in color among themselves. Many of the other minerals, however, which do not have a metallic luster vary very widely; and in the same species, as tourma- line, we may have colorless kinds, also those that are differ- ent shades of red, blue, green, brown, and black. So too we call clear red kinds of corundum ruby, and clear blue kinds sapphire, although they both belong to the same mineral; and in this as in many other cases it is difficult to tell precisely what is the very slight chemical change upon which the color depends. In general, therefore, the color of minerals with metallic luster is a very important and constant character, and such variations as are noted are due chiefly to a little change of the surface, by which it becomes tarnished, as we say; that is, dull in some cases, or perhaps bright-colored in others. Thus galena has a very characteristic bright bluish lead- gray color, which, however, may become quite dull if the surface has been long exposed to the air. Again, the pale brass-yellow of pyrite and the still paler shade of the same color characteristic of fresh marcasite are readily dis- tinguished, but both are subject to tarnish, especially the latter, and this may result in making the distinction be- tween the two impossible in the case of exposed surfaces. Hence it is necessary to be on our guard to insure that what we see is the true color of a mineral, that is, the color which is obtained from a fresh surface by fracture. 92 MINERALS, AND HOW TO STUDY THEM. Another striking example is given by the mineral bornite, which has a very peculiar reddish-bronze color on a fresh surface, on account of which the miners of Cornwall have called it horse-flesh ore. But this changes or tarnishes so easily that it almost immediately becomes colored bluish, purple, etc., the color depending on the time it has been exposed. Because of this character the mineral is some- times called purple copper ore or variegated copper ore. So too chalcopyrite, which is bright brass-yellow when fresh, may become iridescent as in the variety called pea- cock ore a name which, by the way, is also given to bor- nite. It is often important, especially with a mineral having metallic luster, to test the color of the fine powder, or the color of the streak. The slight scratch which is given to test the hardness will often show this, but a better way is to have at hand a piece of rough white porcelain, or one of ground glass upon which the mineral can be rubbed. This method shows, for example, that hematite, sometimes iron-black in color and with a bright metallic luster, has a red streak; this is indeed so important a character that it is the source of the mineral's name (from the Greek capa, meaning blood). Minerals of unmetallic luster usually have a streak which differs but little from white even if the mineral itself is dark-colored or even black; this is true, for exam- ple, of the different varieties of tourmaline. Transparency. A mineral is said to be transparent when it is so clear THE OTHER PHYSICAL CHARACTERS OF MINERALS. 93 that an object can be seen through it with perfect distinct- ness, as a piece of window-glass, a plate of selenite, or a thin sheet of mica. It is semi-transparent or subtranspa- rent when the outlines of an object can be seen, but not distinctly. A mineral is translucent when it transmits light, as a piece of thin porcelain, but does not allow an object to be seen through it. It is subtranslucent when light is trans- mitted only on the edges. When describing the color of a mineral, some peculiar- ities in its distribution may be noted and receive special names. A mineral is said to show a play of colors when, like the opal, it exhibits internally the various prismatic colors when the mineral is turned. A pearly reflection from the interior of a mineral, like the effect of a glass of water to which a few drops of milk have been added, is called opalescence because common with the opal. Iridescence is the presenting of prismatic colors on the surface of a specimen, and then usually due to tarnish; or in the interior of a mineral, and then often explained by the presence of thin twinning lamella or of minute air- spaces, as in a cleavage mass of calcite. Asterism is the name given to the peculiar starlike effect seen sometimes by reflected light, as in certain kinds of sapphire; or by transmitted light, as in some mica when a candle-flame is viewed through it. It is due in the first case to planes of structure intersecting symmet- rically in the crystal; in the second to the presence of 94 MINERALS, AND HOW TO STUDY THEM. minute crystals of a foreign mineral (often rutile) symmet rically arranged between the plates of the mica. Phosphorescence is the property of becoming luminous when slightly heated, as shown by fluorite, especially the variety chlorophane; also upon friction, as some marble, tremolite, etc.; again, when exposed to the sun -rays or to an electric discharge, this last being especially true of some of the gems. Optical Characters. Only a few words can be devoted here to the large and important class of optical characters of minerals, depending upon the action of light on them as determined by their molecular structure. The under- standing of this part of Physical Mineralogy demands first a good knowledge of crystallography, and further a mastery of optics, especially of the difficult subject of polarization. Only one or two points belonging here can be touched upon. 148. Iceland Spar or Double refracting Spar. One of these is double refraction, or the separation of a ray of light passing through certain crystalline substances THE OTHER PHYSICAL CHARACTERS OF MINERALS. 95 into two rays. This is indeed true, in general, of all trans- parent crystals, except those of the isometric system, but the only mineral in which it is noted to a marked degree is calcite, especially in the transparent variety called Iceland spar. Fig. 148 illustrates this property well; there the single cross on the paper beneath appears double to the eye; one cross (to the eye looking perpendicularly down on the sur- face) has its arms in the continuation of the lines beneath, the other is pushed to one side. Neither cross appears quite black except at the two points where they intersect. Another point in this connection is the dichroism of a crystal, or the appearance of different color, as it is viewed by transmitted light in different directions; this is due to varying degrees of light absorption. This is often seen in transparent crystals of epidote; it is also to this cause that the different appearance of a crystal of musco- vite is due, first, in the direction through it and again at right angles to the cleavage. 4. CHARACTERS DEPENDING UPON HEAT. The fusibility of minerals, or their relative power of being melted at a more or less elevated temperature, is the most important character depending upon the action of heat. This is discussed in another place in connection with the description of the use of the blowpipe. The con- ductivity of crystals for heat is another point which some- times is experimented upon. As would be expected, it de- pends upon their molecular structure in different directions. This and other related subjects belong to advanced miner- alogy. 96 MINERALS, AND HOW TO STUDY THEM. 5. CHARACTERS DEPENDING UPON MAGNETISM. A few minerals have the property of being attracted by a magnet. This is true of magnetite, or the magnetic oxide of iron; of pyrrhotite, or magnetic pyrites; also of some specimens of native platinum. A specimen of magnetite sometimes is itself a magnet, and has then the power of attracting little particles of iron or steel; it has a north and south pole, and if hung by a thread will swing around until the poles oome into the magnetic meridian, that is, the direction assumed by a compass-needle. This kind of magnetite is called the lode- stone. Pyrrhotite is much less strongly magnetic than magnetite, and the magnetic varieties of platinum are not common; both may have polarity like the lodestone. A few minerals, as hematite and franklinite, are sometimes slightly magnetic, but probably only because they contain a little admixed magnetite. Most minerals containing much iron become magnetic when heated in the reducing-flame of the blowpipe; this is true also of millerite or the sulphide of nickel. 6. CHARACTERS DEPENDING UPON ELECTRICITY. There are a number of electrical properties of minerals, but these are characters that belong to a more minute study of minerals, and they need be only briefly mentioned here. A number of minerals, like sulphur, the diamond, and THE OTHER PHYSICAL CHARACTERS OF MINERALS. 97 topaz, become rather strongly electric when rubbed, as with a piece of silk, and show this by their power of attracting light substances, such as bits of straw or paper. Again, the crystals of. some minerals, when carefully heated or cooled, become electrified and show opposite kinds of elec- tricity in different parts, as at the two extremities; this is particularly true of tourmaline. It is remarked on p. 318 that when both ends of a tourmaline crystal are developed it is common to find them different in their crystalline faces. This dissimilarity in structure in the opposite direc- tions of the axis is connected with the property of becom- ing dissimilarly electrified by change of temperature. This subject is called pyro-electricity, because the electrical effect is due to the action of heat (nvp, fire). Tourmaline is hence said to be pyro-electric, and the same is true of quartz, and less strikingly of many other species. 7. TASTE AND ODOR. Taste belongs only to the few minerals which dissolve to some extent in water. The terms employed are familiar and hardly need explanation. Saline means the taste of common salt; alkaline, of soda; bitter, of epsom salts; sour, of an acid; astringent, of iron vitriol; sweetish astringent, of alum; cooling, of saltpeter. Odor also belongs to a few minerals only. Some va- rieties of limestone, barite, or quartz have a fetid odor, or odor of rotten eggs, especially if rubbed sharply; this is usually due to the presence of some sulphur compound. Moistened clay and some claylike minerals when breathed 98 MINERALS, AND HOW TO STUDY THEM. upon give off a peculiar argillaceous odor. Bitumen and some allied substances have a bituminous odor. A sharp blow across the surface of a piece of arseno- pyrite often produces a peculiar garlic odor, like that obtained by heating the same mineral on charcoal, and in fact due to the same cause. Similarly a blow on a mass of pyrite may yield a sulphurous odor THE CHEMICAL CHARACTERS OF MINERALS. 99 CHAPTER V. THE CHEMICAL CHARACTERS OF MINERALS. IT has already been stated that every mineral is neces- sarily a definite chemical compound, and that this is the most essential point in the definition of a mineral. But to understand what a chemical compound is, and what re- lations different compounds bear to each other, requires some knowledge of the fundamental principles of chemis- try. In the first place it is necessary to understand what the chemical elements are. The chief work of the chemist in the laboratory is to analyze different substances, or in other words to separate them into the various kinds of matter which they contain. But this process of analysis, or chemical separation, can only be carried a little way, for the chemist soon obtains substances which he is unable to decompose further. Thus if he takes a piece of calcite, it is easy by simply heating it to separate it into a white powder called lime (this is what the mason uses for making mortar) and a gas called car- bon dioxide, or carbonic-acid gas. Then further, if the proper means are taken, the lime can be separated into a metal, called calcium, and a gas, oxygen; while the car- bon dioxide can be separated into the familiar substance carbon and the same gas, oxygen. But these three sub- stances, calcium, carbon, oxygen, cannot be decomposed 100 MINERALS, AND HOW TO STUDY THEM. further; hence they are called elementary substances or elements. Again, common salt can be separated into two kinds of matter, the metal sodium and the gas chlorine; but neither of these can be separated any further, hence they are also put down among the simple or elementary substances. So, too, galena can be separated into its ele- ments, the metal lead and sulphur; sugar is decomposed into carbon and the gases hydrogen and oxygen; and many other illustrations might be given. These substances, then, into which. a given kind of mat- ter can be separated, but which the chemist is unable to decompose further, are the CHEMICAL ELEMENTS. Now the chemist finds that although there is no limit to the different kinds of bodies which he may be asked to analyze or separate into their parts, still they contain but a small number of distinct kinds of matter. If we re- gard only those which are commonly present, they are very few indeed. There are, it is true, about seventy of the elements recognized by the chemist, but many of them are excessively rare, and those which make up the chief part of common minerals are hardly more than twelve or thirteen. The table, p. 101, gives the names of all the common ele- mentary substances and most of the rarer ones. With the names are given also the initial letter or letters by which they are generally represented in the kind of algebraic shorthand that the chemist employs; these letters are called the symbols of the elements. Thus oxygen is repre- sented by the capital letter 0; hydrogen by H; nitrogen by N; calcium by Ca; and so on. In a good many cases the THE CHEMICAL CHARACTERS OF MINERALS. 101 THE CHEMICAL ELEMENTS. Sym bol. Al Sb As Ba Be Bi B Br Cd Cs Ca C Ce Cl Cr Co um. Cu Di Er F Ge lor: Au H I Ir Fe La Pb Li Mg Mn Hg At. Weight. 27 120 74.9 137 9.1 207.5 10.9 79.8 111.7 132.8 39.9 12 141 35.5 52.5 58.7 63.2 142 166 19.1 73.3 Be * 196.7 1 126.5 192.5 55.9 138 2064 7 24 54.8 199.8 Molybdenum Sym- At. bol. Weight. Mo 96 Ni 58.6 Nb 93.7 N 14 Os 191 16 Pd 106 P 31 Pt 194.3 K 39 Rh 104.1 Rb 85.2 Ru 103.5 Sc 44 Se 78.9 Si 28 Ag 107.7 Na 23 Sr 87.3 S 32 Ta 182 Te 125 Tl 203.7 Th 232 Sn 117.4 Ti 48 W 183.6 U 240 V 51.1 Yt 172.6 Y 89 Zn 65.1 Zr 90.4 Antimony (Stibium). . Arsenic Nickel Niobium Barium . Nitrogen Beryllium . . Osmium Bismuth Bromine Phosphorus Cadmium. ....... Platinum CsBsium Potassium (Kalium). . Rhodium Calcium . . Carbon Chlorine Scandium Chromium Selenium Cobalt. ... Silicon Columbium, see Niobi Copper (Cuprum). . . . Didymium Silver (Argentum). . . . Sodium (Natrium). . . Strontium Erbium Sulphur Fluorine Tantalum Tellurium Germanium Thallium Hydrogen Tin (Stannum) Titanium Iodine Iridium Tungsten (Wolf r ami- urn) Iron (Ferrum) Lead (Plumbum) Lithium .... Magnesium Yttrium Manganese Mercury (Hydrargy- rum) Zinc Zirconium See Beryllium. 102 MINERALS, AND HOW TO STUDY THEM. initial letters of the Latin name of a metal are used, as Fe, from the Latin ferrum, for iron; Ag, from argentum, the Latin name of silver; Au, from aurum, gold; Sb, from stibium, antimony, etc. The numbers placed after each name give the atomic weight of each element. What this is will be explained immediately. But first note that the larger part of the elements are metals, having physical properties of luster, malleability, etc., more or less like those of gold, silve^ lead, and iron. There is also a small class of non-metals^ including the gases, hydrogen, oxygen, etc., also sulphur, phosphorus, silicon, and carbon. Further, a few elements standing between the two groups are sometimes called semi-metals, as tellurium, arsenic, antimony. The chemi- cal distinction between the metal and non-metal is spoken of later. Now as to the meaning of the term atomic weight. We have spoken quite particularly of the minute particles, or molecules, of which the physicist believes that a body is made up, and whose relations to each other determine whether the body is a solid, a liquid, or a gas. We have also seen that the regular form of a crystal is due to the arrangement of these molecules as they are marshaled into place by the attractive forces acting between them when the solid is formed. Now these minute molecules, as the chemist believes, are made up of simpler particles, often of several different kinds of elements, which he calls atoms. It is the relative waight of the atom of each substance compared with that of the lightest substance known, hydrogen, that is called its atomic weight. This THE CHEMICAL CHARACTERS OF MINERALS. 103 does not mean that the chemist can actually weigh the atoms themselves that form the minute molecules out of which a body is built up, but he can compare the weights of two masses, for instance of oxygen and hydrogen, under such conditions that he is sure that he is comparing the same number of atoms, and hence he obtains the relative masses of the atoms; or he may obtain the same result in one of several indirect ways. It is found invariably true that when the different ele- ments unite to form a certain compound, there is always a definite relation between the amounts by weight of each element which enters, and that these weights are either the atomic weights or simple multiples of them, as given by the number of atoms present. Just what this means will be shown by some examples. It was stated above that the chemist could decompose common salt into sodium and chlorine. Now in doing this it is possible to find how much by weight of each is present, for instance, in 100 parts. The result is this : Sodium 39.32 Chlorine . . . 60.68 100.00 But the numbers 39.32 and 60.68 are in the ratio of 23 to 35.5 (39.32 : 60.68 = 23 : 35.5), which have been inde- pendently found to be the atomic weights of these two elements, and hence it is evident that there is one part, or one atom, of each present in this compound. The brief expression for the composition of sodium chloride, or the 104 MINERALS, AND HOW TO STUDY THEM. formula as it is called, is NaCl, for Na is the symbol for sodium (natrium) and Cl of chlorine. The formula of a compound, therefore, gives simply the kinds of elements present, represented by their initial let- ters, that is, by their symbols, with small numbers, written usually below, to show how many parts of each, that is, how many atoms, are present. Again, calcium unites with chlorine also, and the com- pound, calcium chloride, analyzed by the chemist, gives: Calcium '. 36.04 Chlorine 63.96 100.00 Here the numbers 36.04 : 63.96, expressing the ratio by weight of the two substances, are in the ratio of 40 : 71 or 40 : 2 X 35.5; hence the compound contains one atom of calcium and two of chlorine, and the brief expression, or formula, for it is Cad,. Two other examples are gold chloride and tin chloride, analyzed with the following results : Gold 64.87 Tin 45.26 Chlorine.. . 35.13 Chlorine.. . 54.74 100.00 100.00 For the gold chloride the ratio of 64.87 : 35.13 is as 196.7:106.5 or 196.7:3x35.5; hence the formula is written AuCl 3 . Similarly for tin chloride the ratio of 45.26 : 54 74 is as 117.4 : 142 or 117.4 to 4 X 35.5; hence the formula is SnCl 4 . THE CHEMICAL CHARACTERS OF MINERALS. 105 These examples illustrate the fact that the atomic weights of the given elements multiplied by the number of atoms gives the amount of each element present in the given compound. They also show another important point: it is seen that one atom of chlorine unites with one atom of sodium, but two atoms of chlorine with one of calcium, three with one of gold, and four with one of tin. Again, in hydrochloric acid the formula can be shown to be HC1; in other words, one atom of hydrogen is pres- ent and one of chlorine. Water, however, has the formula H a O, or contains in a molecule two atoms of hydrogen and one of oxygen; sulphureted hydrogen is similarly H 2 S. In other words, one atom of chlorine is here, as always, equivalent to one of hydrogen, but one of oxygen or one of sulphur is equivalent to two of hydrogen. Further, it is evident, from what has been stated, that the formula of galena, or lead sulphide, must be PbS, since one atom of lead unites with two atoms of chlorine, and one of sulphur is equivalent to two atoms of hydrogen or chlor- ine; therefore one atom of lead (Pb) is equivalent in combining power to one atom of sulphur (S). The same general principle can be extended to all the other elements; or in other words, it can be shown how many atoms of each element are equivalent in forming compounds to one of hydrogen. Thus the equivalence, as it is called, of sodium is one, of calcium two, of gold three (also sometimes one), of tin four. In the compound SbCl 6 the equivalence of the antimony (Sb) is five, but in Sb a O, only three. 106 MINERALS, AND HOW TO STUDY THEM. Some complication comes in from the fact that it is found that the same substance has, within certain narrow limits, different equivalence in different compounds, as noted above of gold and antimony; thus, too, the chemist knows one compound FeO, another Fe.,0, , and a third Fe0 2 . A good deal more attention must be given to the matter be- fore it can be thoroughly understood, and for this the student must have a good course in chemistry, including not only the study of some standard book, but also practical work with a good teacher in the laboratory. But the expla- nations given should suffice to make it pretty clear, first, as to what the elements are ; second, what is meant by their atomic weights; and third, the significance of their com- bining power, or equivalence. Some further explanations are needed as to chemical compounds. The distinction between a chemical compound and a simple mixture of two elements is well illustrated by the air we breathe. The chemist finds by analysis that it is nearly constant in composition, containing essentially in one hundred parts 76.8 by weight of nitrogen* and 23.2 of oxy- gen. A little water vapor is also present, still less carbon dioxide. Is the air a chemical compound ? The answer is given at once that it is not, for the simple reason (and there are others equally conclusive) that the ratio of 76.8 to 23.2 is not that of the atomic weights of the two elements present, namely 14 : 16, nor of any simple multiples of these. * In this approximately one per cent has been shown to be the new element argon, which in many characters is closely related to nitrogen. THE CHEMICAL CHAEACTERS OF MINERALS. 107 There are indeed several compounds of nitrogen and oxy- gen know to the chemist, namely, N,0, N.O., N a 5 ; but if they are analyzed, the relative amounts by weight of nitrogen and oxygen are found to be in the ratio of 2 X 14 : 16 2 X 14 : 3 X 16 2 X 14 : 5 X 16 or 28 : 16 28 : 48 28 : 80 One further point must be mentioned in regard to the compounds taken for illustration: Sodium chloride, NaCl. Calcium chloride, CaCl a . Gold chloride, AuCl s . Tin chloride, SnCl 4 . The first element in these and similar formulas is a metal and the second a non-metal; the first is said to be the positive element, the second is the negative element. Why the terms positive and negative are introduced is known to the student of electricity, for he has learned that in the decomposition of a compound by the electrical cur- rent a very powerful means, often accomplishing the result when other methods fail one element always goes to the positive pole or electrode, the other to the negative; the former is hence called the negative element, its atoms be- ing attracted by the oppositely electrified positive electrode, and the second the positive, since its atoms are attracted by the negative electrode. Corresponding to this the metals, are positive in nearly all their compounds, while the non- metals are negative, and further the semi-metals are some- 108 MINERALS, AND HOW TO STUDY THEM. times positive, sometimes negative. Remember that the positive element is always written first; this will be clear from the examples given above, and in the following: FeO, FeS, PbO, PbS, etc. Again, in As.,0., and As a S 3 arsenic is positive, but in FeAs, and CoAs 3 arsenic is negative. Similarly among the metals there are some which in compounds with certain elements (as oxygen, sulphur) are always positive, while when com- bined with certain other metals they may be negative. The names given to the different chemical compounds are in most cases easy to learn and understand. In the de- scription of minerals, in the pages that follow, both the chemical names and the formulas are given so as to famil- iarize the student with each method. When there are two or more compounds of the same ele- ments, the name is usually such as to distinguish between them. Thus PbO and PbO s may each be called oxide of lead or lead oxide, but the first is properly lead monoxide * or lead protoxide, and the second lead dioxide. The following are other examples: FeO, iron protoxide, or ferrous f oxide. Fe a 3 , iron sesquioxide, J or ferric oxide. *Monoxide means an oxide containing one atom (from //oVoS. single) of oxygen; dioxide one containing two atoms (from Si?, twice); protox- ide means the first oxide (TTpoSro?, first) because the first or lowest of the oxides of the given metal in amount of oxygen present; the high- est oxide is sometimes called peroxide. f The terminations -ous and -ic are frequently used for the lower and higher oxides respectively. \ Sesquioxide means a one-and-half oxide, because the ratio of oxy- gen to metal is 1$ : 1 or 3 : 2. THE CHEMICAL CHARACTERS OF MINERALS. 109 FeS,, iron disulphide. Sb a S 3 , antimony trisulphide. SnCl 4 , tin tetrachloride. The following are a few special names with which it is desirable to be familiar: Na a O, soda, instead of sodium oxide. K 3 0, potash, instead of potassium oxide. CaO, lime, instead of calcium oxide. MgO, magnesia, instead of magnesium oxide. BaO, baryta, instead of barium oxide. A1 Q 3 , alumina, instead of aluminium trioxide. SiO,, silica, instead of silicon dioxide. A few others might be added to this list. It will be helpful to note briefly what are the common kinds of compounds found among minerals, so that the statement of the chemical composition and formula, given under the description of each species, may have a definite meaning. There are three fundamental divisions: I. NATIVE ELEMENTS. This is the simplest case of all, that of the elements, a few of which occur in nature and are hence called native elements, as native gold, native sul- phur, etc. II. SIMPLE COMPOUNDS, usually of two elements; there are four prominent classes. (1) Sulphides, compounds of a metal with sulphur, as: Galena, lead sulphide, PbS. Sphalerite, zinc sulphide, ZnS. Pyrite, iron disulphide, FeS,. Stibnite, antimony tmulphide, Sb,S,. 110 MINERALS, AND HOW TO STUDY THEM. Similar to the sulphides and closely related to them are the rare tellurides, arsenides, antimonides, etc., as: Altaite,* lead telluride, PbTe. , Niccolite, nickel arsenide, NiAs. Breithauptite, nickel antimonide, NiSb. (2) Chlorides, compounds with chlorine, as: Halite (rock salt), sodium chloride, NaCl. Cerargyrite, silver chloride, AgCl. Similarly the rare bromides and iodides are compounds with bromine and with iodine, as: Bromyrite, silver bromide, AgBr. lodyrite, silver iodide, Agl. (3) Fluorides, compounds with fluorine, as: Fluorite, calcium fluoride, CaF 9 . (4) Oxides, compounds with oxygen, as: Cuprite, cuprous oxide, Cu a O. Zincite, zinc oxide, ZnO. Hematite, iron sesquioxide (or ferric oxide), Fe.,0,. Cassiterite, tin dioxide, SnO a . The examples given under some of these heads, as the sulphides and oxides, illustrate the important point already spoken of: that there may be more than one kind of com- pound, varying in the number of atoms present, for example of oxygen, as Cu.,0, ZnO, Fe 3 3 , SnO s . Even with the same metal two or more compounds are often known, though not always occurring in nature. Thus, besides cuprite, *Some rare minerals, not elsewhere metitioned, are for the sake of completeness included in this list. THE CHEMICAL CHARACTERS OF MINERALS. Ill Cu a Q, there is also a mineral called tenorite whose com- position is CuO; also the chemist knows FeO, iron protox- ide (or ferrous oxide), while Fe,0 3 , iron sesquioxide (or ferric oxide), is the common mineral hematite. Further, there is magnetite, whose percentage composition is ex- pressed by the formula Fe s 4 , also written FeO.Fe a O s ; in this last case the chemist's view of the composition is a lit- tle too complex to be explained here. This, however, only illustrates again the limitation to which the beginner is subject, since he cannot expect to master all the relations of a large and difficult subject without much hard study. A little knowledge, however, is useful if it does not make the one who possesses it imagine that he has a deeper un- derstanding of nature's laws than he really possesses. There may be also, under each of these heads, com- pounds containing more than one metal, or, on the other hand, more than one negative element, as arsenopyrite, FeAsS, which is equivalent to FeAs a .FeS a . Another simple example is cryolite, Na 3 AlF s , which is equivalent to 3NaF.AlF 9 . We may provisionally include here a series of rather rare compounds among minerals, of which class, though nu- merous, only a very few are alluded to in this book. The best example is tetrahedrite, whose formula is Cu o Sb a S 7 , which we may write as if made up of two sulphides, thus: 4Cu a S.Sb a S,. Pyrargyrite, Ag,SbS, or 3Ag a S.Sb a S 3 , is an- other example. Strictly, these compounds are regarded by the chemist as similar to those of the class now to be men- tioned called Salts, but containing sulphur in place of oxygen. 112 MINERALS, AND HOW TO STUDY THEM. III. COMPLEX COMPOUNDS, called SALTS. These com- plex compounds may be referred back in each case to some acid, of which the given compound is said to be a salt. In these also the metal, usually written first, is the positive part, the remainder the negative, in the electro-chemical sense in which the terms were used on page 107. The metal is thought of as taking the place of the hydrogen atom or atoms in the formula of the given acid. The principal classes are: Carbonates, salts of carbonic acid, H,C0 3 , in which some metal, as calcium, lead, etc., takes the place of the two hy- drogen atoms, as : Calcite, calcium carbonate, CaC0 3 . Dolomite, CaMg(CO s ) a or CaC0 3 .MgCO,. Cerussite, lead carbonate, PbC0 3 . Sulphates, salts of sulphuric acid, H 2 S0 4 , and similarly chromates, tungstates, molybdates, as: Barite, barium sulphate, BaS0 4 . Anglesite, lead sulphate, PbS0 4 . Crocoite, lead chromate, PbCr0 4 . Scheelite, calcium tungstate, CaW0 4 . Wulfenite, lead molybdate, PbMo0 4 . Phosphates, mostly salts of the phosphoric acid H 3 P0 4 . Closely related are the arsenates and vanadates; less closely the rarer antimonates and nitrates. The mineral triphylite (not common) has the composition LiFeP0 4 or Li 3 P0 4 . Fe,(P0 4 ),; the rare mineral xenotime has the composition yttrium phosphate, YP0 4 ; pucherite, also very rare, is bis- THE CHEMICAL CHARACTERS OF MINERALS. 113 muth vanadate, BiV0 4 . Other examples are afforded by the common minerals: apatite, essentially calcium phos- phate, Ca 3 P 4 O g orCa s (P0 4 ) s ; pyromorphite, lead phosphate, Pb 3 P 2 8 ; mimetite, lead arsenate, Pb,As,,O g ; vanadinite, lead vanadate, Pb 3 V a 8 . In all of these, however, the com- position is in fact a little more complex than these simple formulas would suggest, since chlorine (and, with apatite, fluorine) enters in small amounts; this is again a point which it will not be attempted to explain here. Less common than the preceding classes are the Tanta- lates and Niobates (columbates), as: Tantalite, iron tantalate, FeTa a 6 . Columbite, iron niobate, FeCb s O s . There is also a group of Borates, mostly very rare min- erals. A very large and important but complex group is that of the Silicates. These are salts of several kinds of silicic acids, the commonest of which are metasilicic acid, H a Si0 3 , and orthosilicic acid, H 4 Si0 4 , as: Rhodonite, manganese metasilicate,* MuSiO,. Willemite, zinc orthosilicate, Zn a Si0 4 . Other silicates are: Pyroxene (diopside), CaMg(Si0 3 ) a or CaMgSi a 8 . Beryl, Be,Al a (SiO,) 6 or Be s Al 2 Si g 18 . These are metasilicates, while the two following are ortho- silicates : * The designations meta- and orlJio- are often omitted, and the com- pounds described simply as silicates. 114 MINERALS, AND HOW TO STUDY THEM. Garnet (grossularite), Ca,Al 8 (Si0 4 ) s or Ca.Al.Si.O,, Zircon, ZrSi0 4 . Also Orthoclase, KAlSi.O.. Albite, NaAlSi 3 8 . And again Andalusite, Al a Si0 6 . Besides the two types first mentioned, there are others, as those represented by orthoclase (or albite) and by anda- lusite, and further still others, many of them very com- plex and in some cases not clearly understood, notwith- standing all the study that has been given to the subject. All of these points require much more knowledge than can be demanded of a beginner in mineralogy. As illustrated by the examples given, a silicate com- monly contains more than one metal, perhaps four or five or even more. The same thing is also true of the com- pounds belonging under the other classes of salts, though they are not usually so complex as the silicates; with them also, in some cases, two of the negative elements may be present. In this connection it is also important to understand that the relative amounts of the metals present may in a given case vary widely, as the amounts of calcium, iron, etc., in the different kinds of garnet. For example, besides pure calcium carbonate, CaCO s (calcite), and magnesium carbonate, MgCO, (magnesite), there are many interme- diate compounds, to which the general name dolomite is given, containing more or less of the two metals, and for THE CHEMICAL CHARACTERS OF MINERALS. 115 them the formula is then written (Ca,Mg)CO s . For nor- mal dolomite, however, in which one atom each of calcium and magnesium is present, the formula is written CaMgC a O, or CaMg(C0 8 ) a ; the difference between these forms (with and without a comma) should be noted. So, too, the three compounds of lead mentioned under the head of Phos- phates, namely, the minerals pyromorphite, mimetite, vanadinite, have many intermediate compounds contain- ing in varying amount two of the negative elements phos- phorus, arsenic, vanadium. The formulas of these salts are often written as if sepa- rated into the corresponding simple oxygen compounds or oxides, as CaO.CO Q instead of CaC0 3 . The chemist does not, however, believe that the calcite molecule is made up of these two parts, only this method of writing is conven^ lent because these are the parts readily obtained by analy- sis; in this particular case by simply heating the substance it gives off CO, , and CaO is left behind. So, too, we may write either CaMg(Si0 3 ) 3 or CaMgSi 2 6 or CaO.Mg0.2SiO a ; Ca s Al,(Si0 4 ) 3 or Ca.Al.Si.O,, or 3CaO.Al a O s .3SiO a ; KAlSi 3 8 or K a O.Al 2 9 .6SiO a ; Al,Si0 6 or Al s 3 .SiO a ; and so on. Hydrous Compounds. Finally, there are also a large number of minerals which yield water when heated; some- times this means only that the elements of water (hydro- gen and oxygen) are present in the complex compound and they combine at the high temperature. This is true, for example, in talc. 116 MINERALS, AND HOW TO STUDY THEM. In the majority of cases, however, the water is believed to be present as water of crystallization, as if water mole- cules were present with the other molecules when the compound was crystallized out from the solution. These species are called hydrous compounds, and they as a rule give off water at a comparatively low temperature. Thus gypsum is hydrous calcium sulphate, and its composition is expressed by the formula CaS0 4 + 2H a O. Other examples of hydrous minerals are the zeolites, among silicates, mentioned near the end of the chapter on the Description of Species. Still another important class are the hydrates, or hydrated oxides that is, oxides which yield water when heated. A good example is brucite, whose formula is written Mg(OH) 2 or MgO.H 2 0. Percentage Composition. It was shown on p. 104 that, from the proportion by weight of the different elements ob- tained by the analysis of the chemist, the chemical formula could be deduced by the aid of the table of atomic weights. Conversely, if the formula is given, the percentage composi- tion, or the amount by weight of each element or group of elements present in the compound in one hundred parts, can be easily calculated. Thus sodium chloride, or common salt, has the formula NaCl, as has been shown, and hence, taking the atomic weights of sodium and chlorine, the re- lation of the weight of sodium to that of chlorine is as ths numbers 23 : 35.5. Now adding together 23 and 35.5 we obtain 56.5, which is called the weight of the molecule, or molecular weight. Further, by the rule of three, if 58.5 THE CHEMICAL CHARACTERS OF MINERALS. 117 parts of the compound contain 23 parts of sodium, 100 will contain 39.32 parts: 58.5 : 23 = 100 : 39.32, or = 39.32: 00*0 OK K V 1 00 58.5 : 35.5 = 100 : 60.68, or ^V^ = 60.68. o8.5 The percentage composition of sodium chloride is, there- fore: Na 39.32 Cl 60.68 100.00 Again, the formula of stibnite, Sb a S 3 , means that two atoms of antimony (Sb) unite with three of sulphur (S). But the atomic weights of antimony and sulphur are 120 and 32 respectively. The molecular weight is, therefore, equal to 2 X 120 + 3 X 32 = 240 + 96 = 336. Hence in 336 parts, 240 are antimony and 96 sulphur, and to find the amount of each in one hundred parts we have the proportions 94.0 v 100 336 : 240 = 100 : 71.43, or * = 71.43. ooo 336 : 96 = 100 : 28.57, or 96 * 10 = 28.57. 336 The percentage composition is, therefore: Sb 71.43 S 28.57 100.00 Again, the formula of one kind of garnet is Ca 3 Al a Si 3 lt or, as it may be written, 3CaO.Al a O,.3SiO a . Taking the 118 MINERALS, AND HOW TO STUDY THEM. second form and finding the atomic weights for each ele- ment from the table, adding them together for each group of atoms and multiplying by the factor given, we have: 3CaO = 3(40 + 16) = 3 X 56 = 168 Al,0, = 2 X 27 + 3 X 16 = 54 + 48 = 102 3SiO, = 3(28 + 2 X 16) = 3 X 60 =180 450 Hence again, by the rule of proportions: 450 : 168 = 100 : 37.33 450 : 102 = 100 : 22.67 450 : 180 = 100 : 40.00 The percentage composition is, therefore : Lime, CaO 37. 33 Alumina, A1 2 3 22.67 Silica, SiO a 40.00 100.00 If desired it would have been as easy to deduce the amounts of the elements Ca, Al, Si, present, but, as stated on p. 115, it is more convenient to use the oxides instead. CLASSIFICATION. There are various methods of classification that may be adopted for minerals. The strictly scientific way is to arrange similar compounds together, that is, first, the native elements; then the sulphides, the oxides, the carbonates, and so on. These are further classified by the relationships which a study of the elements and of the crystalline forms of their compounds makes known. THE CHEMICAL CHARACTERS OF MINERALS. 119 For example, the following minerals being all carbonates are, on a strictly scientific method, placed in the same general division: Calcite, calcium carbonate, CaC0 3 . Dolomite, calcium-magnesium carbonate, CaMg(CO s ). Magnesite, magnesium carbonate, MgC0 3 . Siderite, iron carbonate, FeC0 3 . Khodochrosite, manganese carbonate, MnCO,. Smithsonite, zinc carbonate, ZnC0 3 . Further, they are all placed side by side in the same group, called the Calcite group, because they have the same general crystalline form and very nearly the same angles, e.g., all show rhombohedral cleavage with the angle varying from % Q5 to 107. This is called, therefore, an isomorphous group, having like form * and analogous composition. Another series of minerals, also in the same division of carbonates, form a second isomorphous group, the Aragou- ite group: Aragonite, calcium carbonate, CaCO s . Witherite, barium carbonate, BaC0 3 . Strontianite, strontium carbonate, SrCO s . Cerussite, lead carbonate, PbCO s . A third case is the Barite group of sulphates: Barite, barium sulphate, BaS0 4 . Celestite, strontium sulphate, SrS0 4 . Anglesite, lead sulphate, PbS0 4 . Also, a little less closely related, Anhydrite, calcium sulphate, CaS0 4 . * Isomorphous is from i'tro?, like, and tiopjj, form. 120 MINERALS, AND HOW TO STUDY THEM. The Galena group (galena, argentite, etc.), the Apatite group (p. 113), the Feldspar group, the Mica group, are other examples. These and many besides are described in an advanced work on mineralogy. Another method of classification is to place together the different compounds of each metal, as all the compounds of iron, all those of silver, and so on. Still another way would be to put the metallic ores together, the gems, and so on. Of these and still other different methods, the most satis- factory for us is the second, as further explained on a later page (p. 158 et seq.). It will be noticed, in the cases of the two groups taken for illustration, that the same composition, calcium carbonate, CaCO s , belongs to two minerals, calcite and aragonite. These are regarded as distinct species because they have a different crystalline form and different physical characters, e.g., specific gravity. What is true of this chemical com- pound is true of a number of others. Among minerals, such compounds are said to be dimorphous or to have two forma THE USE OF THE BLOWPIPE. 121 CHAPTER VI. THE USE OF THE BLOWPIPE. 1. GENERAL DESCRIPTION OF APPARATUS, THE chemist in the laboratory, as has been already ex- Dlained, can subject a mineral specimen to a process of analysis, and in this way discover, first, what simple sub- stances or elements it contains, and, second, in what pro- portion by weight they are present; in other words, he can analyze it. This has been done many times in the case of all the minerals we know, and the result has been to show what the composition of each species is, and by what for- mula this can be expressed. It is obvious that this method of complete analysis is the only satisfactory way to gain a complete knowledge of the chemical nature of a given mineral. But the work of the chemist is slow and laborious, and it is often important to be able to learn something about the composition of a mineral more quickly and by an easier method. This can be done by the blowpipe, supplemented by some simple chemical tests; and any one who is supplied with a few tools, and who has the patience to learn to use them, can accomplish it. The results of this blowpipe analysis, taken in connection with the study of the physical charac- ters of a given specimen, almost always suffice to enable a mineralogist who has a fair amount of knowledge and ex- 122 MINERALS, AND HOW TO STUDY THEM. perience to determine what it really is, even if at first it was entirely unknown. The following list includes the articles that are most es- sential for this work: 1. Lamp. 4. Charcoal. 2. Blowpipe. 5. Platinum wire. 3. Platinum-pointed forceps. 6. Glass tubes. Also (7) a few chemical reagents as explained beyond. After some words of explanation about each of these, several other appliances which it is also convenient to have will be mentioned. 1. Lamp. The most convenient form of lamp is a Bun- sen gas-burner (Fig. 149); it is provided with a special jet (b in the figure). This burner can be connected with any ordinary gas-jet by a rubber tube, so as to be placed on the 149 table for use. In the Bunsen burnei ^ proper, that is, when the jet b is not inserted, the gas mingles in the tube with the air which enters at , and they P together burn at the top in a very hot flame, but one which gives very little light and which deposits no soot upon a surface of cold glass or porcelain. This flame is used by the chemist in the lab- oratory, and also by the mineralogist in heating glass tubes as described beyond. Instead of the Bunsen burner an alcohol lamp may be employed, and in fact was long used by the early chemists; alcohol, however, is a very inflammable substance, so that THE USE OF THE BLOWPIPE. 123 its use requires much care. One precaution also must be observed with the Bunsen burner: it is best not to turn the flame down low (unless the end of the tube is covered with a cap of wire gauze), for if this is done the flame is liable to " snap down," that is, the gas may ignite within the tube just above a (Fig. 149). It then burns with a feeble yellowish flame, yielding a disa- 150. greeable odor, and the tube becomes im- mediately very hot. This is dangerous, not only because a severe burn may result from touching the tube, but, still more, because if left a few moments the rubber tube may be melted, the gas ignite from it, and a serious fire be caused. Hence it is better never to go out of the room and leave a Bunsen flame burning even for a few minutes. In a laboratory where there is a slate table this precaution is not so important. When the jet b is inserted in the tube of the Bunsen burner the air-supply a from the openings below is cut off and the gas now burns at the top with the usual yellow flame, here flattened by the shape of the jet; the convenient flame for ordinary use is about one and a half inches in height. This is the flame to be used with the blowpipe. Instead of this gas-flame a good stearine candle will answer the purpose sufficiently well, or an oil lamp with a suit- able burner. 2. Blowpipe. A common form of blowpipe is shown in 124 MINERALS, AND HOW TO STUDY THEM. Fig. 150. It may be very simple and inexpensive, but should have an air-chamber, a, to collect the condensed moisture from the breath. A separate tip (b), either of brass or platinum, with a fine hole, is often used, but it is not absolutely necessary. The essential thing is that the hole, whether in the tip or the tube itself, should 151. be large enough and not too large, and also that it should be round and true, so that a moderate pres- sure of air shall suffice to blow a clear blue flame (see Fig. 153). A trumpet-shaped mouthpiece (c) is usu- ally furnished, but some prefer to dispense with it. 3. Forceps. A pair of steel forceps (Fig. 151) is needed, and it is desirable that they should be nickel- plated to prevent rusting. One end has platinum points at d, self-closing by a spring, so that the piece of mineral to be heated, placed between them, is firmly supported. At the other end are ordinary forceps for picking up small fragments; this end should never be inserted in the flame. A caution in regard to the use of the platinum points is given on p. 130, for, though infusible, they can be easily injured. 4. Charcoal. Several pieces of charcoal are needed. These are most conveniently rectangular in shape (see Fig. 156) and about four inches long, an inch wide, and three fourths of an inch thick. The charcoal must burn without snapping and must leave very little white ash. It is so difficult to obtain really good charcoal that it is well worth while to purchase a few pieces expressly pre- pared for the purpose, and with care one piece will last for THE USE OF THE BLOWPIPE. 125 many experiments, the surface being rubbed clean, as by a file or knife, after each use. 5. Platinum Wire. A few inches of platinum wire, of the size designated No. 27, usually sold for this purpose, are needed; directions for its use are given on a later page. In addition to the wire, a small piece of platinum foil is sometimes useful. 6. Glass Tubes. Some tubes of rather hard glass are required; it is convenient to have two sizes, with bores of one sixth and one quarter of an inch, but one will suffice. The larger size can be cut into pieces about five inches in length; the tube will break easily if a single scratch is first made with the edge of a three-cornered file. These tubes are to be used as open tubes, as explained later. Again, pieces a little longer, say six inches, and of the size with the smaller bore, may be taken and held with the middle point in the hot part of the Bunsen flame. When the glass is soft, draw the two ends apart by a quick motion (without twisting), and then heat each long tapering end in the flame and pinch it off short while hot, using for this the steel end of the forceps. In this way two closed tubes will be made from each piece; a considerable number should be made and kept in a closed box for use. A tube must be clean inside and out, and should not be used twice. 7. Fluxes and other Chemical Reagents. The chemical reagents needed are the fluxes * borax (sodium tetraborate), soda (sodium carbonate), and salt of phosphorus, or micro- cosmic salt (phosphate of soda and ammonia). Each of * So called because they help in the melting or fusion of the sub- stance under examination. 126 MINERALS, AND HOW TO STUDY THEM. these may be kept in a round wooden pill-box, or in a small bottle with a glass stopper. A little potassium bisulphate, to be kept in a glass bottle, is occasionally needed. Small bottles of hydrochloric, nitric, and sulphuric acids are also useful, and one of a solution of cobalt nitrate; these bottles may conveniently have a glass dropping-tube with a bulb in the place of the ordinary glass stopper. Test-paper is also required, cut up into small strips, both turmeric-paper and blue litmus-paper. The yellow tur- meric-paper is turned broivn by an alkali, such as soda, while the Hue litmus-paper is turned red by an acid or acid fumes, as of sulphur dioxide in the open tube. Eed litmus-paper turns blue with an alkali, but the turmeric- paper is better. In addition to the above, the following articles will be found very convenient, though not all of them quite so es- sential: A small hammer having a square face with sharp edges; also a steel anvil an inch or two long,. A horseshoe magnet (Fig. 152), the place of which may be taken by a magnetized knife- blade. A small agate mortar and pestle; also a steel diamond mortar (one in which the pestle fits tightly) in which a hard mineral can be pulverized without loss of the fragments. A pair of cutting pliers. A three-cornered file. A few small watch-glasses are convenient; also several small dishes of glass or porcelain (smooth butter-plates are THE USE OF THE BLOWPIPE. 127 very good) to hold the fragments of the mineral under ex- amination; several test-tubes; a porcelain dish, or casser- ole, in which a substance can be heated with acid. Also, if chemical tests proper are to be tried, a wash-bottle (for distilled water), a bottle of ammonia, and some filter-paper. Before beginning to experiment it is best to put a thick sheet of cardboard, covered each time with a fresh piece of white paper, upon the table and place the lamp upon this. A slate or a sheet of plate glass is even better than the cardboard. The student must remember also that the acids men- tioned are powerfully corrosive in their action, staining and finally destroying any fabric, as clothes or the carpet, which they are allowed to touch.* Moreover, the fumes from the acids when hot are injurious; for any extended series of strictly chemical trials it is almost essential, there- fore, to have some of the conveniences of a laboratory. Still another caution is needed: do not put away a piece of charcoal after use until it is quite certain that no fire lingers in it. 2. How TO USE THE BLOWPIPE. The first thing in the use of the blowpipe is to learn to blow a hot, steady flame. Place the tip of the blowpipe close to or just within the flame as shown in Fig. 153, directing it slightly downward, and blow through the tube. The blast of air will direct the flame into a thin cone, and with * In case of accident the effect of the acid can often be neutralized by the prompt application of ammonia or carbonate of soda, which may afterward be washed out with a little water. 128 MINERALS, AND HOW TO STUDY THEM. a little practice a clear blue flame quite free from yellow will be the result. This flame is much hotter than the or- dinary gas-flame, and when the blowpipe is in skillful hands it is hot enough to melt a fine platinum wire. The hottest part is just at the extremity of the blue flame (shaded in Fig. 153). It seems difficult at first to blow a continuous steady flame, but it is really very easy. It is only necessary to continue slowly to breathe through the nose while the pressure of the cheeks upon the reservoir of air kept all the time in the mouth prolongs the blast. This pressure need not be great not enough to tire the cheek-muscles sensibly except after a long time; if fatigue soon comes, it is because the student is unskillful or has a bad blowpipe. It is not wise, however, to give too much thought to the learning of the art of steady blowing; this will come quickly with practice. At the same time it will not do to be careless about the character of the flame; the stu- dent is ready to go on when he can take a thin sliver of orthoclase and without great difficulty melt the edges. An important distinction must be made between the reducing flame and the oxidizing flame. The flame in general consists of two parts: the inner blue cone, and the outer almost invisible envelope extending far beyond. In the former the gas is only partly burned; there is a de- ficiency of oxygen, and a substance which at that tem- perature can part with its oxygen is reduced. Here the reducing effect is to rob of oxygen, as when oxide of nickel, NiO, is changed to metallic nickel (Ni); or iron sesqui- oxide (Fe a O s ) is changed to iron protoxide (FeO). THE USE OF THE BLOWPIPE. 129 In the outer part of the flame, on the other hand, there If an excess of oxygen from the surrounding air, and the tendency is to give oxygen, or to oxidize. Here the lower oxide of manganese, MnO, is changed to the higher oxide., Mn,0 8 . This distinction between the action of the two parts of the flame is very important in a certain class of experi- ments. The student must notice further that to blow a good strong oxidizing flame the tip of the blowpipe should be placed just inside the gas-flame, as indicated in Fig. 153; the flame is then free from any yellow, and the sub- 153. 154. stance under experiment is to be held well beyond the end of the blue cone, at d. For a good reducing flame, on the other hand, the tip should be a little outside of the gas-flame (Fig. 154), so that a little yellow follows the flame down, above the blue cone; the substance is held at d, within the blue cone, and best more or less surrounded by the yellow flame. The experiment described on p. 138 with manganese will show the learner with what success he is following the direc- tions here given. In the following pages the different methods of exami- nation with the aid of the blowpipe are described fully. The student should take them up in order, going through 130 MINERALS, AND HOW TO STUDY THEM. as many as possible of the trials with the minerals sug- gested and endeavoring to obtain the results described as closely as he can. It is essential that the material used for the experiments should be pure. 3. EXAMINATION IN THE FORCEPS. A small fragment of a mineral, held in the platinum points of the forceps, may be tested to see whether it can be melted, and, if so, whether easily or with difficulty. At the same time it may be observed that the mineral imparts a, color to the flame which will give information as to its composition, while other phenomena, as detailed below, may also be noted. And here a few important suggestions must be made. It is very necessary to remember that while platinum can not be injured by the heat of the blowpipe flame, nor attacked by the ordinary acids used by the chemist, it may yet be easily injured. A mineral containing antimony or arsenic, if fused in the forceps, may destroy the platinum points, for these metals form a very fusible alloy with platinum. Hence it is desirable to try minerals about which there is question especially a mineral with metallic luster in the closed tube or on charcoal first, and if there are fumes given off, caution is needed. In any case it is a good rule never to let the fused part of the mineral fragment come in contact with the plati- num; for it may adhere to the points in an inconvenient way, even if not capable of doing any permanent harm, and thus much time be wasted in cleaning them. THE USE OF THE BLOWPIPE. 131 Take now a little sliver, if possible with a fhin edge, of a piece of barite or heavy spar; place it between the platinum points, letting the edge project well beyond them; blow a clean blue flame with the blowpipe, and just in front of this (in the oxidizing flame, see Fig. 153) insert the min- eral. It will be seen to melt rather easily to a white opaque glass; at the same time the flame beyond will be streaked with a pale yellowish green, which is character- istic of the element barium. Further, if the fused end, after it has cooled, be placed upon a piece of moistened turmeric-paper, it will be seen to turn it brown, showing the presence of an alkaline earth. If a piece of a barite crystal is taken, it is very likely to break violently into fragments when the flame is thrown upon it. This is called decrepitation and is not uncom- mon, especially with crystallized minerals. It can often be prevented by heating the fragment quite slowly at first, but in some cases it is necessary to begin by reducing the mineral to a fine powder, then mix it with a drop of water in the agate mortar, and finally support the thick paste so formed on a loop at the end of the platinum wire. Scale of Fusibility. The method of experiment de- scribed gives in the first place an approximate determina- tion of the melting-point or degree of fusibility. The following scale is used to define the fusibility of the differ- ent minerals: 1. Stibnite (must be heated on charcoal) : fusible in the ordinary gas-flame even in large fragments. 2. Natrolite : fusible in fine needles in the ordinary gas- flame, or in larger fragments in the blowpipe-flame. 132 MINERALS, AND HOW TO STUDY THEM. 3. Almandite, or iron-alumina garnet: fusible to a glob- ule without difficulty with the blowpipe, if in quite thiu splinters. 4. Actinolite : fusible to a globule in thin splinters. 5. Ortltoclase: thin edges can be rounded without great difficulty. 6. Bronzite : fusible with difficulty on the finest edges. The following list gives the names of some minerals, most of them common, with the degree of fusibility of each according to this scale. It is repeated here that for miner- als with metallic luster the trial should be in charcoal. Stibnite, galena 1. Cryolite, apophyllite, pyromorphite 1.5 Amblygonite, witherite, prehnite, arsenopyrite 2. Ehodonite, analcite 2.5 Gypsum, barite, celestite, fluorite, epidote 3. Oligoclase 3.5 Albite 4. Apatite, hematite, magnetite 5. Bronzite 6. Infusible : quartz, calcite, topaz, sphalerite, graphite. It may be interesting here to add the temperatures (in degrees Centigrade) at which the prominent metals fuse, that is, pass from the solid to the liquid state: Mercury 39 Antimony... 450 Silver 1020 Tin 230 Copper 1090 Bismuth 320 Gold 1100 Lead 330 Iron 1500 Zinc-. . 420 Platinum . . 2000 THE USE OF THE BLOWPIPE. 133 The student must be warned that the method of express- ing the fusibility of a mineral, by referring it to the scale given, is not exact. The results obtained in different cases will depend upon the size and shape of the fragment taken, the conductivity for heat, also obviously upon the skill of the experimenter. Flame coloration. Besides the fusibility, this experi- ment with a fragment of barite in the forceps serves to prove the presence of barium by the color given to the flame. It is found that a considerable number of sub- stances are characterized in the same way, hence the flame coloration becomes a simple and important means of quali- tative blowpipe chemical analysis. Color of the Flame. The following is a list of the colors likely to be observed and the substances to which they are due: ( Carmine-red... . Lithium. RED ... \ Purple-red Strontium. I Yellowish red .Calcium. YELLOW Sodium. ( Yellowish green. Barium. I Siskine-green . . . Boron. GREEN. y the combination of the sulphur and oxygen, sulphur dioxide (SO,), goes off as a gas in the air. If now the reducing flame is thrown for a moment against the white coating, it is burned off with a bluish flame. The action of the flame is to reduce the oxide to the metal (Sb), which is instantly volatilized, and as it goes off it is again oxidized. Again, take a fragment of orpiment, sulphide of ar- senic (As a S 3 ), and treat it in the same way. The result is somewhat similar; it fuses easily, giving white fumes (of As 2 3 ), and it is also entirely volatile. But now a strong disagreeable odor will be perceived as the fumes are formed; this is usually described as a garlic, or alliaceous, odor; it is characteristic of arsenic, and is always produced when the metal is volatilized and arsenic trioxide (As.,0,) formed. The odor serves to distinguish the two cases just described; but more than this, the white coating will be THE USE OF THE BLOWPIPE. 143 perceived to lie this time much farther from the flame than the oxide of antimony, because it is more volatile and can be deposited only where the coal is comparatively cool. A third trial may be made with arsenopyrite. It gives off as it is heated a cloud of white fumes with the same peculiar penetrating garlic odor, and the white coating of arsenic trioxide forms at a distance on the coal. There is, however, a residue in this case which soon fuses to a grayish black globule which when cold is found to be magnetic, proving the presence of iron. The mineral con- sists of iron, sulphur, and arsenic (the formula is FeAsS). Part of the sulphur is driven off (as S0 2 ), and after some time all the arsenic, while a magnetic compound of iron and sulphur (with perhaps a little residue of arsenic) is left behind. Another trial may be made with sphalerite or zinc blende, but to succeed now the mineral should be pulver- ized first, since it is infusible before the blowpipe and the compound is only with difficulty decomposed on char- coal. A little of the powder placed in the scratch and carefully heated (lest it be blown away) will cohere together and presently, if the flame is hot, a coating will be formed over the powder and just about it on the coal. If the powdered mineral is first mixed with two or three times its volume of sodium carbonate, it is then more easily decomposed and the sublimate obtained. This coating, which consists of the oxide of zinc (ZnO), has a bright canary-yellow color when hot, but becomes white on cooling. If the coating is thin, it might sometimes be 144 MINERALS, AND HOW TO STUDY THEM. mistaken for the charcoal ash (although good charcoal gives very little), and confirmatory evidence can be ob- tained by letting a drop of cobalt nitrate fall upon it. when, if again heated in the oxidizing flame, it will assume a bright green color characteristic of zinc. Still again, if the coating of zinc oxide, as at first obtained, is heated with the reducing flame, it is reduced, the zinc volatilized yielding a characteristic bluish-green flame. It may be repeated here that a fragment of calamine (zinc silicate) yields a similar sublimate, but the mineral itself becomes blue when heated after being moistened by the cobalt solution. A fragment of galena should also be tried on charcoal. It will fuse very easily, and immediately about it there will form a yellow coating of the oxide of lead (PbO), while farther off there will be a white coating of lead sulphate (PbS0 4 ) formed by the union of the PbO and S0 a in the presence of the oxygen of the air. Further, lead is what is called an easily-reducible metal; that is, its compounds are rather easily changed to the metallic state under the action of heat, as on charcoal; hence continued heating yields globules of metallic lead. A little soda on the galena hastens the production of the metal, and at the same time it is noted that the yellow coating is more distinct, while the white fumes are nearly absent, for now the soda unites with the sulphur of the galena. Again, a fragment of ruby silver either the dark red (pyrargyrite) containing silver, antimony, and sulphur, or the light red (proustite) containing silver, arsenic, and THE USE OF THE BLOWPIPE. 145 sulphur may be tried. It will fuse easily, and give off white fumes of either the oxide of antimony or arsenic according as to which mineral was in hand, and the dis- tinction is easily noted as before described. A black globule will be left behind; and if now some sodium car- bonate be poured over this on the coal, and the mass heated persistently till the globule is fused in it, presently, after rather long-continued blowing, a white globule, or perhaps several, will be seen moving about in the fused soda. By removing the fused mass from the charcoal and crushing it in a mortar, the metallic silver is readily separated. It will be found that the globule is white like silver, and remains bright (not oxidizing readily), and it is malleable. That it really is silver may be proved by chemical means, for it dissolves easily in nitric acid, and the addition to the solution of a drop of hydrochloric acid causes a white curdy precipitate of silver chloride to separate at once. The reddish coating, on the coal, of the silver oxide formed during the process is sometimes distinct. From some silver minerals, as the chloride, cerargyrite (also called horn-silver), the metal is obtained at once by heating on charcoal. A mineral containing copper will yield metallic copper on charcoal when heated with the soda. This may be either in small globules or as a thin crust. When exposed to the air the copper becomes coated with the black oxide, but it is easily recognized, being malleable in the anvil, and showing when rubbed its peculiar red color. Cassiterite or tin-stone, first powdered, for it is an infusi- ble and refractory mineral, and then heated with soda, will 146 MINERALS, AND HOW TO STUDY THEM. give minute malleable globules of metallic tin. These are at first nearly as white as silver, but soon oxidize and become dull; with a little nitric acid in a watch-glass they yield an insoluble white powder of tin dioxide. The reduction is more easily accomplished if potassium cyanide is added to the mixture, but it is a very poisonous sub- stance, and its use hardly to be recommended outside of the laboratory. The tin globules may not be very con- spicuous in the soda, but are easily separated from the soda by crushing and washing in a mortar; the soda and char- coal are washed off and the heavy tin particles left behind. Besides the coatings mentioned in these examples, the following must be mentioned : Bismuth gives a volatile sublimate, which is dark orange- yellow when hot and lemon-yellow when cold. Mixed with equal parts of potassium iodide and sulphur and heated in the 0. F., a beautiful red sublimate of bismuth iodide is deposited. Molybdenum gives a sublimate which is yellow when hot and white on cooling; this is volatile in the 0. F., leaving a copper-red stain of the oxide; if touched for a moment with the R. F. a beautiful azure-blue is obtained. Cadmium gives a sublimate red-brown near the frag- ment and orange-yellow at a distance from it; the subli- mate is volatile. Selenium and selenides give a very disagreeable odor (like decaying horse-radish) which is highly characteristic. Soda is also used on charcoal with the group of com- pounds called sulphates, to prove the presence of sulphur. A little of the pulverized mineral (as barite) fused with THE USE OF THE BLOWPIPE. 147 the soda yields a mass of a liver-brown color (called hepar), which, removed from the coal and placed with a drop of water on a silver coin, will stain it black. This is explained by the action upon the silver of the sodium sulphide formed on charcoal. It is essential that the soda itself should be free from sulphur; and further, since this may be contained in the illuminating gas employed, a prelimi- nary trial should be made with soda alone; if this gives no action on the silver, then the final result with this, if show- ing the presence of sulphur, can be trusted. 6. USE OF THE CLOSED AND OPEN" TUBES. The tubes in blowpipe work are chiefly used in the examination of minerals which yield on heating a volatile substance; this in most cases is condensed in the colder part of the tube. There is an important distinction to be observed between the use of the closed and the open tube. The closed tube contains but very little air, and this is driven out with the first puffs of gas from the heated mineral, and hence what goes on takes place without much effect from the oxygen of the air. In the case of the open tube, on the contrary, if held in the proper inclined position, there is a constant stream of hot air (that is, of oxygen) which passes up the tube and over the heated mineral fragment. A few examples will show how this principle is applied. Place a little fragment of sulphur in the closed tube and heat it gently. At once it is fused and converted into sulphur vapor which rises in the tube and soon condenses, 148 MINERALS, AND HOW TO STUDY THEM. giving a dark orange-red ring of liquid sulphur, which becomes light yellow as it cools and solidifies. Here there has been no change, simply the volatilization of the sulphur. Now place a fragment in a rather large open tube, about an inch from the end; incline the tube as much as possi- ble without causing the fragment to slip out, and heat it very slowly. The sulphur fuses as before, but the hot oxygen which passes over it unites with it, forming sulphur dioxide (SOJ, an invisible gas which rises through the tube and comes out of the open end, giving the usual sulphur odor (it should be 'inhaled with a little caution); further, the acid fumes of this gas will turn a piece of blue litmus-paper bright red. It is difficult to heat the sulphur slowly enough to prevent the formation also of a ring, as in the closed tube, simply because it is easily volatile; that is, it goes off into gas very readily, and the oxygen can hardly be supplied fast enough to oxidize it all. As a second example, take a small piece of as pure cinna- bar as can be obtained (it often occurs with clay as a gangue, as it is called, and this may give off water and obscure the result). The cinnabar is sulphide of mercury, HgS, a substance which is converted into vapor when heated out of contact with the air. In the closed tube we get at once a black ring, or sublimate, of mercury sulphide which, like the sulphur, was first volatilized and then condensed where the tube was cooler. This black coating becomes reddish if rubbed. In addition to it there may be also a faint gray deposit above of metallic mercury, because of the small amount of air in the tube at the start (see beyond). THE USE OF THE BLOWPIPE. 149 Now place a fragment in the open tube and heat it, this time also very slowly and carefully. Gradually the cinna- bar disappears, while the sulphurous fumes can be per- ceived at the end of the tube, as in the other case. But more than this, a little above the fragment a faint deposit begins to form, growing more and more distinct, and finally, when seen by reflected light, it appears as a shining mirror. This is metallic mercury in the form of minute globules coating the glass; that it is mercury can be proved even to the skeptical by cutting the tube carefully near the deposit (by first scratching it with a file) and then rub- bing the deposit with a match-stick. The minute globules unite to form a few large ones which will run out of the tube, when tipped up, and on to the hand. As before remarked, a little of the metallic mercury may be noted for the same reason in the closed tube. It is not difficult to explain what has happened in this case. The hot oxygen passing over the heated mineral has united with the sulphur to form sulphur dioxide (SOJ, while the mercury thus left alone has been driven off as vapor by the heat and collected where the cube was cool enough to allow of its condensation. Very likely in this case too, unless the heating is very slow, a little sulphide of mercury will go off without change and form a black ring in the closed tube, but by gradually heating this, keeping the tube in the same position, it is driven up the tube, more and more of the sulphur being oxidized, until nothing but the pure metallic mirror of the mercury is left. This experiment succeeds best if the tube is first heated 150 MINERALS, AND HOW TO STUDY THEM. quite hot a little above the mineral and then the heating of the fragment carried on very slowly and carefully. If the powdered cinnabar be mixed with soda (first dried to expel the water), and then introduced into the dosed tube* and heated, a sublimate of metallic mercury is very readily obtained. Again, take a fragment of galena; in the closed tube it undergoes no change and no sublimate is formed. If, however, another fragment is placed in the open tube, al- though no sublimate is produced here, some of the sulphur is oxidized and the sulphurous fumes can be perceived by the odor or by their reddening effect on litmus-paper. This method is consequently a general method of testing for sulphur in the class of compounds called sulphides. A fragment of orpiment, sulphide of arsenic, As 2 S,, heated in the closed tube is melted, volatilized, and forms a beautiful red ring of sulphide of arsenic. Heated in the open tube (very slowly), both sulphur and arsenic are oxidized; the sulphur gives as always S0 a , while the ar- senic yields a white deposit of minute octahedral crystals of arsenic trioxide (As 2 3 ) spangling in the light. This sublimate is very volatile and hence may be driven farther and farther up the tube when heated. Arsenopyrite, FeAsS, in the closed tube gives a trace of * In a case like this where the substance is in powder, it can be introduced into the tube without soiling the tube (which is quite an important matter) if a little trough be made by folding once a narrow strip of paper ; then place the substance in this and insert it in the tube carefully, this being held in a horizontal position ; now when the tube is turned into a vertical position the powder will fall to the bottom and the paper can be removed. THE USE OF THE BLOWPIPE. 151 a white sublimate of the oxide of arsenic, but more dis- tinctly at first a dark red deposit of sulphide of arsenic (As a S 3 ), which, if the heating is stopped and the tube allowed to cool, becomes a rich red color. 1 1 the heating is continued, the arsenic now goes off alone and forms a shining mirror of crystalline scales of metallic arsenic. The residue is magnetic and consists of iron and sulphur chiefly. In the open tube, heated slowly, part of the sulphur goes off as sulphur dioxide, while the arsenic gives a white crystalline deposit of As 2 s , and the same magnetic res- idue as before is left behind. This case illustrates again the important difference between the use of the open and closed tube. Another good example is given by stibnite, Sb a S s . In the closed tube it is all volatilized and gives a dark red sublimate, most of which is a complex compound called an antimony oxysulphide (2Sb.,S 3 .Sb,0 3 ). The presence of this is explained by the fact that the oxygen in the small amount of the air contained in the tube is enough to unite with the sulphide of antimony and form the com- pound named. In the open tube both sulphur and antimony are oxi- dized. The sulphur gives sulphur dioxide (SO,) and the antimony gives antimony trioxide (Sb.,0 3 ), which forms as a dense white powdery deposit which is not volatile when heated by the flame. It is thus easily distinguished from the arsenic trioxide, which is crystalline and spangles in the light instead of being a dull powder; while the oxide of arsenic too is, as before stated, very volatile. The dis- 152 MINERALS, AND HOW TO STUDY THEM. tinction between the sublimates of antimony and arsenic formed on charcoal should be recalled (see pp. 142, 143). Of other rarer substances it may be mentioned that selenium gives a dark red, nearly black, sublimate in the closed tube with its peculiar disagreeable odor at the open end; in the open tube the sublimate is steel-gray, the upper edge red with perhaps white volatile crystals of the oxide. Tellurium in the closed tube condenses in small drops with metallic luster; in the open tube a gray sublimate is formed which fuses to colorless drops, becoming solid on cooling. Pyrite, iron disulphide, FeS,, when heated in the closed tube gives off about half its sulphur which condenses in a ring like that just described. Heated slowly in the open tube, the sulphur which is driven off may be all oxi- dized to S0 2 . A magnetic residue is left in both cases. A few other uses of the tubes must be mentioned here. A mineral containing water, when heated in the closed tube, gives off the water vapor which condenses as drops of water in the upper part of the tube. A change in the appearance of the mineral may take place at the same time. Thus a piece of limonite, or hydrated oxide of iron, gives off its water and turns red, for it is now the an- hydrous oxide of iron, like hematite, which has a red powder. In a few cases, as with some sulphates, the water has an acid reaction and turns blue litmus-paper red. It may be added that the higher oxide of manganese (MnO a the mineral pyrolusite) gives off oxygen in the closed tube. THE USE OF THE BLOWPIPE. 153 Fluorine is usually tested for in the closed tube, the powdered mineral being mixed with previously fused bi- sulphate of potash and then heated; the hydrofluoric acid given off attacks the glass or, as it is usually ex- pressed, etches it. A few minerals, as fluorite, phosphoresce in the tube, that is, give out a yellow or green light when held, after slight heating, in a dark spot. Also, as another phenom- enon sometimes noted, the fragment when heated in the tube may glow brightly. The more of the experiments with the minerals named the student performs the better, for knowledge thus ob- tained by experience is much better than knowledge learned from a book. A list of the minerals particularly useful for blowpipe work is given in an Appendix at the close of this book. Any one who has mastered the elements of blowpipe work and who is interested in learn- ing more should turn to a manual of blowpipe analysis, where he will find many more tests and reactions and more minute directions for the work in general. The work of Professor Brush on Determinative Mineralogy may be particularly recommended. 7. CHEMICAL EXAMINATION BY ACIDS AND OTHER REAGENTS. In addition to the various methods of chemical examina- tion .already described which can be made by means of the blowpipe, there are a few other chemical tests so easy to apply that the mineralogist should be in a position to use 154 MINERALS, AND HOW TO STUDY THEM. them. The reagents most needed are the three acids, hydrochloric, nitric, and sulphuric, perhaps also a little ammonia. In most cases it is best to use the strong acids, but often these diluted with an equal volume of water answer every purpose. A few test-tubes are ulso required, and sometimes a porcelain dish or casserole. The caution in regard to chemical reagents already mentioned (p. 127) is to be carefully observed. Solubility in Acid. The question as to whether a min- eral is soluble in one of the acids named is often of great importance. To test the solubility hydrochloric acid is generally used, except with metallic sulphides and some other minerals containing prominently one of the heavy metals (lead, copper, silver, etc.); for these latter nitric acid is usually better. The mineral should in general be pulverized as finely as possible in the agate mortar and in- troduced into a large test-tube, some acid poured on, and the whole carefully heated over the Bunsen flame, the tube being shaken gently during the process. It must be remembered here that the acid fumes in the air are injurious to breathe and will act corrosively upon surfaces of brass in the neighborhood; hence such tests can only be tried with caution unless the conveniences of the laboratory are at hand. Various results may be noted during this trial: A. The mineral may dissolve quietly with or without coloring the solution; this is true, for example, of hema- tite, also of many of the sulphates and phosphates. B. There may be a bubbling off or effervescence of a gas. This gas is usually carbon dioxide or carbonic-acid gas THE USE OF THE BLOWPIPE. 155 (CO,); but may be hydrogen sulphide or sulphuretted hy- drogen (H a S). Also chlorine may be liberated, or reddish fumes of nitrogen. C. There may be a separation of some insoluble sub- stance, as sulphur, silica, etc. These points will now be spoken of more in detail. Effervescence with Carbon Dioxide. This resembles the bubbling observed, for example, in a glass of soda-water, due to the escape of this same gas liberated because of the relief of the pressure which kept it dissolved in the water in the tank. This is an easy and important test for the carbonates. Some of them dissolve in cold acid and even in lumps without being first pulverized. This is true of calcite, but is not true of dolomite and siderite, which re- quire to be pulverized or heated, or both; hence this is used as a means of distinguishing between them. The carbo- nates of copper and lead should be tried with nitric acid. Effervescence with Hydrogen Sulphide. Most metallic minerals, as stated above, will be treated with nitric acid, but some not having a metallic luster, sphalerite for exam- ple, may be put into hydrochloric acid. In this case the reaction produces the gas hydrogen sulphide, while zinc chloride goes into solution. This gas bubbles off like car- bon dioxide, but its disagreeable odor, resembling that of rotten eggs, shows at once what it is. Chlorine, easily detected by its peculiar odor, is given off in some cases, as when the oxides of manganese are heated in hydrochloric acid. Nitrogen Peroxide, giving a peculiar red color and suffo- cating odor, is liberated when many metallic sulphides (as I5e MINERALS, AND HOW TO STUDY THEM. chalcopyrite), also a few other compounds, as cuprite, are treated with nitric acid. Separation of Sulphur. A number of sulphides, as for example pyrite, dissolve in nitric acid with the separation of particles of sulphur which usually cling together and float on the liquid. It may be added that this is also true of chalcopyrite, or copper pyrites, but this, like other cop- per sulphides, gives a green solution which turns a deep fine prussian blue when ammonia is added in sufficient quantity to dissolve the precipitate that forms at first. Separation of Tin Dioxide. When metallic tin is treated with nitric acid, tin dioxide (SnOJ is formed, which sepa- rates as an insoluble white powder. Separation of Silica. A number of silicates dissolve in hydrochloric acid with the separation of the silica, some- times as a powder, sometimes as a slimy mass. Other sili- cates dissolve entirely; but if the solution is gently heated until part of the liquid has been evaporated off, a thick jelly is finally formed, so that the test-tube can be partially in- verted without its flowing out. Such silicates are said to gelatinize with acid. This is true of calamine and a num- ber of the zeolites; chabazite, on the other hand, is de- composed with the separation of slimy silica. Difficultly -soluble or Insoluble Minerals. A large num- ber of minerals, even when pulverized, dissolve very little or not at all in strong hot acid. Quartz and corundum, for example; also the silicates, orthoclase, topaz, and many others, even when finely pulverized and long heated in strong acid, are not at all or only very slightly attacked. The question whether there has been partial solution is not THE USE OF THE BLOWPIPE. 157 always easy to answer, but can be decided if the liquid takes a distinct color, or more fully by filtering off the liquid from the undecomposed mineral, and then adding to it a few drops of ammonia, which, in general, will cause the bases which have gone into solution to separate as precipi- tates. To explain the various ways simple, too, many of them in which the bases present in the solution can be identified would take us too far into the subject of Chemis- try. Do not forget, however, the test foi copper just men- tioned (p. 156), or that for silver given on an earlier page (p. 145). Further, attention may be called to the fact that, as a test for sulphuric acid or a sulphate, the addition, to a solution containing them, of a little barium chloride will cause a heavy white precipitate of barium sulphate to form. 158 MINERALS, AND HOW TO STUDY THEM. CHAPTER VII. DESCRIPTION OF MINERAL SPECIES. THE following chapter gives descriptions of all the com- mon species of minerals, with remarks, more or less brief, about many of those which are rarer. The system of classification is that spoken of on p. 120, in which the different compounds of the same metallic element are grouped together. The Silicates, however, many of which are complex in composition, containing more than one metal, are, with the exception of a few valuable ores, most conveniently included in a common section at the close of the chapter. The several characters for each mineral species are enumerated in the following list : .Crystalline system ; the characteristic angles and the common form, or habit, of the crystals; also thfe structure of the crystalline aggregates and massive varieties. Cleavage; also fracture and tenacity. Hardness (H.). Specific gravity (G.). Luster, color, streak, degree of transparency. Other physical characters, as magnetism, etc. Chemical composition and blowpipe characters.* * These last are also called pyrognostic characters because depend- ing upon the application of heat (nv/j, fire); this word is often con traded to Pyr. DESCRIPTION OF MINERAL SPECIES. 159 The order in the above list is that which is at once the most convenient and scientific. In the account given of each species in the following pages, however, it is not at- tempted to adhere to this order strictly, as would be done in an advanced scientific work. On the contrary, so far as is possible in the brief space available, the aim is to make this account readable and to call attention especially to the characters most easy or most important to remember. Further, in the description of many species no men- tion is made in regard to certain characters, which are relatively unimportant in these particular cases. Thus if the cleavage is not mentioned, it is because it is either not observed or too imperfect to be an important character. So, too, nearly all minerals are brittle, hence it is unneces- sary to repeat this word in each case; but if the mineral is not brittle but malleable or sectile, this is stated and to be carefully noted. Again, if the streak is not given, it is to be understood to be white or nearly white, like that of most non-metallic minerals, even when the mineral itself in the mass has a deep color. Also all minerals if having a metallic luster ire opaque. The localities of the species are men- tioned, if at all, very briefly. The student will find it easier to remember the charac- ters of the different minerals, and a 'help in other ways, if. after studying the descriptions in the book and comparing them with such specimens as he has access to, he will make a brief tabular list of the characters for each species, some- thing like that on the following page. It is very easy to arrange a note-book (conveniently of the square letter size) for this purpose by ruling a series d* 160 MINERALS, AND HOW TO STUDY THEM. Diamond. Graphite. Galena. Sphalerite. Cryst.systemA common form Cryst & mass Isometric octahedron Hexagonal tabular Foliated Isometric cube Granular- cleavable Isometric tetrahedral Granular- cleavable Cleavage Octahedral Basal Cubic Dodecahedral Hardness, etc. 10! 1-2! flexible 2.5- 3.5-4 Gravity 3.5 2.2 7.51 4 Luster Adamantine Metallic Metallic Ruinous Color Colorless, yel'w Black Lead-gray Yellow, brown, black, etc. Str&ak White Black Dark gray White to brown Comp Carbon Carbon PbS ZnS Pyr. etc. Infusible Infusible Easily fusible Infusible parallel vertical columns, and the trouble of writing the list of characters over each time may be avoided if they are written on the edge of the first left page and the corresponding strip from a sufficient number of the sheets following neatly cut off. A little contraction of some common words will save space : hardness is often indicated by the letter H.; specific gravity by G.; yellow may be written yw, and so on. When a character is particularly important it may be underscored or followed by an exclama- tion point. It is not worth while to repeat in tabular form the entire description in the text; a little experience will soon show how much may be advantageously written down. It will also be a useful exercise to fill out a similai column, so far as the individual case allows, for any specie from the specimen itself, and then it may be compared with the description in the book, or the list in the stu~ dent's note-book made out from the book. If the species was not known at first, this list of characters will often suffice to enable the student to determine it. DESCRIPTION OF MfifERAL SPECiES. 161 It is not necessary to learn by sheer effort of memory all the characters from the book at once; this would be diffi- cult and tiresome; the most important can be learned (and first the chemical composition), while the knowledge of most of the physical characters is rather to be acquired gradually by the repeated handling of the specimens themselves. The following is a summary of the species included in the pages which follow,* arranged, except for the silicates, under the prominent element of which they are com- pounds. Muny other species are mentioned briefly in the text, though not included here. The student should read again the brief statements in regard to the classification of the chemical elements and the prominent groups of chemical compounds given on pp. 109 to 116. It may be interesting here to recall the old alchemistic method of designating the chief metals by referring them to one of tin members of the Solar System, as follows : the Sun, gold; the Moon, silver; the planet Mercury, mercury; Venus, copper ; Mars, iron ; Jupiter, tin ; Saturn, lead. Other prominent metals are platinum, zinc, and aluminium. CARBON: Diamond. Graphite. SULPHUR: Native Sulphur. HYDROGEN: ice (and water). * A list is given in the Appendix of those of the species here enumerated which it is most important for the student to have in hie collection. 162 MINERALS, AND HOW TO STUDY THEM. ARSENIC : Native Arsenic. Realgar and Orpiment, Arsenic sulphide* ANTIMONY : Native Antimony. Stibnite, Antimony sulphide. BISMUTH: Native Bismuth. MOLYBDENUM: Molybdenite, Molybdenum sulphide. GOLD: Native Gold. Sylvanite, Gold telluride. PLATINUM: Native Platinum. SILVER: Native Silver. Argentite, Silver sulphide. Pyrargyrite, Sulphide of silver and ant: mony. Proustite, Sulphide of silver and arsenic. Cerargyrite, Silver chloride. MERCURY: Native Mercury. Cinnabar, Mercury sulphide. COPPER: Native Copper. Chalcocite, Copper sulphide. Bornite and Chalcopyrite, Sulphides o: copper and iron. Tetrahedrite, Sulphide of antimony anc copper. Cuprite, cuprous oxide. Malachite and Azurite, Carbonates of co[ per. Dioptase and Chrysocolla, Silicates of cop- per. LEAD: Native Lead. Galena, Lead sulchide DESCRIPTION OF MINERAL SPECIES. 163 Jamesonite and Bournonite, Sulphides of antimony and lead. Pyromorphite, Lead phosphate. Mimetite, Lead arsenate. Vanadinite, Lead vanadate. Cerussite, Lead carbonate. Anglesite, Lead sulphate. Also Crocoite, Lead chromate; Wulfenite, Lead molybdate, etc. TIN : Cassiterite, Tin dioxide. TITANIUM: Rutile; also Octahedrite and Brookite, all alike Titanium dioxide, TiO,. URANIUM: Uraninite. Torbernite, Autunite, Uranium phosphates. IRON: Native Iron. Pyrrhotite, Iron sulphide. Pyrite and Marcasite, Iron disulphide. Arsenopyrite, Iron sulph -arsenide. Hematite, Iron sesquioxide. Magnetite, Magnetic iron oxide. Franklinite, Iron-zinc-manganese oxide. Chromite, Iron-chromium oxide. Limonite, Hydrated iron oxide. Siderite, Iron carbonate. Also Columbite, Iron niobate (columbate) and Wolframite, Iron tungstate ; Triphy lite, Phosphate of iron and lithium. NICKEL: Millerite, Nickel sulphide. Niccolite, Nickel arsenide. Genthite and Garnierite, Nickel silicates. 164 MINERALS, AND HOW TO STUDY THEM. COBALT: Linnaeite, Cobalt sulphide. Smaltite and Cobaltite, Arsenides of co- balt. Erythrite, Cobalt arsenate. MANGANESE: Pyrolusite and Manganite, Oxides of man- ganese. Khodonite, Manganese silicate. Rhodochrosite, Manganese carbonate. ZINC: Sphalerite, Zinc sulphide. Zincite, Zinc oxide. Willemite and Calamine, Zinc silicates. Smithsonite, Zinc carbonate. ALUMINIUM: Corundum, Aluminium oxide. Spinel, Oxide of magnesium and alumin- ium. Cryolite, Fluoride of aluminium and so- dium. Turqnois and Wavellite, Aluminium phos- phates; Amblygonite, Phosphate of aluminium and lithium. CALCIUM: Fluorite, Calcium fluoride. Calcite and Aragonite, Calcium carbonates. Apatite, Calcium phosphate. Anhydrite, Calcium sulphate. Gypsum, Hydrated calcium sulphate. Scheelite, Calcium tungstate. MAGNESIUM: Brucite, Hydrated magnesium oxide. Magnesite and Dolomite, Magnesium car- bonates. Boracite, Magnesium borate. DESCRIPTION OF MINERAL SPECIES. 165 BARIUM: Barite, Barium sulphate. VYitherite, Barium carbonate. STRONTIUM : Celestite, Strontium sulphate. Strontianite, Strontium carbonate. SODIUM and POTASSIUM: Halite or Rock Salt, Sodium chloride. Borax, Sodium borate. Sylvite, Potassium chloride. SILICON : Quartz, Silicon dioxide. Opal, Hydrated silicon dioxide. SILICATES : * Feldspars : Orthoclase (and Microcline), Albite, Anorthite ; also Oligoclase, Labradorite. Pyroxene (Diopside, Salite, Augite, etc.). Amphibole or Hornblende (Tremolite, Actinolite, Asbestus, etc.). Beryl. Garnet (Grossularite, Almandite, etc.) Micas: Muscovite, Biotite, Phlogopite, Lepidolite. Chlorites : Clinochlore, etc. Chrysolite. Zircon. Scapolite. Vesuvianite. Epidote. Tourmaline. Topaz. * The composition of the following minerals is in many cases too complex to be given briefly here. 166 MINERALS, AND HOW TO STUDY THEM. Titanite or Sphene. Andalusite, Sillimanite and Cyanite. Staurolite. Talc. Serpentine. Datolite. Prehnite. Apophyllite. Pectolite. Zeolites : Thomsonite, Natrolite, Aualcite, Chabazite, Stilbite, Heulandite. CARBON. Diamond. Carbon, C. The DIAMOND is usually found in distinct isolated crys- tals, most of them very small, but sometimes as large as a robin's egg or even larger. The crystals are commonly octahedrons, though less often some of the other forms of the isometric system (p. 22) are observed. The natural crystals before cutting " rough diamonds " they are called frequently have rounded edges and curved faces, or the faces show little pits like the etchings spoken of on p. 64. This is illustrated in Fig. 157, while Fig. 158 shows a hex- octahedron with convex faces. There are also forms with irregular structure, occasionally as round as peas, and one peculiar kind is massive and dull black in color. The crystals have perfect cleavage parallel to the octa- hedral faces, which the lapidary makes use of to bring a stone into the form best suited for cutting. The hardness DESCRIPTION OF MINERAL SPECIES. 167 is 10, or higher than that of any other species, and the specific gravity is also high, 3.5 (see p. 84). The luster is very brilliant and of tho peculiar character named (from this species) adamantine; the brilliancy of the diamond, however, is much greater when cut with many facets than in the natural crystals. The most highly prized stones are colorless and clear as water (then said to be " of the first water") ; a pale yellow color is very common, and some- times other colors, in pale shades, as green, pink, and blue, are observed; rarely it is black and dull. The diamond consists of pure carbon, and has thus the 157. 158. same composition as a piece of charcoal. It is infusible as is charcoal, and is not acted upon by acids; but it is unlike charcoal in that it does not burn. When heated very hot, however, as in the electric arc, it is slowly consumed, form- ing, like burning charcoal, carbon dioxide or carbonic-acid gas (CO,). Heated out of contact with the oxygen of the air it is converted into a mass resembling coke. The diamond has been found mostly in gravel deposits, or the rocks formed by their consolidation, and but little is known about its real home or the way it was made. For- merly it was obtained in great quantities in India; later Brazil afforded many of the gems, but both these countries 168 MINERALS, AND HOW TO STUDY THEM. now yield comparatively few. The great region which pro- duces the diamond at the present time is in South Africa, some eight hundred miles from Cape Town, where it occurs along the Vaal River, and in larger quantities especially in the neighborhood of Kirnherly, in peculiar oval regions called " pans." Here the diamond mines have been worked for about twenty-five years and a vast number of stones have been found and brought to market. Think of more than eight tons of diamonds obtained during this time! Everybody knows of the use of the diamond for jewelry, for which its brilliancy, hardness, and comparative rarity peculiarly fit it. Many of the great diamonds of the world have a long and fascinating history of their own, which would fill a large volume if all were told of the way in which they have repeatedly changed hands until they have come into the possession of royalty, as the famous " Kohinoor " among the crown jewels of England, or the " Florentine " of Austria and the great " Orlov " diamond of Russia. Diamonds are also used for cutting glass and, in the form of powder, in grinding diamonds and other hard gems. The black coal-like diamonds, set in a collar and rotated rapidly by machinery, as a diamond drill, cut quickly through the hardest rocks, leaving a core behind, which is raised at in- tervals; a well-boring is thus easily made. Graphite or Plumbago. Carbon, C. GRAPHITE, or Plumbago as it is often called, is usually found in massive forms which may be separated easily into thin leaves or plates and hence are said to be foliated; sometimes also it is finely granular and compact. Rarely DESCRIPTION OF MINERAL SPECIES. 169 the plates are distinct and separate and show a regular six- sided outline, whence it is referred to the hexagonal system; it is then seen to ha\e perfect basal cleavage. It is sectile and so soft as to make a mark on paper and to feel greasy to the hand ( H. = 1 to 2), and its specific gravity is only 2.2. It has a metallic luster and an iron- black or steel-gray color and streak; it is perfectly opaque. Graphite has the same composition as the diamond, con- sisting also of nearly pure carbon; it is, however, a differ- ent substance in its physical characters and is hence a dis- tinct mineral. Note that they differ in crystalline form; also the diamond is hard and heavy, while graphite is soft and light. It is also infusible like the diamond, and is not at- tacked by acids, but may be converted into carbon dioxide (C0 2 ) by heating to a very high temperature in the air. Graphite is commonly found in the crystalline rocks called gneiss, sometimes scattered in scales, but occasionally in large beds that can be mined; it is also found in scales in crystalline limestone, and is often formed in an iron fur- nace. It is largely mined at Ticonderoga, N. Y. ; also in Eastern Siberia, and in Ceylon. Graphite is the so-called black lead of our " lead-pencils " (but it is only like lead in its color), and would be mined for this purpose if for no other. It is used as an excellent lubricator because of its smooth soapy character when pul- verized ; also, mixed with clay, for making crucibles because it is infusible and not affected by the heat of an ordinary furnace; in electroplating because it is a conductor of elec- tricity. 170 MINERALS, AND HOW TO STUDY THEM. CARBON is also the element which forms the essential part of the different kinds of coal and of mineral oil or petro- leum. Anthracite, the coal of eastern Pennsylvania, contains 85 to 95 per cent of carbon and has a bright shiny surface and conchoidal fracture; it burns with a pale feeble flame without smoke. Bituminous coal is black to dark brown in color, often dull and with a pitchy luster; it contains less carbon than anthracite (usually 75 per cent) and more hydrogen and oxygen; it burns with a yellow smoky flame. Brown coal, or lignite, has a brown color, dull luster, often retains the structure of the original wood and contains still less carbon, sometimes only 50 per cent. These different kinds of coal and others related to them, though of great economic value, are not properly mineral species, since they have no definite chemical composition. The same remark applies to asphaltum, bitumen, mineral wax or ozocerite, the many kinds of mineral resins including amber, and finally min- eral oil or petroleum, all of which consist chiefly of carbon. The element carbon is also present in the large group of minerals called carbonates, of which calcite, including com- mon limestone, is much the most important. SULPHUR. Native Sulphur. S. SULPHUR is another of the chemical elements occurring in nature. It is found in crystals of the orthorhombic system; a common form is an acute rhombic pyramid (Fig. 159) with terminal angles of 106 and 85. Figs. 160, 161 DESCRIPTION OF MINERAL SPECIES. 171 are also common forms. It also occurs in masses and in powder. It is soft (H. = l.o to 2.5) and, though brittle un- der the blow of a hammer, is easily cut by the knife; the specific gravity is about 2. It has a resinous luster and a bright sulphur-yellow color and streak. The crystals are often clear and transparent. It consists of pure sulphur, and is remarkable among minerals because when heated it takes fire and burns with a pale blue flame, giving a gas (sulphur dioxide, S0 2 ) which has a very suffocating odor familiar to all who use sulphur matches. Sulphur is for the most part found in volcanic regions, as in Sicily and the Sandwich Islands; also in beds associated 159. 160. 161. with gypsum. It is used for making sulphur matches; it is one of the three substances of which gunpowder is made (with charcoal and niter) ; it is used in preparing the rubber gum for overshoes and other purposes; also in making sulphuric acid and in other ways. It is in fact a most important mineral. Sulphur also occurs abundantly in nature, not as an ele- ment, but in combination with the metals forming the very large and important class of sulphides, as lead sulphide, PbS, the mineral galena. It also forms the acid, sulphuric 172 MINERALS, AND HOW TO STUDY THEM. acid, H a S0 4 , the salts of which are the important class of sulphates, as barium sulphate, BaS0 4 , the mineral barite. Ice. Hydrogen oxide, H 2 0. Although it cannot be preserved in a mineral cabinet, ICE, the solid form of water, is as truly a mineral as diamond or quartz. It occurs in crystalline forms of the hexagonal type, often of great complexity and beauty, as seen in snow-crystals. These, as stated on p. 17, are formed in the atmosphere direct from the water vapor. Some of the forms are shown in figure on p. 17. The ice-grains that make the pellets of hail, not infrequently occurring with summer thunder-storms, are also occa- sionally in clusters of crystals, somewhat resembling the hexagonal pyramids of quartz, though this is the exception; generally there is simply a concentric con- cretionary structure. The ice of the pools and ponds is always crystalline, though it is usually only in the first stages of the process of freezing that the crystals are separately visible. This process of solidification goes on, as every one knows, at a temperature of 32 Fahrenheit (0 Centigrade). The hardness of ice near the freezing- point is 1.5, but this increases at lower temperatures. The specific gravity is about 0.92, so that it floats in the water with a little more than nine tenths of its bulk submerged. Water expands, therefore, largely on freezing and exerts a great force on confining surfaces. One consequence Oi this is the breaking of vessels, water-pipes, etc., when the water they contain is frozen. In nature ice is on account of this DESCRIPTION OF MINERAL SPECIES. 173 property a powerful agent in pulling rocks to pieces, the water creeping into the cracks, especially into the narrow ones, by capillarity, and when it solidifies the rock masses are slowly but surely wedged apart. Water consists chemically of hydrogen and oxygen, combined in the ratio of 2 : 1 by volume, or 11.1 : 88.9 by weight. TELLURIUM. Carbon in its two forms, the diamond and graphite, and sulphur belong, as was stated on p. 102, to the non- metals among the chemical elements. Intermediate be- tween them and the true metals, like gold and silver, come several elements which occur in nature, namely, tellurium, arsenic, antimony, and bismuth. TELLURIUM has a bright tin-white color and a metallic luster, though, unlike the true metals, it is rather brittle; it is occasionally found in Colorado. This element is of little economic importance, but is interesting because it is the only one with which gold occurs combined in nature, in some of the rare minerals called tellurides (see p. 181). ARSENIC. ARSENIC is found occasionally as a mineral and then called NATIVE ARSENIC. It has a metallic luster and tin- white color, but soon tarnishes on the surface to a dull dark gray; it is also brittle. It generally occurs showing a fine granular structure when fractured, and the masses commonly have a reniform or botryoidal surface. Arsenic is used with copper and tin to form the alloy called speculum metal, useful for metallic mirrors because 174 MINERALS, AND HOW TO STUDY THEM. of the brilliant surface it takes when polished. The lead employed for making shot contains a small amount of arsenic. The compounds of arsenic find various uses, as pigments, (sulphide); as a preservative; a poison for in- sects (white arsenic and Paris green); also in dyeing, medicine, etc. Kealgar, Orpiment. Sulphides of Arsenic. Two important but rather rare minerals containing ar- senic are the sulphides, Realgar, AsS, and Orpimeut, As a S 8 . EEALGAR is found in transparent monoclinic crystals and massive forms of a beautiful aurora-red color. It is soft and sectile (H. = 1.5-2) and has a specific gravity of 3.5; the luster is resinous. Its composition is AsS, or arsenic monosulphide, which gives the percentage com- position: Sulphur 29.9, arsenic 70.1 = 100. ORPIMENT, named from the Latin auripigmentum, or Gold pigment (also called King's yellow), is of a beautiful golden yellow. It is generally found in masses showing a foliated structure and with one perfect cleavage so that it can be split off into thin flexible leaves. Distinct crystals of orpiment are very rare; they belong to the ortho- rhombic system. It is soft (H. = 1.5-2), sectile, and the specific gravity is about 3.5. The composition is As a S 3 , or arsenic trisulphide, which gives: Sulphur 39.0, arsenic 61.0 100. Its behavior in the closed and open tubes is mentioned on p. 150; on charcoal it is all volatilized, giving the characteristic garlic odor of arsenic and white fumes of the oxide (As a 8 ). Arsenic is present in a great many other minerals. It DESCRIPTION OF MINERAL SPECIES. 175 forms with the metals a series of compounds called arsen- ides, of which arsenopyrite and cobaltite are examples. It forms with sulphur a number of compounds of the metals, as proustite. There are also a series of salts called ar senates, one of which is the lead arsenate mimetite; an- other is the cobalt arsenate, erythrite. White arsenic, or the "arsenic of the druggist," is the oxide, As a O s , which occasionally occurs as a mineral (then called ARSENOLITE). It is formed whenever metallic ar- senic or an arsenide is roasted in the air. In the open tube it is often obtained in spangling octahedral crystals (see pp. 17 and 150). ANTIMONY. ANTIMONY, like bismuth, is usually included among the metals, for it has a high metallic luster, although its structure is crystalline and it is quite brittle. It is a very easily fusible metal and is useful in the arts because of the alloys which it forms with lead and tin, to which it imparts greater hardness and durability. Thus type-metal is an alloy of one part of antimony to three or four of lead. Britannia metal, often used as the base of plated silver- ware, is an alloy of antimony with brass, tin, and lead. Babbitt metal, used for bearings, is another alloy of anti- mony with tin and copper. Tartar emetic, used in medi- cine, is tartrate of antimony and potassium. Native Antimony, Sb. NATIVE ANTIMONY is a bright tin-white mineral with metallic luster, and commonly showing brilliant cleavage 176 MINERALS, AND HOW TO STUDY THEM. surfaces; rhombohedral crystals are rare. Its hardness is 3 to 3.5, and the specific gravity 6.7. It is not a common mineral, but is found in New Brunswick in some quantity, in California and elsewhere. Heated on charcoal it fuses and goes off entirely in white fumes of the trioxide, Sb a O,; in the open tube dense white fumes of this oxide are also deposited (see p. 151). Stibnite, or Antimony Glance. Antimony Sulphide, Sb 2 S,. STIBNITE, the sulphide of antimony, is its commonest and most important ore. It is. found in prismatic crystals of the orthorhombic system, often spear-shaped at the ends (Fig. 162). These crystals are frequently acicular and arranged in radiating groups, or again they may be very large; the mines in Japan have afforded specimens magnificent in size and brilliancy of luster. The crystals have very perfect cleavage, parallel to one vertical edge, and the surfaces formed by this are smooth and highly polished. Besides the prismatic crystals, stibnite also occurs in massive forms, generally columnar in structure and then also showing the perfect cleavage ; but also sometimes compact and granular and then the cleavage is not apparent. The hardness is only 2, so that it is scratched by the nail and leaves a mark on paper; it is quite sectile. It is not, however, to be confounded with graphite, which is much more soft and greasy in feel and marks the paper without the slightest tendency to tear it. The specific DESCRIPTION OP MINERAL SPECIES. 177 gravity of stibnite is about 4.6. The luster is metallic and on a fresh surface particularly a cleavage surface it is very brilliant, as already noted. The color is a bluish gray, but less blue than galena, with which it is sometimes confounded (but note the difference in cleavage); the streak is nearly black. Stibnite is the sulphide of antimony (antimony tri- sulphide), Sb,S s ; this gives the percentage composition: Sulphur 28.6, antimony 71.4 = 100. Heated on charcoal it fuses very easily and gives off fumes of the oxide of antimony (Sb 2 3 ), which form a thick coating at a little distance; after a few moments the fragment is entirely volatilized. If the reducing flame is thrown for a moment on the coating, it is burned off with a greenish-blue flame. In the open tube, heated slowly, the same dense deposit or sublimate is formed in the cold portion ; this is powdery and not readily volatile like the somewhat similar white oxide of arsenic. In the closed tube a dark red sublimate of antimony oxysulphide is formed (cf. p. 151). Antimony also enters into a number of other minerals, as pyrargyrite, or dark red ruby-silver; also tetrahedrite, or gray copper, jamesonite, bournonite, etc. These are further mentioned under the metals of which they are compounds; for a description of the other related minerals reference must be made to larger works on mineralogy. BISMUTH. BISMUTH is silver-white in color with a reddish tinge and has a bright metallic luster; it is rather brittle and shows a crystalline structure with perfec 4 cleavages; it is. 178 MINERALS, AND HOW TO STUDY THEM. however, nearer to the true metals than either arsenic or antimony. Native bismuth is a rare mineral, and its com- pounds, chiefly among the sulphides, are also too rare to be particularly mentioned here. The sulphide of bismuth, or bismuthinite, resembles stibnite rather closely in phys- ical characters. Bismuth is an even more fusible metal than antimony, and the alloys which it forms are remarkable for their low melting-points; an alloy of bismuth with lead and tin fuses at a temperature below that of boiling water; an- other alloy of the same metals in different proportions is used as a kind of solder. Some bismuth alloys have the curious property of contracting instead of expanding with heat. Bismuth is also employed in medicine in the form of the subnitrate; another compound is used as a cosmetic; other uses are in calico-printing, to give luster to porcelain, etc. MOLYBDENUM. Molybdenite. Molybdenum sulphide, MoS,. MOLYBDENITE is the sulphide of the rare element mo- lybdenum. It is not a common mineral, but is found in small quantities in a good many localities, chiefly in crys- talline rocks like gneiss. Like graphite, which it much resembles, it occurs in foliated masses or in crystalline plates having a hexagonal outline; rarely in distinct hex- agonal crystals. It is also very soft (H. = 1-1.5) with a soapy feel and leaves a trace on paper. It has a bluish- black color and metallic luster. The color, however, is distinctly bluer and the specific gravity (G. = 4.7) is higher than that of graphite. DESCRIPTION OF MINERAL SPECIES. 179 The composition of Molybdenum disulphide, MoS 9 , gives: Sulphur 40.0, molybdenum 60.0 = 100. Heated in the open tube or on charcoal it gives off strong sulphur fumes and yields a deposit, which is pale yellow or white, of molybdic oxide; this coating on charcoal, if touched with an intermittent blowpipe flame (reducing flame) be- comes a bright blue (see p. 146). Molybdenum also occurs in the salts called molybdates, of which lead molybdate, the mineral wulfenite, is the most common. GOLD. Native Gold, Au. GOLD is the most highly prized of the metals, valued because it serves as the money of all civilized people,* and because of its use for ornaments, as watches, rings, etc. It is sometimes found in isometric crystals, as in octa- hedrons, but usually in plates or scales or wirelike forms; also in larger masses sometimes very large called nug- gets (see Fig. 163). It is soft (H. = 2.5 to 3) and can be cut by the knife. It is highly malleable and ductile and especially remarkable because it can be hammered out into very thin sheets; the skillful gold-beater can make the plates so thin as to transmit a faint greenish light. Gold is very heavy and when pure has a specific gravity a little over 19. The luster is metallic and the color the familiar gold-yellow, but varying with the other metals * The gold coin of the United States and France contains gold and copper in the ratio of 9 to 1; that of England in the ratio of 11 to 1. 180 MINERALS, AND HOW TO STUDY THEM. alloyed with it. The native gold practically always con- tains some silver and often a good deal, and then it has a paler color and lower density; with sixteen per cent of silver the specific gravity is only 17. The gold used for watch-cases and for ornaments, on the other hand, is often alloyed with copper and hence has a reddish color. Gold is not attacked by the ordinary acids, but is dissolved in a mixture of nitric and hydrochloric acids (called aqua regia). Gold occurs mostly in veins in the older crystalline 163. Figure of a model of a large Australian gold nugget weighing 2166 ounces and valued at about 40,000 dollars. rocks, especially associated with quartz; gold quartz is quartz often milky which either shows little particles of gold scattered through it, or from which gold can be obtained even if not visible to the eye after the rock is crushed to powder and then washed to remove the lighter material. A large part of the gold of the world has been obtained from the sands and gravels produced by the disintegration of gold-bearing rocks. These gravels in the bed of a stream may be washed by the miner in his pan; or, on a large scale, where a powerful stream of DESCRIPTION OF MINERAL SPECIES. 181 water is thrown against the gravel bank, carrying away the lighter rock and leaving the heavy gold particles be- hind, usually in the form of little flattened scales. The finest particles preserved are called " gold-dust." The chief gold-producing countries at the present time are the United States, especially in the State of California, where gold was discovered in 1848; in Australia, Eussia, South Africa, where recent discoveries have proved to be very important. Gold is also produced in South America, China, British India, Canada; to a limited extent in Ger- many and Austria-Hungary and some other countries. It is remarkable that almost all the gold of the world and an amount valued at about $180,000,000 was mined in 1894 is obtained from the native metal; for minerals containing gold are very rare. The only ones known, be- sides the auriferous pyrite, arseuopyrite, etc., are a few compounds with tellurium called tellurides. The best known of these gold tellurides is SYLVANITE, a silver-white mineral with brilliant metallic luster, soft (H. = 1.5-2) and heavy (G. = 8.0). It was long since found in Transylvania (whence it takes its name), but also occurs in Colorado. Another name for it is Graphic Tel- lurium, because of the curious forms, resembling written characters, that the crystals sometimes take on a rock surface. PLATINUM. Native Platinum, Pt. PLATINUM is reckoned among the nobler metals with gold, and like it is not attacked by any of the single acids. 182 MINERALS, AND HOW TO STUDY THEM. It has a rather dull gray color, and is not a beautiful metal, although now more highly valued because of its practical uses than any of the metals except gold. It is rarely found in isometric crystals, as in cubes, more commonly in scales or in larger masses (up to twenty pounds) called nuggets, washed out of the gold sand. It has a hardness of 4 to 4.5, and a specific gravity varying from 14 to 19 according to the amount of other metals alloyed with it chemically. Pure platinum, as obtained in the laboratory, has a specific gravity of 21 to 22, for native platinum is not the pure metal, but is found by the chemist to contain iron, sometimes in large amount (nearly 20 per cent), and also a number of rare metals, as palladium, rhodium, and others. Platinum is a highly useful metal. The fact that it is fused with great difficulty and is not attacked by ordinary chemical reagents makes it very valuable- both to the chemist in the laboratory and in the chemical manufac- tories, where crucibles and dishes are made of it. It is also largely used by dentists. It has come into use of recent years for the attachments to the ends of the carbon wire in the incandescent electric lamp. Only a very minute quantity is required in each case, but so many lamps are called for that the demand is very great, and as only a small amount is mined chiefly in the Ural Mountains in Russia the price, has risen much higher than formerly. Platinum has been used to a small extent for coins. Between the years 1828 and 1845 in Russia a considerable amount was in circulation, but the coins were recalled and the experiment has not been repeated. DESCRIPTION OF MINERAL SPECIES. 183 Platinum, like gold, does not readily combine with other elements, and in nature the only compound known is an arsenide (PtAsJ, called SPERRYLITE; this is found in very small quantities in a mine near Sudbury, Ontario, Canada. It is interesting to note that the name platinum is derived from plata, the Spanish word for silver, since it was re- garded in South America at the time of its discovery (1735) as an impure ore of that metal. IRIDOSMINE is a compound of the rare metals iridium and osmium resembling platinum, but of a whiter color. It is found under similar conditions in the form of flattened scales in gold-washings. It is very hard, and on this account has been used for the points of gold pens. SILVER. SILVER is one of the precious metals, useful alike as money,* for ornaments of many kinds, and for utensils. The color is a fine silver-white when perfectly fresh, but unfortunately it is very easily tarnished, and the presence of a very little sulphur or sulphur gases in the atmosphere soon turns it black. Native Silver, Ag. NATIVE SILVER is not an uncommon mineral, although the world's supply of the metal comes chiefly from its ores. It is like gold in its occurrence, sometimes, though rarely, in distinct isometric crystals, more frequently in arbores- * The silver coin of the United States and France contains silver and copper in the ratio of 9 to 1 ; that of Eugland in the ratio of 12$ tol. 184 MINERALS, AND HOW TO STUDY THEM. cent or branching groups, in plates and scales or wirelike forms (Fig, 164); sometimes in fine threads. Its hardness is 2.5 to 3; it is highly malle- able and ductile, and is the best known con- ductor for both heat and electricity. Its specific gravity is 10.6 when pure, but higher when alloyed with gold, as often in nature. Native silver occurs rather abundantly in nature, as in Mexico, Arizona, Norway, also in South America and Australia. Silver is readily dissolved by nitric acid,, forming silver nitrate, and from its solution the addition of any compound containing chlorine, as hy- drochloric acid or sodium chloride, causes part to separate as a white curdy deposit of silver chloride. This is a very delicate test for silver. Argentite, or Silver Glance. Silver sulphide, Ag a S. Argentite is named from the Latin word of silver, argentnm. It is a very valuable though not very common ore, since when pure it contains 87 per cent of metallic silver. It is found in .cubic or octahedral crystals, often growing together in branching forms; more commonly it occurs simply in masses. The hardness is about 2, and the specific gravity 7.3. It is readily cut with the knife, almost like lead, and hence is said to be eminently sectile, also flattening to some extent under the hammer, while almost all other sulphides are brittle and break at once with a blow into fragments. The luster is metallic, and the color and streak grayish black. DESCRIPTION OF MINERAL SPECIES. 185 The formula is Ag,S, or silver sulphide, which gives: Sulphur 12.9, silver 87.1 = 100. Heated by the blowpipe flame on charcoal, the sulphur is easily roasted off and a little silver ball left behind, which can be tested chemically by dissolving in nitric acid, and adding a drop of hydro- chloric acid, as before mentioned. There are a number of other sulphur compounds of silver, but most of them are too rare to be mentioned fully here. The most interesting of these are the two beautiful minerals called red-silver ore or ruby-silver, that is, the dark red-silver ore, PYRARGYRITE, which contains sulphur, antimony, and silver, am" the light red-silver ore, PROUS- TT~E, which contains sulphur, arsenic, and silver. Both these minerals crystallize in hexagonal prisms with rhombohedral or scalenohedral faces, and they resemble each other closely in their characters, as hardness 2.5, specific gravity 5.8 pyrargyrite and 5.6 proustite. The color of pyrargyrite is dark red, often black, with nearly metallic luster on the surface, while proustite is bright red. Both have a red streak. Heated on charcoal, pyrargyrite gives off dense antimony fumes (Sb 2 3 ), while proustite yields arsenical fumes (As 2 3 ) easily recognized by their garlic odor. Both min- erals give a globule of silver if roasted with soda on charcoal (see p. 144). Cerargyrite, or Horn-silver. Silver chloride, AgCl. The name CERARGYRITE, translated into English, means horn-silver, and it is so called because of its appearance and the ease with which it is cut by a knife. 186 MINERALS, AND HOW TO STUDY THEM. It is found in cubic crystals rarely, more commonly in scales, plates or masses. The hardness is 1 to 1.5, and the specific gravity 5.5. It is remarkable for being perfectly sectile, cutting with a knife like a piece of lead or wax. The luster is adamantine and the color white or pale gray or green; it is transparent to trans- lucent. It is a rather rare but highly valuable silver ore, the per- centage composition being: Chlorine 24.7, silver 75.3 = 100. Eoasted alone on charcoal the chlorine is easily driven off and a globule of silver left behind. MERCURY. MERCURY is a remarkable metal, because it is a liquid at all ordinary temperatures, only freezing, or becoming solid, at 40. It has a silver-white color and brilliant metallic luster, and is so mobile that from early times it has been called quicksilver. Its density is high, 13.6, or higher than silver (10.6) and lead (11.4), and for this reason and because of its liquid form it is of great value for scientific purposes. It is used in most thermometers and barome- ters and is employed in many experiments in the physical and chemical laboratories. It also has the property of forming a pasty mass or amalgam with some of the other metals, as gold and silver (also copper, zinc, tin, etc., but not iron), and is hence of great value in separating them from the rock in which they occur. For this purpose the rock is ground into powder, the greater part of the loose material washed off, and then the remainder is agitated with mercury. The amalgam, which forms, is collected, DESCRIPTION OF MINERAL SPECIES. 187 and by heat the mercury is driven off to be collected again in cool chambers for further use, and the gold and silver are left behind. Ordinary mirrors are made of glass backed with an amalgam of mercury and tin. The sul- phide of mercury is the valuable pigment called vermilion. Mercury in various forms is also used in medicine, but in minute doses, for it is an active poison. Corrosive sub- limate is a chloride of mercury. NATIVE MERCURY is a rare mineral in nature, though occasionally found in minute globules scattered through the rock; the common ore is cinnabar. NATIVE AMALGAM is a rather rare mineral containing mercury and silver, but in very varying amounts. Cinnabar. Mercury sulphide, HgS. CINNABAR, the sulphide of mercury, and sometimes called natural vermilion, is found in masses of a fine red color, and sometimes also in small rhombohedral or pris- matic crystals. The hardness is 2 to 2.5, and the specific gravity is about 8, or above that of metallic iron (7.8). The great weight of a specimen cannot escape the observer and is a striking character; in some cases, however, if the cinnabar is not a pure solid mass, but only scattered through a light clayey gangue, the density of the whole may be much lower than 8. The luster is adamantine and the color bright cochineal- red, sometimes becoming dull and dark; the streak is scarlet; crystals are usually perfectly transparent. The formula for mercury sulphide, HgS, gives the per- centage composition: Sulphur 13.8, mercury 86.2 = 100. 188 MINERALS, AND HOW TO STUDY THEM. If heated on charcoal, a piece of pure cinnabar is volatilized entirely; if anything is left behind, it is only the gangue. In the closed tube it is also sublimed entire, but here it collects again in the cold part of the tube above as a black ring of sulphide of mercury, which has the same compo- sition as the original mineral, for the chemist knows both a black and a red sulphide. In the open tube, if heated very slowly, so as to avoid forming a black ring in other words, so as to give the sulphur time to oxidize (go off as S0 a ) a ring of metallic mercury is formed in the cold part of the tube (see also p. 148). Cinnabar is mined at Almaden in Spain, Idria in Car- niola, also at New Almadeu and other points in California, and less abundantly elsewhere. COPPER. COPPER is one of the most useful of the metals, having been employed for utensils and in other forms, both as a metal and in different alloys, since very early times. Of recent years its use has been increased very largely be- cause of its good conductivity for electricity. It thus forms the material of the wires of the dynamo machines, those by which the electrical current is carried for the elec- tric light, the trolley, etc. Copper is also extensively used for electroplating, as in making stereotype plates. It forms further a large number of useful alloys, of which brass an alloy of copper and zinc in the ratio of about 2 : 1 is the best known. In the various kinds of bronze (bell-metal, gun-metal, antique and medal bronze, etc.) copper is also the prominent metal, alloyed with tin; in DESCRIPTION OF MINERAL SPECIES. 189 aluminium bronze it is alloyed with aluminium; in german silver it is alloyed with zinc and nickel. Copper is obtained in nature in the native state, and also from a variety of valuable ores, which are some of the most interesting and beautiful minerals. Native Copper, Cu. NATIVE COPPER is found sometimes in isometric crys- tals, but they are not often distinct, and the common rorms are strings or wires which have a crystalline form but are difficult to decipher (see Figs. 127, 128, p. 61). It is also in grains, plates, and masses, sometimes very large. The hardness is 2.5 to 3, and the specific gravity 8.8. The luster is metallic and the color that peculiar reddish hue called copper-red. It is highly malleable and ductile, so that it may both be rolled out into sheets and drawn into fine wires. It is an excellent conductor of both heat and electricity. Copper is easily dissolved by nitric acid, giving a blue solution, and ammonia in excess (enough to dissolve the precipitate first formed) turns it a deep azure- blae. The most celebrated locality for native copper is in the upper peninsula of Michigan on the shores of Lake Su- perior, where it has been mined for many years. The total production has been very large. Beautiful crys- tallized specimens have been found here where it is asso- ciated with calcite, datolite, and a number of the zeolites. Sometimes it is inclosed in the crystals, as of calcite, so that they are colored bright red from the internal reflec- 190 MINERALS, AND HOW TO STUDY THEM. tions. Great masses of native copper hare also been found; one of them weighed 420 tons. Native copper is further found in Arizona, in Siberia, South America, and Australia. Chalcocite, or Copper Glance. Cuprous sulphide, Cu.S. CHALCOCITE is one of the most valuable ores of copper, for when pure it contains about 80 per cent of the metal. It is found in orthorhombic prisms or pyramids, occasion- ally having a hexagonal aspect; more commonly in mas- sive forms of a nearly black or bluish-black color. When fresh it has a brilliant metallic luster, which it loses easily, becoming a little dull and tarnished on the surface. The hardness is 2.5 to 3, and the specific gravity about 5.6. It is brittle when struck with the hammer, but can be cut a little with the knife. The formula for chalcocite (cuprous sulphide), Cu a S, gives the composition : Sulphur 20.2, copper 79.8 = 100. On charcoal it is easily reduced by the blowpipe flame alone to metallic copper. Fine specimens come from Cornwall in England (often called redruthite) ; it was also formerly obtained at Bristol, Conn. Bornite, or Erubescite. Sulphide of Copper and Iron, Cu 3 FeS 8 . BORNITE was named after the Austrian mineralogist von Born, but it has a variety of other names purple cop- per ore, variegated copper ore, peacock copper, erubescite all of which suggest a character by which it is easily recognized: the bright iridescent tarnish of the surface. DESCRIPTION OF MINERAL SPECIES. 191 A fresh fracture gives a color of a peculiar reddish bronze and a bright metallic luster, which has led the Cornish miners, a little fancifully, to call it horse-flesh ore. This fresh surface soon becomes slightly colored even after a day or two, and gradually the color changes and becomes more variegated, until it is indeed a peacock-copper ore. This character, with the peculiar color of the fresh frac- ture, makes it always easy to recognize. It is sometimes found in cubic crystals, but usually it is simply massive as imbedded particles or larger pieces. The hardness is 3, and the specific gravity about 5. Bornite contains both copper and iron, but not always in the same proportions; the formula Cu 3 FeS s gives: Sulphur 28.1, copper 55.5, iron 16.4 = 100. When heated in the open tube it gives off fumes of sulphur dioxide, which are recognized by the odor and their effect in red- dening litmus-paper. On charcoal it fuses to a brittle magnetic globule; after roasting it reacts with borax for iron and copper. It dissolves in nitric acid with separation of sulphur, giving a blue solution. Chalcopyrite, or Copper Pyrites. Sulphide of Copper and Iron, CuFeS,. CHALCOPYRITE, or Copper Pyrites, is the beautiful deep brass-yellow copper mineral, often called yellow copper ore. The color is so golden that it is not infrequently mistaken for gold, especially when scattered in small par- ticles through a mass of quartz; but, as we shall see, it can be easily distinguished, though the name "fool's gold/' 192 MINERALS, AND HOW TO STUDY THEM. which it shares with the less golden iron pyrites, is still not inappropriate. It is generally found massive, sometimes in large speci- mens, sometimes only in specks in the inclosing rock, but 165 it is also found in crystals which com- monly are either like octahedrons (though belonging to the tetragonal system), or in wedge-shaped forms called sphenoids (Fig. 165). The hardness of chalcopyrite is 3.5 to 4, so that, unlike pyrite, it can be easily scratched with a knife. It is brittle, and its specific gravity is a little over 4. The luster is brilliant metallic, and the color, as we have seen, deep brass-yellow; the streak is greenish black. It is often tarnished on the surface, sometimes so as to deepen the color, sometimes variegated so as to rival bor- nite, with which it might then be confounded, only that the breaking off of a scale so as to show the color on the fresh fracture serves to distinguish them at once. It is a sulphide of both copper and iron, and the formula CuFeS, gives: Sulphur 35.0, copper 34.5, iron 30.5 = 100. Heated on charcoal, a fragment fuses to a black ball which is strongly magnetic, and this roasted with soda gives metallic copper. A fragment in nitric acid dissolves, giv- ing a blue solution which turns azure-blue when ammonia is added in excess. Chalcopyrite can be easily distinguished from pyrite (iron pyrites) because of its inferior hardness, as noted be- fore; its color, too, is deeper. It is distinguished from gold by its being brittle, breaking into fragments under DESCRIPTION OF MINERAL SPECIES. 193 the point of the knife, while the gold is cut; a particle of gold, too, is not attacked by nitric acid, while the chalco- pyrite is easily dissolved with the separation of particles of sulphur. It is a very common mineral, often occurring in /eins of quartz with galena and pyrite, though sometimes cnly as minute specks. When present in large masses, as iu Montana, it is one of the valuable ores of copper. Tetrahedrite, or Gray Copper. Sulphide of Antimony and Copper, 4Cu a S.Sb a S s . TETRAHEDRITE is so named because the crystals are Commonly tetrahedral in habit (Figs. 166, 167), and often highly modified. Good crystals, as with many of these 166. ' 167. metallic minerals, are rare and the mineralogist has often to content himself with massive pieces. These he recog- nizes by the brilliant metallic luster and dark grayish-black color and streak. The hardness is 3-4.5, so that it is easily distinguished from magnetite, which is too hard to be scratched by the knife; the specific gravity varies from 4.4 to 5.1. The ordinary tetrahedrite contains sulphur, antimony, and copper, but there are a great many varieties, some of which contain arsenic in place of part of the antimony, and others silver or mercury in place of part of the copper. 194 MINERALS, AND HOW TO STUDY THEM. The typical composition is given by the formula Cu g Sb.,S, or 4Cu,S.Sb,S s ; this requires: Sulphur 23.1, antimony 24.8, copper 52.1. There is also a related mineral, containing sulphur, arsenic, and copper, which is called tennantite. In the closed tube tetrahedrite gives a dark red subli- mate of antimony oxysulphide; in the open tube sulphurous fumes and a white coating of antimony trioxide. If arsenic is present, it is detected by the odor when the mineral is heated on charcoal; with soda it yields a globule of metallic copper. Cornwall, Bohemia, Hungary, also Col- orado, afford fine specimens. Cuprite, or Eed Copper Ore. Cuprous oxide, Cu,0. CUPEITE is called red copper because of the fine red color which the clear crystals show, and because of the red color of the streak. The crystals (Figs. 168-170) are often 168. 169. 170. cubes or octahedrons or combinations of them and other forms; sometimes they are highly modified (see Fig. 29, p. 28). In one kind the cubes are spun out into long threads, forming a matted mass of bright red hairs which look very pretty in the cavities of the rock; examined closely with a glass, it is often seen that these threads cross each other at right angles as if trying to build up skeleton cubes, the threads taking the direction of the cubic edges. Commor DESCRIPTION OF MINERAL SPECIES. 195 cuprite is a massive mineral, and it is in its cavities that the crystals are usually found. The hardness of cuprite is 3.5 to 4, and the specific gravity about 6. The luster is adamantine, but on some dark surfaces may look almost metallic; again it is dull and earthy. The color, as remarked above, is bright cochi- neal-red in the clear transparent crystals, but the surface is often darkened and may appear nearly black. The streak is always brownish red. The composition of cuprous oxide, Ci^O, gives: Oxygen 11.2, copper 88.8 = 100. A fragment on charcoal is easily robbed of its oxygen and reduced to metallic copper. Cuprite is not only a beautiful mineral, but also a valuable ore of copper, occurring usually with malachite and othef ores, as in Arizona, Cornwall in England, in Australia, etc. Malachite. Green Carbonate of Copper, CuCO,. Cu(OH),. MALACHITE, the carbonate of copper, is a bright green mineral, often found with native copper, cuprite, and othef copper ores because of the readiness with which they are converted into the carbonate by the action of the carbon dioxide present in the air or dissolved in the water. It may be found in acicular crystals (monoclinic), but onlj rarely, and the common forms have a rounded or mammil- lary surface and a concentric fibrous structure (see Figs. 135, 136, p. 68). When close and compact it can be cut and polished and thus form a handsome ornamental stone The malachite of Siberia is used in this way, table-tops vases, and columns having often been veneered with it. 196 MINERALS, AND HOW TO STUDY THEM. The hardness is 3.5 to 4, and the specific gravity about 4. The color is a bright green; the streak a little paler; it is transparent only in minute crystals. The formula of malachite is CuC0 8 .Cu(OH), or 2CuO, CO..H.O, which gives: Carbon dioxide (CO,) 19.9, cuprio oxide (CuO) 71.9, water (H a O) 8.2 = 100. A fragment heated in the forceps gives a green flame characteristic of copper, and in the borax bead the reactions described on pp. 136, 137. It yields a good deal of water in the closed tube, and in nitric acid dissolves with the effervescence of carbon dioxide. Malachite is found in fine specimens at many localities, as in the Siberian mines, in Cornwall, Australia, and Arizona. Azurite, Blue Carbonate of Copper. 2CuC0 3 .Cu(OH) 3 . AZURITE, or the Blue Carbonate of Copper, is not so common as malachite, but it is also a beautiful mineral, and when in large transparent crystals of a fine deep blue it forms one of the most attractive specimens in a cabinet. The crystals are oblique rhombic prisms. The hardness is 3.5 to 4, and the specific gravity is 3.8. The luster is vitreous, the color azure-blue, and the streak somewhat lighter. The composition is expressed by the formula 2CuC0 3 . Cu(OH) a or SCuO.gCOj.H.O; this gives: Carbon dioxide (C0 a ) 25.6, cupric oxide (CuO) 69.2, water (H 3 0) 5.2 = 100. It hence differs from malachite in containing less water; it is not uncommon to find crystals which are blue on the outside but have changed within to a fibrous mass of green malachite. The most famous localities are those DESCRIPTION OF MINERAL SFtiCIES. 197 of Chessy, near Lyons in France, and the Copper Queen mines and elsewhere in Arizona. DIOPTASE, or Emerald Copper, the silicate of copper, has a beautiful emerald-green color. It is a rare mineral, only known to occur at a few localities, one of which is in Asia, another in Arizona, another in the French Congo re- gion in Africa. The crystals are commonly hexagonal prisms with rhombohedral faces on the ends. The formula is H.O.CuO.SiO,, which gives : Silica 38.2, cupric oxide 50.4, water 11.4 = 100. CHRYSOCOLLA is another silicate of copper of a bluish- green or sky-blue color. It occurs in massive forms some- times earthy, also looking a little like malachite. The hard- ness is 2 to 4; the specific gravity 2.2. It contains a good deal of water, which it gives off in the closed tube. The formula is CuSi0 3 .2H a O. It is a not uncommon product of the alteration of other copper minerals. There are many more copper minerals, most of them too rare to be described here. They include a number of hydrous sulphates, of which the most important is CHAL- CANTHITE or blue vitriol, a common substance at the druggist's and often to be seen in clusters of large crystals, but rare in nature. The sulphate, BROCHANTITE, may also be mentioned. There are further several arsenates of copper, including OLIVENITE ; several phosphates, includ- ing LIBETHENITE; several chlorides, as ATACAMITE; and so on. LEAD. LEAD is one of the most important of the metals, used for many purposes familiar to all, as for pipes, to convey water, 198 MINERALS, AND HOW TO STUDY THEM. for shot and rifle-balls, etc. It has a dull blue-gray color. It is very soft and malleable, and fuses readily at a com- paratively low temperature (see p. 132.) It is often alloyed with other metals; thus with tin in common solder and pewter; with antimony in type-metal ; with arsenic in small amount for making shot. White lead (the carbonate) :s largely used in making paint, also the oxide, red lead. NATIVE LEAD is a very rare mineral, though occasion- ally found in small amount, particularly in Sweden. The supply of the metal, which is used so largely in the arts, is obtained from its ores, especially the culphide, galena. Other important lead minerals are the phosphate, pyro- morphite; the sulphate, anglesite; the carbonate, cerussite. Galena. Lead sulphide, PbS. GALENA crystallizes in the isometric system, and occurs commonly in cubes ; it is also found in octahedrons and very frequently in combinations of these two forms as, too, of other forms of this system (Figs. 171-173, also Fig. 20, 171. 172. 173. p. 25). It has very perfect cubic cleavage, and a mass often breaks up into a multitude of little rectangular blocks (see Fig. 143, p. 71). This cubic cleavage is readily seen in the common coarse-granular kinds, and is revealed also by the spangling of the surface in those which are fine-granular. DESCRIPTION OF MINERAL SPECIES. M* The hardness is 2.5, and the specific gravity is 7.5 or nearly as high as metallic iron, for lead being a metal of high density (G. = 11.4), all its compounds have the same prop- erty. The luster is metallic and usually very brilliant; the color is a bluish lead-gray, but the exposed surface of a specimen is often somewhat dull from tarnish. Galena is lead sulphide, PbS, which gives the percentage composition: Sulphur 13.4, lead 86.6 = 100. On charcoal a fragment fuses easily, yielding finally a globule of metallic lead and a yellow coating of lead oxide near it and a white coating at a distance from it (see p. 144). With soda on charcoal metallic lead is readily obtained. Galena is the most important ore of lead and one of the commonest of minerals, occurring in large deposits in many mining regions; for example, in Missouri, Illinois, Iowa, and Wisconsin; also in Colorado and abroad in Derbyshire, England; Freiberg, the Harz, and soon. It also occurs, but less abundantly, with the ores of other metals. Spha^ lerite, calamine, smithsonite, also pyrite and chalcopyrite are common accompanying metallic minerals; quartz, cal- cite, barite, also fluorite, are common non-metallic minerals associated with it and then called the gangue. As resulting from its own decomposition, lead carbonate (cerussite) and also lead sulphate (anglesite) are often found with galena; less often pyromorphite and other lead minerals. Much galena carries a small amount of silver, and when this is sufficient in quantity to justify its being worked for the precious metal, it is regarded as a silver ore and called argentiferous galena. 200 MINERALS, AND HOW TO STUDY THEM. Galena is used not only as a source of lead or sometimes of silver, but also for glazing common stoneware; it is hence called potter's ore. JAMESONITE is a rare sulphide of lead and antimony (2PbS.Sb a S s ) occurring in acicular crystals, also in fibrous or compact masses. Hardness 2 to 3; specific gravity 5.5 to 6. The luster is metallic and the color is steel-gray to dark lead -gray ; it resembles stibnite both in form and color. BOURNONITE is another Bather rare sulphide of lead and antimony containing also copper (3(Pb,Cu)S.Sb 2 S g ). It occurs in short prismatic or tabular crystals, often grouped in wheel-shaped forms ; also massive and compact. Hardness 2.5 to 3 ; specific gravity 5.7 to 5.9 ; luster metallic ; color dark steel-gray, inclining to iron-black. Pyromorphite. Lead phosphate, 3Pb 3 P 3 8 .PbCl 4 . PYROMORPHITE is found in small hexagonal prisms which are sometimes cavernous in form, also often rounded into barrel-shaped forms, or even nearly spheri- cal. The crystals are frequently clustered together in a curious way, branching out from a slender stem, as shown in Fig. 174. It also occurs as a thin crust or coating, which may be drusy on the surface, or simply globular or mammillary. Pyromorphite has a hardness of 3.5 to 4, and, like all compounds of lead, a high specific gravity, viz., 6.5 to 7. The luster is resinous and the color is commonly green, varying from grass-green to both darker and lighter shades; it is also sometimes pale brown. The DESCRIPTION OF MINERAL SPECIES. 201 streak is not far from white even in the deep green varieties. It consists essentially of phosphate of lead, Pb,(P0 4 ) s , but contains also some chlorine. Hence when heated in the tube a little lead chloride is driven off and forms a white coating above. The same white coating is also de- posited on charcoal at a distance from the fragment which is being heated ; more conspicuous than this is the yellow coating of lead oxide (PbO) which is formed just about the fused fragment. Also if the fragment, after it is com- pletely fused, is examined, it will be seen that it is nearly spherical, has a brilliant luster, and sparkles on the surface from the reflection of light from a multitude of crystalline facets; it is this that gives the name to the mineral from the Greek words meaning fire and form (nvp and fiiop and this resemblance has given it the name chias- tolite. The hardness is 7.5, and the specific gravity 3.2. The 324 MINERALS, AND HOW TO STUDY THEM. luster is vitreous, and the color varies from white or gray to pink, brown, or green. The chemical formula is Al 2 Si0 6 or Al 2 3 .SiO.,, and the percentage composition is: Silica 36.8, alumina 63.2 = 100. It is infusible before the blowpipe, and a fragment becomes blue when moistened with cobalt solution and ignited. Andalusite is not uncommon in crystalline or partly crys- talline schists, as those of New England ; sillimanite is a frequent associate. In the White Mountain region in New Hampshire, as on Mount "Washington, andalusite is con- spicuous in rough crystalline forms. Cyanite and Sillimanite are two other minerals having the same composition as andalusite. CYANITE, named from the Greek for blue (xvavos), because of its characteristic color, is usually found in thin- bladed crystals, showing a fine blue sometimes over the whole, sometimes as a central strip between paler or even colorless sides. There are also gray and green varieties and those which are columnar to fibrous. The hardness is only 5 on the flat side of the blades, but on the edges a little over 7; specific gravity 3.6; luster vitreous to pearly. Fine specimens come from North Carolina. SILLIMANITE occurs rarely in prismatic crystals; usually it is in fibrous or columnar forms, and then the crystals are not distinct. There is a very perfect cleavage in a direction parallel to the length of the prisms. The hard- ness is 6 to 7, and the specific gravity 3.2; luster vitreous or subadamantine; color pale brown to gray and green. It is found in crystalline schists, often with staurolite. PYROPHYLLITE is a silicate of aluminium containing DESCRIPTION OF MINERAL SPECIES. 325 water (H a O.Al 2 O s .4SiO a ). It often occurs in foliated masses with radiated lamellar structure; also granular to compact (pencil-stone). Hardness 1 to 2; specific gravity 2.8 to 2.9; luster pearly; color white to green or yellow. In the for- ceps all but the compact varieties exfoliate, but fuse only on the edges with difficulty and give a blue (alumina) with cobalt solution. AXINITE is a rather rare silicate of aluminium, calcium, iron and manganese, containing also boron. It occurs in triclinic crystals, often with a sharp edge (hence named from a^ivrj, an ax) ; one form is figured on page 46, Fig. 98. Hardness 6.5 to 7; specific gravity 3.27; luster vitre- ous; color clove-brown to yellow and gray, also blue. It fuses easily and gives a green flame in the forceps, due to the boron. Staurolite. STAUROLITE, or Cross-stone,* is remarkable for the vari- ety of its compound or twin crystals. The simple form is a rhombic prism with the angles of 129 20' and 50 40', and with these a dome, d, often occurs, making an angle of 124 44' with the top plane. This is not rare, but it is more common to find two crystals crossing each other, sometimes at right angles (Fig. 308, also Fig. 119, p. 58), sometimes at an angle of nearly 60, as shown in Fig. 309; more complex twins, of the same interpeuetration type, also occur where three or even four crystals are grouped together. The hardness is 7 to 7.5, and the specific gravity about * The name is derived from the Greek words oravpoS, a cross, aud Ar&oS, a stone. 326 MINERALS, AND HOW TO STUDY THEM. 3.7; the fracture is subconchoidal, and cleavage parallel to the side plane is sometimes noted. The luster is vitreous, and the color reddish, yellowish brown, brownish black, or gray. Staurolite is a silicate of aluminium, iron, and magnesium, but its formula is quite complex. It occurs especially with garnet, tourmaline, cyanite, or sillimanite, in mica schist and gneiss; many localities are known in New Eng- land, very perfect crystals are found in Fanniii County, Georgia, and in North Carolina. CHONDRODITE is a silicate of magnesium and iron con- taining fluorine. It occurs in yellow grains imbedded in crystalline limestone, also in deep red crystals associated, for example, with magnetite, as at Brewster, N. Y. Hard- ness 6 to 6.5; specific gravity 3.1 to 3.2. The mineral HUMITE, occurring in honey-yellow crystals at Vesuvius (Mt. Somma), is closely related to chondrodite. Talc. Magnesium silicate, TALC is remarkable among minerals because of its soft- ness; on this account it is placed at the beginning of the scale of hardness. It is easily scratched by the nail and has a soapy, unctuous feel. The common form of talc is that in plates or leaves, foliated it is called, which separate DESCRIPTION OF MINERAL SPECIES. 327 easily because of the perfect cleavage; these leaves are not elastic like those of mica, but flexible. There are also kinds of talc which are compact and show but little of the foliated character; this is especially true of the kind called steatite or soapstone, which is sawed into slabs and used for hearths and furnaces, also, when pulverized, as a lubricator. The Chinese make im- ages and other small articles out of a fine-grained com- pact kind, and a similar kind is used for slate-pencils. Another variety, derived, however, from the alteration of a different mineral (Enstatite, p. 295), is fibrous, a little like asbestus, and is used, when ground up, for giving a silky gloss to wall-paper and to mix with wood-pulp in making paper; it is obtained from Edwards, N. Y. The hardness of talc is 1, as stated above, and its specific gravity is 2.8. The luster is pearly, especially in the foli- ated kinds, and the color in the finest of these a beautiful sea-green; there are also white foliated kinds, and the massive varieties may be dark gray. Talc is a silicate of magnesium, with the formula H 4 0.3Mg0.4SiO. The percentage composition is: Silica (SiOJ 63.5, magnesia (MgO) 31.7, water (11,0) 4.8 = 100. Upon being heated quite hot in a closed tube it gives off a small amount of water. It is a common mineral, often associated with serpentine and chlorite rocks; many locali- ties are known in the eastern United States and Canada. Serpentine. Magnesium silicate, H 4 Mg,Si,0, . SERPENTINE is a remarkable mineral because of the variety of massive forms it assumes, although it is not 328 MINERALS, AND HOW TO STUDY THEM. known to occur in crystals of its own. The crystals of serpentine which are found are what are called pseudo- morphs (p. 55), having been derived from some other species by chemical change. Thus the magnesium-iron silicate chrysolite a mineral found especially in basaltic rocks is often changed to serpentine, and then the ser- pentine is said to be a pseudomorph after the chrysolite, since it retains its form. The translucent masses of serpentine of a deep oil- green color are called precious serpentine. A clouded or mottled variety, either green or red, is called ser- pentine marble or verd - antique marble or ophiolite; this last name, like that of the species, refers to the serpent-like markings so commonly observed. This kind is used as a building stone in Philadelphia and Bal- timore. Other kinds of serpentine are foliated or lamellar and separable into oil). brittle leaves. The most pe- culiar variety is the fine fibrous kind called chrysotile (not to be confounded with chrysolite). This usually occurs as thin seams in the massive mineral (Fig. 310). Chrysotile may be separated into fibers, very flex- ible and as soft as the finest silk. This variety is popu- larly called asbestus, but it must be remembered that there is another kind of asbestus of rather similar appear- ance which is a variety of the mineral amphibole (p. 297). The serpentine asbestus is extensively mined in the prov- DESCRIPTION OF MINERAL SPECIES. 329 ince of Quebec, as at Thetford, where it occurs in seams sometimes 3 or 4 inches in thickness. It is ground up and used (since it is a non-conductor of heat) for packing steam-pipes, etc.; also as asbestus roofing; as a lubricator; and so on. The hardness of serpentine is usually from 2.5 to 3; it is hence easily scratched and often has a smooth feel, sometimes almost greasy; some varieties are harder than 3, up to 4 or even 5.5. The specific gravity is about 2.50. The luster is usually feeble and greasy or waxlike; the color, as stated, some shade of green to gray or nearly white. The composition of serpentine is given by the formula 3Mg0.2SiO a .2H 9 0, which requires: Silica (SiOJ 44.1, magnesia (MgO) 43.0, water (H a O) 12.9 = 100; iron is often present in small amount. Heated in the closed tube it yields considerable water, but it fuses in the forceps only on the edges. Besides the two minerals talc and serpentine there is also another important magnesium silicate, the mineral SEPIOLITE, better known as meerschaum (from the Ger- man). This is a soft white mineral, so light, because of its loose texture, as to nearly float on water when quite dry, and this is what gives it its familiar name, which means sea- foam. It is much used for the bowls of tobacco-pipes, and for this purpose is mined in Asia Minor. Datolite. DATOLITE is found in clear glassy crystals having usu- ally a faint green tinge. The crystals are monoclinic in 330 MINERALS, AND HOW TO STUDY THEM, crystallization, and are complex and difficult to decipher even for one who has had a good deal of experience (Fig. 311, also Fig. 97, p. 45). It is noticed at once, however, on examining the surface spangled with crystals that there is little apparent uniformity in the shape of the faces, which is in agreement with their monoclinic char- acter. There is also a kind of datolite occur- ring in forms resembling a pinkish porcelain, but it is only known from the Lake Superior copper-mining region; also another kind in botryoidal masses; but both of these are rare. The hardness is 5 to 5.5, and the specific gravity about 3. The luster is vitreous, and the color white to greenish or pale reddish. Datolite is a silicate of boron and calcium. It yields a little water when heated in the closed tube, and gives a green flame in the forceps (boron), at the same time fusing It occurs chiefly in the kind of eruptive rock found in New Jersey, Connecticut, and Massachusetts, and is often associated with the various minerals belonging to the Zeo- lite family. Like them it is a secondary mineral, that is, formed after the rock which encloses it, and usually out of the chemical material the rock affords by partial decom- position. DESCRIPTION OF MINERAL SPECIES. 331 Prehnite. PREHNITE is another mineral occurring like datolite under the same conditions as the zeolites and associated with them. It is seldom in distinct crystals, usually in crystalline masses with a botryoidal or mammillary surface (Fig. 138, p. 68), or in groups of tabular crystals showing a series of little ridges in parallel position. The hardness is 6 to 6.5, and the specific gravity about 2.9. The luster is vitreous, and the common color green, but it is sometimes white or gray. Prehnite is a silicate of alumina and lime yielding a little water when heated in the closed tube. It fuses in the forceps rather easily, and is slowly decomposed by hydrochloric acid. Apophyllite. APOPHYLLITE stands still closer to the Zeolites than either of the two preceding minerals; it is not only re- lated to them in the way it occurs and in association, but also in its chemical composition in fact it is sometimes called a zeolite. It occurs in square prismatic or pyramidal crystals, but of a considerable variety in habit. The crystals may be simple square prisms terminated by the basal plane with its characteristic pearly luster, and then often looking like a cube, or the form may resemble a cube whose angles have been replaced, like Fig. 313 (compare Fig. 107, p. 50). Here it is to be noted that the angle made by the pyramid (p) on c (119 28') is not the same as that on the prism a (128); further, it is noticed on close ex- MINERALS, AND HOW TO STUDY THEM. animation that the faces, a, have a vitreous luster and are often striated vertically, while the base, c, has a pearly luster and is often dull. Another form of crystal is sharply terminated by the pyramid p, and still another is in flat tables (Fig. 312). The cleavage is perfect parallel to the top or base of the crystals (c), and hence the peculiar pearly luster noticed 312. 314. 813. in this direction; elsewhere the luster is vitreous. The pearly luster is so peculiar that an old name of the mineral (ichthyophthalmite) means fish-rye *tone; the name apo- phyllite refers to the character of exfoliating before the blowpipe. The hardness is 4.5 to 5, and the specific gravity 2.3 to 2.4. The crystals may be entirely colorless, or they may be white or rarely of a beautiful rose-pink. Apophyllite is a silicate containing lime and potash with a good deal of water and a small amount of fluorine. Be- fore the blowpipe it exfoliates, whitens, and yields acid water, fusing to a vesicular enamel and coloring the flame a violet (potash). Beautiful specimens have been obtained from Bergen Hill, New Jersey. DESCRIPTION OF MINERAL SPECIES. 333 Pectolite. PECTOLITE is often found with prehnite, datolite, and various of the zeolites. The common form is massive and radiated or stellate, or in similar forms made up of acicular crystals (Fig. 315). Distinct crystals are rare; these are 315. monoclinic and show two perfect cleavages. The hardness is 5, specific gravity 2.68 to 2.78; luster silky to subvit- reous; color white or grayish. Pectolite is a silicate of lime and soda having the for- mula HNaCa 2 (Si0 3 ) 3 or H a O.Na a 0.4Ca0.6Si0 9 , which re- quires: Silica (SiO s ) 54.2, lime (CaO) 33.8, soda (Na.,0) 9.3, water (H a O) 2.7 = 100. It yields water in the closed tube and fuses easily (at 2) to a white enamel. It is de- composed by hydrochloric acid with separation of silica. THE ZEOLITES. The ZEOLITE FAMILY includes a number of beautiful minerals having a close relation to each other in manner 334 MINERALS, AND HOW TO STUDY THEM. of occurrence and in their chemical composition. They are all hydrous silicates, that is, they contain water, which is given off when a fragment is heated in a closed tube, and, like other hydrous silicates, they are of inferior hard- ness, chiefly 3.5 to 5.5, and low specific gravity, chiefly 2.0 to 2.4. They are readily decomposed by hydrochloric acid, some of them forming a jelly. Many of them bubble up, or intumesce when heated before the blowpipe, and this has given the name to the family from the Greek (C,eiv) to boil. The Zeolites are all said to be secondary minerals, which means that they were made subsequent to the time of for- mation of the rock in which they occur, unlike the feld- spar, quartz, etc., which are part of the rock. They have been formed in most cases out of the materials of the feld- spar or related minerals in the rock itself, and hence occur rather in crevices, seams, or in cavities, instead of in the solid mass. They are all silicates of alumina, and with this lime or soda or potash; they do not contain iron or magnesia; hence they are like the feldspar and, indeed, are often called hydrous feldspars. The zeolites, and also the minerals datolite, prehnite, apophyllite, pectolite {also calcite), which occur with them, are found frequently in connection with the dark- colored "trap rock," such as that which forms the Palisades of the Hudson, and is found also at various points in Con- necticut and Massachusetts, also in Nova Scotia. Famous localities have been developed where railroad cuts or tun- nels have been cut through ridges of this and similar DESCRIPTION OF MINERAL SPECIES. 335 igneous rocks (basalt, etc.), as at Bergen Hill, New Jersey, also similarly in British India. Beautiful specimens come from Nova Scotia. The zeolites are also found, but not so commonly, in granitic rocks. Thomsonite. THOMSONITE is usually found in columnar masses with radiated structure, also in radiated spherical concretions, as in the beautiful water-worn pebbles found on the shores of Lake Superior at Grand Marais. Crystals are rare. The hardness is 5 to 5.5; specific gravity 2.3 to 2.4; luster vitreous or slightly pearly; color snow-white, pale red or green. It is a silicate of alumina, lime, and soda. Before the blowpipe it fuses easily (2) with intumescence to a white enamel; with hydrochloric acid it gelatinizes. Natrolite. NATROLITE is sometimes called the needle zeolite because it is so common to find it in very fine acicular, or needle- like, crystals. These crystals are often arranged 316. in radiating tufts; sometimes they line oval cavities in the enclosing rock. When the crys- tals are larger, they are seen to have the form of a nearly square prism (angles 91 15') with alow pyramid on the summit (Fig. 316). There are also massive varieties having a fibrous or fine-columnar radiated structure. The hardness is 5 to 5.5; specific gravity 2.2 to 2.25; luster vitreous, sometimes a little pearly; colorless or white, also gray, yellowish, and reddish. It is a silicate of alumina and soda (natron), as its name indicates. Before 336 MINERALS, AND HOW TO STUDY THEM. the blowpipe it fuses quietly at 2 to a colorless glass; with hydrochloric acid it gelatinizes. SCOLECITE is a rarer zeolite resembling natrolite in its massive forms, but it contains lime instead of soda. It takes its name from its behavior before the blowpipe, curling up as a worm before fusion. Analcite. ANALCITE is found in crystals having usually the form of a trapezohedron, shown in Fig. 317, resembling one of the common forms of garnet. It also less frequently occurs in cubes with three faces on each solid angle, like Fig. 16, page 24. The hardness is 5 to 5.5; specific gravity 2.22 to 2.3; luster vitreous; colorless, white, or pale yellow or gray. It fuses before the blowpipe at 2.5 to a colorless glass and gelatinizes with hydrochloric acid. Chabazite. CHABAZITE is found in rhombohedrons, but, as the angle between two faces (95) is not far from 90, their aspect is often that of cubes, and it is possible 318 . for one not experienced to make a mistake. The crystals often interpenetrate each other (Fig. 318) in twining position a little like the cubes of fluorite. The hardness is 4 to 5; specific gravity 2.08 to 2.16, Luster vitreous; color white, yellow, also flesh-red. In composition it is a silicate of alumina and lime with small amounts of soda and potash. Before the blowpipe it intu- DESCRIPTION OF MINERAL SPECIES. 337 mesces and fuses to a blebby, nearly opaque glass; it is decomposed by hydrochloric acid with the separation of slimy silica. Acadiullte is a reddish variety of chabazite from Nova Scotia; pliceolite a colorless kind from Bohemia; and hay- denite a yellowish kind from Jones Falls near Baltimore. GMELINITE is another rhombohedral zeolite closely re- lated to chabazite, but much rarer. Stilbite. STILBITE is named from one of its most important char- acters, its beautiful pearly luster on the side cleavage face. 319 It is usually found in bundles of crystals, often looking like a sheaf of wheat tied tightly about the centre (Fig. 319); this has given it another of its names, desmine, from the Greek (dea-fiios) for bundle. There are also radiated forms, seen, for example, on a flat surface of rock, but distinct crystals are rare. The cleavage is very perfect on the side face. The hardness is 3.5 to 4, and the specific gravity 2.1 to 2.2. The luster, as already stated, is brilliant pearly on the cleavage surface, the side face of the bundles which have been described. The color varies from white to yel- low, and also to red or brown. Stilbite is a silicate of alumina and lime. Before the blowpipe it exfoliates, swells up, and fuses (2 to 2.5) to a white enamel. HARMOTOME is a rather rare zeolite, remarkable for con- taining 20 per cent of baryta. It is usually found in white 338 MINERALS, AND HOW TO STUDY THEM. to yellow or brown crystals of complex twinned structure. PHILLIPSITE is a related species, not, however, containing baryta; the crystals are similar to those of harmotome, but the color does not often vary from white. Heulandite. HEULANDITE, named after the owner of the famous Heuland cabinet, is found in fine monoclinic crystals with perfect cleavage parallel to the side plane, on which it has also a marked pearly luster that is very characteristic. It is like stilbite in this respect, but very different in form. The hardness is 3.5 to 4, and the specific gravity 2.2. The color is usually white, often milk-white, but red, gray, or brown varieties also occnr. It resembles gypsum a little, but is much harder. LAUMONTITE is a lime zeolite containing a large amount of water, part of which it is apt to lose on exposure to a dry air, whence it frequently falls to pieces in the cabinet. The common color is white (also red) ; the form mono- clinic. ON THE DETERMINATION OF MINERALS. 339 CHAPTER VIII. ON THE DETERMINATION OF MINERALS. IT will seem to the beginner a difficult thing to become so familiar with the many different mineral species as to be able to recognize each of them at sight; and it is diffi- cult, in fact impossible, even for the trained mineralogist to be always prompt and sure in his determination. For there are a large number of distinct species, between 800 and 1000, many of them very rare, while not a few appear in a great variety of forms. In the latter case the varie- ties sometimes depend upon fundamental differences of chemical composition, as among the garnets; and some- times upon less essential distinctions of structure, color, and so on, as with the varieties of quartz, calcite, and fluorite. Hence it is obvious that the characters that can be perceived at once, without the aid of careful tests, are often insufficient to fix a mineral positively. The experi- enced mineralogist, while he learns to know minerals so well that he can name most of them at sight and seldom blunders, is ever distrustful of himself and often hesitates to give the name quartz to a specimen having to the eye the external characters of this common species without, for example, a confirmatory test of hardness. Confidence and hasty judgment belong to those who have little ex- perience and a scanty knowledge of the difficulties of the subject. 340 MINERALS, AND HOW TO STUDY THEM. But, on the other hand, to recognize most of the min- erals, which are likely to be collected on a mineralogical excursion or to be obtained by exchange with other col- lectors, is generally easy even for the beginner, if he goes at the subject in the right way. For the number of common species is small, and quartz, feldspar, mica, calcite, and barite, also galena, sphalerite, pyrite, chalcopyrite, among metallic species, are constantly presenting themselves, and though their characters vary somewhat widely in different specimens, these are usually distinct, and almost always a simple test will make the matter sure. First of all, then, the mineralogist should know these and other common species well, for the chances are many times greater that an unknown specimen is one of them than that it is a rare and little-known species. It may be rare, even a new one not before described and not given in any of the books; but this is a chance that does not often happen. A real difficulty, that even much experience does not entirely remove, lies in the fact that at any large mineral locality there are likely to be many nondescript specimens which show few distinct characters. Some- times these are mixtures of several species, and often they arise from chemical decomposition of well-known min- erals. About such specimens it may perhaps be impos- sible to say anything definite; in fact, exhaustive micro- scopic and chemical work is often needed to settle their character. In such cases the beginner may well turn to some one more experienced for counsel. The best way, then, for one with a specimen of an un- known mineral in hand is to think of the common species ON THE DETERMINATION OF MINERALS. 341 first, and afterward of others which may suggest them- selves, running over in mind or by reference to the book the characters observed and those pf the species to which it is provisionally referred, but with care not to decide too hastily, but to give each character full weight. Do not give the name albite to a specimen of barite, either the tabular glassy crystals or the white massive granular kind, because both species are often white and also resemble each other in form, and overlook the fact that it is much too heavy as well as too soft. Do not give the name beryl to a crystal of apatite because it is a green hexagonal prism, and overlook the fact that it is quite too hard. Finally, do not hesitate to confess ignorance that the ex- perienced mineralogist is ever ready to do, and it is thia fact that enables him from time to time to identify some rare and interesting species and perhaps occasionally one new to science. As the student goes on to learn minerals better and bet- ter,the knowledge of the commoner species becomes so im- pressed upon his mind that he seldom hesitates, when a specimen of one of them is put in his hands, but its name suggests itself at once, and this without the careful sum- mary and comparison of characters which the beginner must go through. Then a confirmatory test clinches the matter. But this power only comes with long experience, and even then it is sometimes necessary to carry on an exhaustive examination before a result is reached. In the systematic determination of an unknown speci- men the first thing to do, as was insisted upon in Chapter II, is to learn all that is possible about it by looking at and 342 MINERALS, AND HOW TO STUDY THEM. handling it. It has already been shown that in this way its form and structure may be at least partially deter- mined; also its cleavage, if it shows any; and finally its luster, color, degree of transparency, etc. But at the same time the other senses must be kept on the alert, so that, for example, if the specimen is particularly heavy or light, greasy to the touch, etc., all these points will be quickly perceived and duly regarded. Then a touch with the point of a knife-blade will show something as to the hardness. This, it must be repeated, should be done carefully so as not to spoil the specimen, and the student must be on his guard not to make any of the mistakes easily possible in such trials, as before pointed out (pages 76, 77). If the mineral is not scratched by the knife it will be well to see whether it will scratch a piece of window-glass, and then whether it is scratched by or will scratch the smooth surface of a quartz crystal for the number of minerals as hard or harder than quartz is very small, as further stated on p. 357. At the same time with the test for hardness, the streak, or color of the powder, left by the knife or on a surface of ground glass or unglazed porcelain, must be noticed. Also if, as is most desirable, the blade of the knife is a magnet, the distinguishing character of magnetite and pyrrhotite will show itself at once. The careful determination of the specific gravity requires more time, and may be post- poned till the blowpipe has been used, but the hand should have already made a rough estimate of this, as has been before remarked (p. 79 et seq.). Further, when the characters mentioned have all been ON THE DETERMINATION OF MINERALS. 343 noted it Will be often necessary to make some chemical tests (read carefully pp. 153 to 157). A fragment for examination can generally be obtained by a careful blow with a light hammer without injury to the specimen. This, placed in a test-tube or on a watch-glass, with a little strong hydrochloric (or nitric) acid (p. 155), will effervesce with a nearly odorless gas (carbon dioxide, CO,) if it is a carbonate. Calcite effervesces at once, even in large fragments, in dilute acid, and other carbonates will act in the same way in strong acid, and also in dilute if they are first pulverized or the tube is warmed (p. 155). But remember that sulphureted hydrogen or hydrogen sulphide (H 3 S) may be liberated from a sulphide by warm hydrochloric acid (p. 155), and do not decide a given specimen is siderite instead of spha- lerite hastily because effervescence is observed, and over- look the strong offensive odor of the hydrogen sulphide; in other words, at all times the different senses should act together. The solution obtained will give the chemist the means of learning more (e.g., the presence of copper, p. 156), and if the specimen is insoluble, even when finely powdered and heated in acid, that is an important point (pp. 156, 157). The blowpipe tests may come before or after the other chemical examination, and these have been so fully ex- plained in Chapter VI that they need not be repeated here. A careful study of this chapter will have given the student full command of this part of the subject, and his experience should have taught him what order is best for the different tests. He will have learned, for example, that a mineral with metallic luster should first be tried in 344 MINERALS, AND HOW TO STUDY THEM. the open tube. If sulphur is present (the mineral being a sulphide) it will be given off as sulphur dioxide (pp. 149, 150), and at the same time arsenic, antimony, and mercury will show themselves (p. 148 et seq.}. The closed tube may be taken next and then the charcoal (p. 140), which last will confirm the results already obtained, and also show by the coating the presence of zinc, lead, etc. ; also, by adding soda in most cases, the presence of a reducible metal (p. 139) may be proved, as lead, silver, tin; while a magnetic residue will indicate iron. Further, after roast- ing off (p. 139) the sulphur, arsenic, or antimony the residue may be tested for copper, cobalt, etc., with borax on the platinum wire. A mineral with unmetallic luster may be tried first in the forceps (but always with caution : it may contain anti- mony, for example), and the degree of fusibility, the flame coloration, and other phenomena noted (p. 130, et seq.}, and then an examination made on the platinum wire (p. 136 et seq.}, in the bead of borax or salt of phosphorus. In the latter a silicate leaves a skeleton of undissolved silica having the form of the individual fragment (p. 140). Note, finally, that to obtain correct and concordant results the pure mineral must be experimented upon. In many specimens two or more species are so closely mixed together that it needs sharp eyes, aided by a magnifying- glass,* to separate them; this is particularly true of metal- lic minerals. Also many species commonly occur in an * Every mineralogist should have a pocket magnifying-glass, for even good eyes often need assistance, especially in examining small crystals. ON THE DETERMINATION OF MINERALS. 345 earthy mass, or gangue, so that it is difficult to obtain absolutely pure material. In such cases the quartz or clay will often do no harm if its presence is noted and the results interpreted correctly. A fragment of cinnabar is entirely volatile on charcoal or in the tube, but frequently it is associated with a gangue of clay, and then this will of course be left behind; also in such cases a fragment heated in the glass tube often yields water which comes from the nonessential gangue. Even if at the commencement it seemed as if very little was known about a specimen, the careful use of the eyes, the hand, and the various tests which may be made in a few minutes will have given a pretty complete table of its characters, and these may be used to fill out the blank list as suggested on page 160. In most cases, unless the speci- men is quite rare and unusual, it will be possible to suggest the name of a species with the description of which it is to be compared. Where this method of attack yields no defi- nite result complete determination tables* may be em' ployed, and in the hands of one who is skillful in the use of the blowpipe and in the simple chemical tests they will quite surely make it possible to identify any distinct species. In order to facilitate the work of determination, and at the same time to emphasize prominent characters of many minerals, the following notes are given. It is intended, as a rule, to mention only the prominent species under each head and those showing the given characters most dis- tinctly; to enumerate all which might be fairly included would deprive the lists of their value. * As those given in Brush's Determinative Mineralogy. 346 MINERALS, AND HOW TO STUDY THEM. 1. CRYSTALLINE FORM. Cubes. Fluorite (p. 245) is the common mineral, with unmetallic luster, which is likely to occur in cubes; it is easily recognized further by its octahedral cleavage. Halite (p. 268), or rock salt, also occurs in cubes, but has cubic cleavage and its slightly sticky feel makes it natural to test its taste, which at once removes all doubt. Pharmocosiderite (p. 225) is a rare arsenate of iron which also occurs in yellow or greenish cubes; the blow- pipe (e.g., on charcoal) shows the presence of both iron and arsenic. Galena (p. 198), of metallic minerals, is frequently in cubes, and is easily recognized by its cubic cleavage, high specific gravity, and lead-blue color. Pyrite (p. 213) in cubes is known by its light brass-yel- low color, brilliant metallic luster, and hardness; further- more, the cubes usually show fine lines or striations parallel to one pair of edges (see Fig. 183, p. 213). There are also some minerals crystallizing in other forms nearly like the cube. Apophyllite (p. 331) may have a form resembling a cube, though it is really a square prism. This is distin- guished by the pearly luster on one face, parallel to which there is easy cleavage, while the four other faces show fine lines or striations in one direction. Chabazite (p. 336) is often in rhombohedrons not far from a cube in angle. Calcite (p. 247) too, though the common rhombohedron cannot be mistaken for a cube, occasionally takes a form very near it in angle. This form has been ON THE DETERMINATION OF MINERALS. 347 called a cuboid. Even quartz (p. 273) appears, though rarely, in forms resembling cubes when the fundamental rhombohedron is present almost alone; the same can be said of hematite (Fig. 192, p. 218). A rare sulphate of alu- minium, called alunite (p. 244) (it becomes blue after ignition if moistened with cobalt solution) also occurs in cubelike rhombohedrons. Cryolite (p. 242), though crystallizing in the monoclinic system, has often a form deceptively like a cube to the eye, and the actual variation from this in angle is not great; it is recognized by its ready fusibility and bright yellow flame (sodium). Octahedrons. Fluorite (p. 245) sometimes occurs in octa- hedrons, though the cube is much more common. Cuprite (p. 194) often takes this form and is at once told by its red color and streak. Spinel (p. 241 ) is another octa- hedral mineral remarkable for its hardness. Alum and diamond (p. 166) are common in octahedrons, but each has other characters by which it is readily recognized. Among METALLIC species magnetite (p. 219) and frank- Unite (p. 221) are also often in black octahedrons; but though they look alike, the former is strongly magnetic, the latter only very feebly so if at all ; the former has a black streak, while that of the latter is brown. Chromite (p. 221) is another mineral sometimes found in black octa- hedrons. Pyrite (p. 213) is often in brass-yellow octa- hedrons, and chalcopyrite (p. 191) sometimes appears in forms resembling them. Galena (p. 198) occasionally occurs in octahedrons. A number of minerals occur in square pyramids looking 348 MINERALS, AND HOW TO STUDY THEM. more i,r less like regular octahedrons, as noted below, and occasionally a rhombohedral mineral resembles one; this is true, for example, of some dark colored dolomite crys- tals from Spain. Dodecahedrons. Garnet (p. 300) is frequently in dodeca- hedral crystals, which are hard (unless altered on the out- side), and commonly dark red to black in color. Magnetite (p. 219) also occurs in black magnetic dodecahedrons, and sometimes cuprite (p. 194) has this form. It is to be remembered that, even if the whole crystal cannot be seen, the diamond face with angles of 60 and 120 is very characteristic and rarely belongs to any other crystalline form; moreover the angle between two adjacent faces of a dodecahedron is always 120. Trapezohedrons. Garnet (p. 300) is also often in trape- zohedrons; it is easily distinguished from analcite (p. 330), which may have the same form, but whose hardness is only 5-5.5. Further, the rarer mineral leucite (p. 291), found in Vesuvian lavas, is also trapezohedral. Do not fail. to notice that the quadrilateral face of this form is very characteristic and will often make it possible to recognize it when only a small part of a crystal is visible. Pyritohedrons. The pyritohedral form is characteristic of pyrite (p. 213), known also by its pale brass-yellow color. Cobaltite (p. 228) has the same form, but it is a rare mineral with a tin-white color; it is an arsenide of cobalt. Tetrahedrons. Among minerals with unmetallic luster the tetrahedron is common with zinc blende or sphalerite (p. 233) and the rare minerals boracite (p. 262) and helvite ON THE DETERMINATION OF MINERALS. 349 (p. 303). This is also true of tetrahedrite (p. 193\ recog- nized by its black color and brilliant metallic luster. Further, chalcopyrite (p. 191) not infrequently occurs in forms called sphenoids, which resemble a tetrahedron closely and only differ a little from it in angles. Square pyramids are common with zircon (p. ;>12) and vesuvianite (p. 315), also xenotime (p. 271), octahedrite (Fig. 43, p. 32), wulfenite (p. 203), and a few other rare species. Some of these forms look a little like a regular octahedron. Square prisms are common with zircon (p. 312), vesu- viauite (p. 315), apophyllite (p. 331), scapolite (p. 314). Square tables, often very thin, are characteristic particu- larly of wulfenite (p. 203) ; they also occur with apophyl- lite. Hexagonal Pyramids. Quartz (p. 273) is the mineral most often found in hexagonal pyramids, but these usually show the planes of the hexagonal prism also (see figures on pp. 273, 274), and often one set of three alternate planes at one end are larger than the other set, as explained on the pages referred to. What appears to be the same form belongs to witherite (p. 265), but it is really a compound twinning form; it is easily distinguished by its softness. Corundum (p. 239) also has this form, but is recognized by its hardness and adamantine luster. A scalenohedron sometimes looks like a hexagonal pyramid, but is easily distinguished, as noted below. Hexagonal Prisms. Beryl (p. 298) is often in hexagonal prisms; the color is usually green (also blue and yellow), and it is hard (H. = 7-7.5). Apatite (p. 254) has nearly 350 MINERALS, AND HOW TO STUDY THEM. the same form and color, but is soft enough to be scratched by the knife (H. = 5). The prisms of apatite are often terminated by the planes of a hexagonal pyramid ; this is less often observed with beryl. Pyromorphite, mimetite, and van- adinite (pp. 200, 201) are also found in small hexagonal prisms, but the crystals are often bundled together and not seldom hollow or cavernous. Quartz (p. 273) is often in hexagonal prisms, and these commonly show fine horizontal lines or striations. This form is also shown by the different species of mica (p. 303), but they have other distinctive characters. Calcite (p. 247) has often this form, sometimes long prisms, or again short six-sided tables; it is soft (H. = 3) and usually has its terminal planes in threes; even if it has only a single flat basal plane, the cleavage on three alternate edges of the prism is characteristic. Tourmaline (p. 317) is often in hexagonal prisms with three rhombohedral faces at each end; three sided and nine-sided prisms are also common with tourmaline. The usual color is black. Willemite (p. 236), the silicate of zinc, also appears in hexagonal prisms. Trigonal Prisms. This form is characteristic of tourma- line (p. 317); if the crystals are imbedded in the rock, some of the cross-sections made by fracture will usually show the shape of an equilateral triangle. It must be noted that an isometric dodecahedron, placed with the line joining the two trihedral solid angles (those formed by three adjoining planes) vertical, has the form of a hexagonal prism with three rhombohedral faces at each ON THE DETERMINATION OF MINERALS. 351 end, also meeting at angles of 120 (compare Figs. 104, 105, p. 49). Aragonite (p. 252) has a form which is often an appa- rent hexagonal prism, but it is really due to twinning. Rhombohedrons. A rhombohedral form, with cleavage parallel to faces making angles of 105 to 107 with each other, is characteristic of calcite (p. 247) first of all, also dolomite (p. 260), siderite (p. 223), rhodochrosite (p. 232). Smithsonite (p. 237) belongs to the same group, but is seldom in distinct crystals. Chabazite (p. 336) is also often in rhombohedral crystals, but they are near a cube in angle, and it shows no distinct cleavage. The same is true of alunite. Scalenohedrons. Calcite crystals (p. 247) are not in- frequently complete scalenohedrons (dog-tooth spar}, or the form may be a hexagonal prism with scalenohedral faces at one end. A scalenohedron, if complete, is at once distinguished from a hexagonal pyramid by its zig- zag basal edge, or in any case by the fact that the angle of one set of three alternate terminal edges is greater than that of the other set. Rhombic Prisms. The following minerals, among the many orthorhombic species, often show a distinct pris- matic habit : topaz (p. 320), staurolite (p. 325) ; also barite (p. 262) and celestite (p. 266). The same is true of the following monoclinic species: pyroxene (p. 292), orthoclase (p. 285). More or less slender crystals of prismatic form or aspect occur often with the varieties of amphibole (p. 296) and with epidote (p. 316). Other species might be included here. 352 MINERALS, AND HOW TO STUDY THEM. Tabular Crystals. Crystals flattened parallel to one pair of faces are common with barite (p. 262); they are often clustered in divergent groups. This is also true of celestite (p. 266). Something of a similar form is seen with crystals of albite (p. 288). Acicular Crystals. Very slender, needlelike crystals, often in radiating groups, are characteristic especially of some of the zeolites, as natrolite (p. 335) and the carbon- ate, aragonite (p. 252). Both are white, but the latter effervesces in hydrochloric acid. Cuprite (p. 194) some- times appears in bright red capillary crystals. Of metal- lic species, stibnite (p. 176) often occurs in groups of radiating acicular crystals. Jamesonite (p. 200), a rare sulphide of antimony and lead, looking much like stibnite, also occurs occasionally in capillary crystals. Millerite (p. 227) is found in small radiating tufts of slender crystals and in capillary forms resembling a bunch of stiff hairs. STRUCTURE. Fibrous. (a) With Separable Fibers. Asbestus, a variety of amphibole (p. 296), and chrysotile (often also called asbestus), a variety of serpentine (p. 328), belong here. The latter contains considerable water and is more silky than the other species. This fibrous character also belongs to a rare mineral called crocidolite (p. 298), which in its unaltered form has a bright blue color. (b) Fibers Not Separable. Of the many species hav- ing fibrous varieties the most important are calcite and gypsum, each of which has a variety called satin spar j ON THE DETERMINATION OF MINERALS. 353 also aragonite, barite, celestite, anhydrite, brucite, wavel- lite. The following are more commonly COLUMNAR rather than fibrous: amphibole, epidote (and zoisite), sillimanite, tourmaline, natrolite, and several other zeolites ; also strontianite and witherite. Cyanite is bladed rather than columnar. Of metallic species, stibnite is the most conspicuous example of columnar structure. Radiated. A radiated and more or less fibrous struc- ture in massive varieties is seen conspicuously in some varieties of the following species : natrolite (p. 335), thomsonite (p. 335), stilbite (p. 337); also amphibole (p. 296), wavellite (p. 243, also Fig. 133, p. 68). Pyrophyllite (p. 324) and gypsum (p. 256, and Fig. 134, p. 68) also have forms that are made up of radiating or, better, stellated plates. Micaceous. The micas (p. 303 et seq.), muscovite, bio- tite, phlogopite, etc., separate readily into thin, usually tough, flexible, and often elastic laminae or leaves. Clinochlore (p. 311) and some related minerals, com- monly green in color, give tough, inelastic laminae. Some other species give soft, more or less brittle leaves, but cleave in the same way with the micas; this is true of talc (p. 326), brucite (p. 260), and pyrophyllite (p. 324). One variety of gypsum (selenite, p. 256) also separates by cleavage into soft, brittle laminae, but it is not properly micaceous. Orpiment (p. 174) yields thin, flexible plates of bright yellow color and brilliant luster. Foliated. Some of the minerals just mentioned, as talc, 354 MINERALS, AND HOW TO STUDY THEM. pyrophyllite, and orpiment, are foliated rather than mica- ceous. Graphite (p. 168) and molybdenite (p. 178), among minerals with a metallic luster, are conspicuous for their foliated character. Mammillary. A mammillary, botryoidal, or globular surface is often seen with prehnite (p. 331, and Fig. 138, p. 68), calamine (p. 237), smithsonite (p. 237), chalcedony (p. 278, also Fig. 137, p. 68), hyalite (opal, p. 283); hematite (p. 217) may be included, though a reniform surface is for it particularly characteristic (Fig. 139, p. 68). Limonite (p. 222) is often in stalactitic forms (Fig. 140, p. 68); also gibbsite (p. 241) and occasionally marcasite (p. 215). CLEAVAGE. Cubic Cleavage. This is exhibited conspicuously by halite or rock-salt (p. 268); also, among metallic species, by galena (p. 198, and Fig. 143, p. 71). Anhydrite (p. 258) and cryolite (p. 242) sometimes show cleavage in three directions, which sometimes resembles cubic, though not so in fact. Octahedral Cleavage. This is usually very distinct with fluorite (p. 245 and p. 71). Dodecahedral Cleavage. This is characteristic of spha- lerite or zinc blende (p. 233). Rhombohedral Cleavage. This is conspicuous with the species calcite (p. 247, and Fig. 144, p. 72), dolomite (p. 260), siderite (p. 223), rhodochrosite (p. 232). The basal cleavage of the micas (p. 303), chlorites (p. 310), brucite (p. 260), and other species, having on this account ON THE DETERMINATION OF MINERALS. 355 a foliated or "micaceous" structure, is an important char- acter. Orpiment (p. 174) has also a foliated structure. Gypsum (p. 256) yields large thin plates by cleavage. Topaz (p. 320) has perfect basal cleavage. Pyroxene (p. 292) often shows a basal " parting " resembling cleavage. The feldspars (p. 284) have cleavage in two directions at right angles to each other, or nearly so. Corundum (p. 239) sometimes shows a rhombohedral "parting" resem- bling cleavage, in directions inclined about 94 to each other. Barite (p. 262) and celestite (p. 266) have basal and prismatic cleavage. Amphibole (p. 296) has prismatic cleavage (124|). The scapolites (p. 314) have cleavage parallel to the two square prisms. Among minerals with metallic luster the following have conspicuous cleavage: graphite (p. 168), stibnite (p. 176). HARDNESS AND TENACITY. (See pp. 74-78.) Very soft : having a greasy feel. Here belong talc (p 326) and pyrophyllite (p. 324); also kaolin (p. 239). Graphite (p. 168) and molybdenite (p. 178), among minerals with metallic luster, have also a greasy feel and soil the fingers. Soft: scratched by the nail. Here belong gypsum (p. 256), brucite (p. 260), orpiment (p. 174), sulphur (p. 170), cerargyrite (p. 185), cinnabar (p. 187), also some chlorite. Further, among minerals with metallic luster may be mentioned: stibnite (p. 176), color lead-gray and luster metallic; argentite (p. 184), sectile, yields silver. 356 MINERALS, AND HOW TO STUDY THEM. Hard Minerals. It should be noted that most of the hard minerals belong to either the class of the Oxides or that of the Silicates. The sulphides are mostly soft, that is, H. = 4 or below; the exceptions are the minerals of the Pyrites Group, of which the common members are pyrite, marcasite, arsenopyrite (pp. 213-215); these are hard enough (6 to 6.5) to scratch glass. Some related sulphides (and arsenides) of cobalt and nickel have H. = 5 to 55. The Carbonates, Sulphates, Phosphates, etc., are also mostly soft, rarely up to 5. Of the Silicates, those yield- ing water, like the zeolites, are relatively soft, rarely up to 6. Again, another distinction partly contained in the above is that minerals of metallic luster are not often hard. The most conspicuous exceptions are the members of the Pyrites Family among sulphides, alluded to in the paragrah above, and hematite, magnetite, franklinite (pp. 217-221) among oxides. The following minerals are hard, mostly falling between 6 and 7: Prehnite (p. 331). Rutile (p. 209). Epidote and zoisite (pp. Cassiterite (p. 207). 316, 317). Diaspore (p. 240). Feldspars (p. 284). Chrysolite (p. 312). Vesuvianite (p. 315). Sillimanite (p. 324). Further, spodumene (p. 295), axinite (p. 325), iridosmine (p. 183), danburite (p. 321), chondrodite (p. 326); finally, cyanite (p. 324), H. = 5 to 7.25. ON THE DETERMINATION OF MINERALS. 35? The following are very hard hardness equal to that of quartz or greater (H. = 7 or aoove): H. H. Quartz (p. 273), 7 Beryl (p. 298), 7.5-8 Garnet (p 300), 6.5-7.5 Spinel (p. 241), 8 Tourmaline (p. 317), 7-75 Topaz (p. 320), 8 Staurolite (p. 325), 7-7.5 Chrysoberyl (p. 242), 8.5 Zircon (p. 312), 7.5 Corundum (p. 239), 9 Andalusite (p. 323), 7.5 Diamond (p. 166), 10 Also the rarer species boracite (p. 262), H. = 7; iolite (p. 322), H. = 7 to 7.5; euclase (p. 299) and phenacite (p. 300), H. = 7.5 to 8. Malleability. The minerals which are malleable include the native metals (see p, 78) ; less perfectly so argentite (p. 184), cerargyrite (p. 185): these last are conspicuously sectile. The rare silver minerals hessite (silver telluride), petzite (gold-silver telluride), are somewhat sectile. Talc (p. 326) and orpiment (p. 174) are flexible; also some mica and chlorite. SPECIFIC GRAVITY. (See pp. 79 to 88.) The importance of the specific gravity as a character ID the determination of minerals has been repeatedly insisted upon, and the subject has been so fully discussed on pp. 79 to 88 that to give a list of minerals of conspicuously low or high density would be unnecessary repetition. The student should in this connection read again carefully the pages referred to. 358 MINERALS, AND HOW TO STUDY THEM. LUSTER. Metallic. A metallic luster belongs to all the Native Metals, as gold, silver, copper, etc. ; also to many of the Sul- phides, as stibnite, galena, pyrite, but not to sphalerite, cin< nabar, and some few others; finally, to a few of the Oxides, as magnetite, hematite (some varieties), ilmenite, chromite, franklinite. The Silicates, Phosphates, Sulphates, Carbon- ates, etc., have, with very few exceptions, an unmetallic luster. Outside of these classes the only minerals having a metallic, or in most cases more strictly a submetallic, lus- ter are a very few rare silicates not described in this work ; also the species columbite (and tantalite, p. 224) and wolframite (p. 225), briefly mentioned, and a few others related to them. It should be noted that all the sulphides having an un- metallic luster (sphalerite, cinnabar, etc.) are soft. A hard mineral (H. = 6 or above) having an unmetallic lus ter is either an oxide or a silicate. There are only one or two rare exceptions (as boracite, H. = 7). Adamantine. An adamantine luster (as explained on p. 89) belongs to some hard minerals, as diamond, corundum, zircon, cassiterite; also to a number of minerals containing lead, as cerussite, anglesite, and other rarer ones, also cerargyrite, cinnabar, cuprite; further, some light-colored specimens of sphalerite and titanite. A metallic-adamantine luster belongs often to pyrargy- rite and some specimens of cerussite and cuprite. Resinous. Sphalerite (p. 233) is a striking example ol resinous luster; many Phosphates belong in this class. ON THE DETERMINATION OF MINERALS. 359 Vitreous or Glassy. Quartz, beryl, garnet are familiar examples of vitreous luster; most silicates belong here. Pearly. Talc (p. 326) and brucite (p. 260) have con- spicuous pearly luster, also pyrophyllite (p. 324) in foliated varieties. A pearly luster is noted on the basal plane of apophyl- lite (p. 331), also on the side planes of perfect cleavage of gypsum crystals (p. 256) and those of stilbite (p. 337) and heulandite (p. 338). Barite (p. 262) and celestite (p. 266) often show pearly luster on the basal plane; so, also, some kinds of feldspar. Other minerals belong in this same class. Silky. Fibrous gypsum (p. 257) and fibrous calcite (p. 249) (each called satin spar), also asbestus (p. 297) are good examples of silky luster. COLOR. (a) Metallic Luster. The following lists may be helpful, although it has not been attempted to make them complete: Silver-white or tin-white: Native silver; arsenopyrite, cobaltite, and, further, several rare minerals containing cobalt and nickel; also native antimony, arsenic, tellurium, and some compounds of tellurium, as the tellurides of gold (sylvanite), silver, or lead. The color is often dull on the surface in consequence of tarnish. Steel-gray : Stibnite, oxides of manganese, as pyrolusite and manganite; native platinum. Lead-gray : Galena, molybdenite (both bluish). StibuiU 360 MINERALS, AND HOW TO STUDY THEM. is also often lead-gray. Argeutite and chalcocite are blackish lead-gray. Copper-red : Native copper. Bronze-red : Bornite (with variegated tarnish), niccolite. Bronze-yellow : Pyrrhotite, millerite. Brass-yellow : Chalcopyrite, brittle, dissolves in nitric acid. Pale brass-yellow, pyrite; also still paler, marcasite. Millerite (see above) has more of a bronze color. Gold-yellow : Native gold (malleable). Black or nearly so: Tetrahedrite, chalcocite, graphite, magnetite, hematite, ilmenite, and sometimes limonite; also (luster submetallic) columbite, wolframite. The following are conspicuous for the tarnish (often bright-colored) of the surface : Bornite, chalcopyrite, tetra- hedrite, hematite, some limonite. The STREAK is to be noted particularly in the case of some minerals with metallic luster. The majority have a streak which differs but little from black, but it is usually dull, not shining. The streak of hematite is brownish red, of pyrargyrite, cochineal-red. The streak is bright and shining with graphite and molybdenite. (b) Vnmetallic Luster. Colorless : Quartz, not cleavable, hard ; calcite, if crys- tallized, shows rhombohedral cleavage, soft; gypsum, very soft. Also cerussite and anglesite, some crystallized varie- ties of albite, barite, apatite. White : Many massive minerals, especially the feldspars, quartz, calcite, barite, cerussite, scapolite, several of the zeolites, calamine, talc, meerschaum. ON THE DETERMINATION OF MINERALS. 361 Blue: Azurite, usually dark blue to blackish blue; also sapphire, cyanite, some celestite, lazulite, lapis lazuli. One variety of tourmaline has an indigo-blue color. Amethyst and some fluorite are violet-blue. Some beryl is blue or greenish blue, also amazon-stone. Turquois varies from robin's-egg blue to greenish blue and bluish green. Lapis lazuli is a bright blue. Chrysocolla and some varieties of calamine and calcite belong here, also some more or ^ss rare copper minerals (as chalcanthite and some other sul- phates, etc.). Green. Emerald-green: This is characteristic of some beryl (emerald), also malachite, dioptase, spodumene (hiddenite); also some other minerals containing copper, most of them not described in this book (as atacamite). Bluish green : Much beryl, apatite, fluorite, amazon-stone, tourmaline, chlorite, prehnite, calamiue, smithsonite, chrysocolla. Apple-green: Talc, some garnet, chrysoprase, willemite, nickel silicate. Yellowish green : Some beryl and apatite, chrysoberyl, chlorite, also (olive-green) chry- solite, datolite, and some serpentine, vesuvi- anite, titanite. Epidote is pistachio-green ; pyromorphite is grass-green. Some varieties of amphibole, pyroxene, also serpentine, are dall grayish or blackish green. Wavellite has green varieties of several shades. Gahnite is dark green. 32 MINERALS, AND HOW TO STUDY THEM. Yellow. Sulphur-yellow : Sulphur, some vesuvianite. Orange-yellow: Orpiment, wulfenite. Straw-yellow, also wine-yellow, wax-yellow: Topaz, sulphur, fluorite, cancrinite, wulfen- ite, vanadinite, willemite, calcite, barite, chrysolite, chondrodite, etc. Broivnish yellow : Much sphalerite, siderite. Ocher-yellow : Yellow ocher (limonite). Red. Ruby-red : Ruby (corundum), ruby spinel, much garnet, proustite, vanadinite, sphalerite. Cochineal red : Cuprite, cinnabar. Orange-red : Zincite. Crimson - red : Tourmaline (rubellite), spinel, fluorite. Orange-red : Realgar (to aurora-red). Scarlet-red : Cinnabar. Brick-red: Some hematite (red ocher). Rose-red: Rose quartz, rhodonite, rhodochrosite, erythrite, some scapolite and apophyllite. Peach-blossom red to lilac: Lepidolite, rubellite. Flesh-red: Some feldspar, willemite (the variety troostite), some chabazite and stilbite, apatite, less often calcite. Brownish red: Jasper, limonite, garnet, sphaler- ite, siderite, etc. Brown. Reddish broiun : Some garnet, some sphalerite, cassiterite. Clove-brown : Axinite, zircon, pyromorphite. Yellowish brown : Siderite, sphalerite, jasper, limonite, goethite, tourmaline. ON THE DETERMINATION OF MINERALS. 363 Blackish brown : Titauite, some siderite, spha- lerite. Smoky brown: Quartz. Slack : Tourmaline, black garnet (melanite) ; also (mostly greenish or brownish black) some amphibole, py- roxene, and epidote; further, some sphalerite and some kinds of quartz (varying from smoky brown to black); also allanite, samarskite. Some black minerals with sub- metallic luster are mentioned on page 360. The STREAK is to be noted in the case of some minerals with unmetallic luster. By far the majority have, even when deeply colored in the mass, a streak differing but little from white. The following may be mentioned : Orange-yellow : Zincite, crocoite. Cochineal-red : Pyrargyrite and proustite. Scarlet-red : Cinnabar. Brownish red : Cuprite, hematite. Brown : Limonite. MAGNETIC PROPERTIES. (Seep. 96.) Magnetite (p. 219) is always strongly magnetic, so that a fragment jumps to a good magnet even when separated by a little distance. Pyrrhotite (p. 212) is also magnetic, but not so strongly so as magnetite; the test often requires some care and the use of small fragments. Some native platinum (p. 181), especially the variety containing con- siderable iron, is also magnetic. The following minerals are slightly magnetic in some varieties: Hematite, franklinite. This seems to be due to 364 MINERALS, AND HOW TO STUDY THEM. an admixture of a little magnetite, for when pulverized a little magnetic powder can be separated. In general a fragment of a mineral containing iron becomes magnetic when roasted on charcoal or when held in the forceps and heated in a strong reducing flame. Thus pyrite, arsenopyrite, chalcopyrite, yield a magnetic globule on charcoal; also a splinter of iron garnet in the forceps fuses to a black bead which is more or less mag- netic. APPENDIX. THE following list includes the names of the species which it is most important that the young mineralogist should have in his collection; they are printed in SMALL CAPITALS. To these are added, in ordinary type, a number of others which are also important but not quite so much so; they may well be present in the cabinet of the school or academy. GRAPHITE. SULPHUR. Orpiment. STIBNITE. Molybdenite. SILVER. GOLD in quartz. An ore of silver. CINNABAR. COPPER. Chalcocite. Bornite. CHALCOPYRITE. TETRAHEDRITE. CUPRITE. MALACHITE. Azurite. GALENA. PYROMORPHITE. Mimetite. Vanadinite. CERUSSITE. Anglesite. Wulfenite. CASSITERITE. Rutile. PYRRHOTITE. PYRITE. MARCASITE. ARSENOPYRITE. HEMATITE. MAGNETITE. Franklinite. Chromite. LIMONITE. SlDERITE. Colninbite. MlLLERITE. Niccolite. Garnierite. Manganite (or Pyrolusite). RHODONITE. 366 APPENDIX. Rhodocrosite. SPHALERITE. Zincite. Willemite." Calamine. SMITHSONITE. CORUNDUM. Spinel. Cryolite. FLUORITE. Wavellite. CALCITE (several varieties). ARAGONITE. APATITE. Anhydrite. Brucite. GYPSUM. DOLOMITE. BARITE. Witherite. CELESTITE. Strontianite. HALITE. QUARTZ (several varieties.) OPAL. ORTHOCLASE. ALBITE. Oligoclase. Labradorite. PYROXENE (several var.). Spodumene. AMPHIBOLE (several var.). BERYL. GARNET. MUSCOVITE. BIOTITE. Lepidolite. Clinochlore. Chrysolite. Zircon. Scapolite. Vesuvianite. EPIDOTE. Zoisite. TOURMALINE. Topaz. Titanite. Andalusite. Cyanite. STAUROLITE. TALC. SERPENTINE. Datolite. PREHNITE. APOPHYLLITE. Pectolite. NATROLITE. Analcite. CHABAZITE. STILBITE. Heulandite. If the student limits himself to small specimens, as ad- vised on page 13, a collection including the species men- tioned will not occupy a great deal of space, and, if APPENDIX. 367 desired, can be purchased at no great cost. From time to time additional specimens can be obtained by exchange or purchase. Of the minerals in the above list the following are most desirable for the blowpipe and other chemical trials de- scribed in Chapter IV. Suitable fragments, of the needed purity, can be obtained for a very small expenditure of money. Stibnite, molybdenite, an ore of silver, cinnabar, chalco- pyrite, tetrahedrite, cuprite or malachite, galena, pyro- morphite, cassiterite, rutile, pyrite, arsenopyrite, hema- tite Or siderite, millerite, rhodonite, sphalerite, corundum, cryolite, fluorite, calcite, apatite, brucite, barite, celestite, orthoclase, amphibole (actinolite), garnet (almandite), tourmalme, natrolite. Also in addition to these : a mineral containing lithium, as either lepidolite, spodumene, amblygonite, or triphylite; one containing cobalt, as linnaeite; chromium, as chromite or crocoite; vanadium, as vanadinite; uranium, as uranin- ite (pitchblende) or autunite. GENERAL INDEX. Acicular crystals, 63, 352 Acid aud salt, 112 Adamantine luster, 89, 167, 358 Aluminium, aluminum, 238; test for, 135 Amalgam, 186, 187 Amorphous structure, 63 Antimony, 175; tests for, 133, 142, 151 Aragonite group, 119 Arborescent structure, 61 Arsenates, 112, 175 Arsenic, 173; tests for, 133, 142, 150 Arsenides, 175 Aslerism, 93, 308 Atomic weight, 102 Barite group, 119 Barium, 262; test for, 133 Basal plane, Base, 31 Beryl loid, 37 Bismuth, 177; test for, 146 Blebby bead, 135 Blowpipe, description, 123; use of, 127 Borates, 113 Borax, use of, 136 Boron, test for, 133 Botryoidal structure, 67 Brittle minerals, 78 Bromides, 110 Bunsen burner, 122 Burner for blowpipe, 122 Cadmium, test for, 146 Calcite group, 119 Calcium, 244; test for, 133 Capillary crystals, 63 Carbon, 166 Carbonates, 112 Carlsbad twin, 286 Charcoal, 124 use of, 140 Chemical compound, 106, 109 elements, 100 formula, 104 symbol, 100 tests, 153 Chlorides, 110 Chlorine, test for, 133, 155 Chromates, 112 Chromium, tests for, 138, 139 Classification of minerals, 118, 161 Cleavable, 66 Cleavage, 65, 70, 354 Basal, 72, 354 Cubic, 70, 354 Dodecabedral, 71, 354 Octahedral, 71, 354 Prismatic, 72, 355 lihombohedral,71, 354 Closed tube, 125, 147 Cobalt, 228; test for, 138 nitrate, use of, 135, 144 Cohesion, 15, 70 Collection of minerals, 11 Color, 90, 359 Color of blowpipe flame, 133 Columbates, 113 Columbiura, test for, 139 Columnar structure, 67, 353 Concentric structure, 67 Couchoidal fracture, 74 Concretionary structure, 69 Concretions, 69 369 370 GENERAL INDEX. Contact-twin, 57 Copper, 188; tests for, 133, 138, 145, 156 Copper chloride, test for, 133 Coralloichil structure. 69, 253 Crypto-crystalline, 65 Crystal, detiuition, 14 distorted, 48 how formed, 1 7 twin, 5? Crystalline structure, 63 Crystallization, systems of, 21 Cube, 22, 346 Decrepitation, 131 Deudrites, 69 Dendritic structure, 61, 69 Density, 79 Description of species, 158 Determination of minerals, 339 Dichroism, 95 Dimorphous compound, 120 Dioxide, 108 Distorted crystals, 48 Dodecahedron, 22, 348 Domes, 42 Double refraction, 94 Drusy, 63 Ductile, 78 Earthy fracture, 74 Effervescence, 155 Elasticity, 78 Elements, chemical, 99 equivalence of, 105 Etching, 64 Exfoliation, 135 Ferrornangnnese, 229 Fibrous structure, 67, 352 Flame coloration, 133 Flame, oxidizing, 128 reducing, 128 Flexible, 78 Fluorides, 110 Fluorine, test for, 153 Fluxes, 125 Foliated structure, 67, 353 Forceps, 124, 130 Formula, chemical, 104 Fracture, 73 Fusibility, scale, 131 Glass tubes, 125; use of, 147 Globular structure, 67 Gold, 179 Goniometer, contact, 47 Granite veins, 287, b07 Granular, 66 Greasy luster, 89 Grouping, irregular, 62 parallel, 60 Hackly fracture, 74 Hammer, 11 Hard minerals, 356, 357 Hardness, 74, 355 Heat, 95 Hexagonal prism, 36 pyramid, 36 Hexoctahedron, 25 Hydrates, 116 Hydraulic lime, 252 Hydrous compounds, 116 Impalpable structure, 66 Inelastic, 78 Intumescence, 135 Iodides, 110 Iridescence, 93 Iron, 210 ; test for, 138 Isometric system, 22 Isomorphous group, 119 Lamellar structure, 66 Lamp for blowpipe, 122 Lead, 197; test for, 144 Left-handed crystal, 275 Lithium, test for, 133 Luster, 88, 358 Magnesium. 259 : test for, 136 Magnetic, 96, 363 Magnifying glass, 344 Malleable, 78, 357 Mammillary structure, 67, 354 Manganese, 2^9 ; test for, 138 Mercury, 186; test for, 148, 149 sulphide, test for, 148 Metallic-adamantine luster, 89, 358 Metallic luster. 88, 357 Metals, 102, 107 Metasilicates, 113 Micaceous structure, 67, 353 GENERAL INDEX. 371 Microcosmie salt, 125, 139 Mineral, artificial, 7 definition of, 5 Mineral kingdom, 1 Mineralogy, science of, 5 Molybdates, 112 Molybdenum, 178 ; tests for, 134, 139, 146 Mouoclinic system, 44 Monoxide, 108 Mossy, 63 Native elements, 109 Negative element, 107 Nickel, 226 : test for, 138 Niobates, 113 Niobium, test for, 139 Non-metals, 102, 107 Nugget, gold, 180 Octahedron, 22, 347 Odor, 97 Opalescence, 93 Opaque, 93 Open tube, 125, 147 Orthorhombic system, 41 Orthosilicates, 113 Oscillatory combination, 53 Oxides, 110 Oxidizing flame, 128 Parallel grouping, 60 Paramorph, 56 Pearly luster, 89 Penetration-twin, 57 Percentage composition, 116 Peroxide, 108 Phosphates, 112 Phosphorescence, 94, 153 Phosphoric acid, test for, 133 Pinacoids. 42. 44 Plaster of Paris, 257 Platinum, 181 Wire, 125, 136 Play of colors, 93 Polysynthetic twinning, 59 Positive element, 107 Potassium, 268 ; test for, 133 Prisms, 31, 32, 41, 44, 349 Protoxide, 108 Pseudomorph, 55 Pyramid, hexagonal, 36, 349 Pyramid, rhombic, 42 square, 31, 32, 349 Pyritohedrou, 30, 213, 348 Pyro electricity, 97, 318 Quartzoid, 274 Radiated structure, 67 Reducing flame, 128 Refraction, double, 94 Reniform structure, 67 Resinous luster, 89, 358 Reticulated, 63 Rhombic prism, 41, 351 pyramid, 42 Rhombohedral system, 39 Rhombohedrou, 39, 351 Right -handed crystal, 275 Roasting, 139 Salt of phosphorus, 139 Salts, 112 Saturation, 137 Scale of fusibility, 131 hard ness, '75 Scale IK >hedron, 39, 351 Secondary twinning, 60 Sectile, 78 Selenium tests for, 134, 146, 152 Semi-metals, 103 Semi transparent, 93 Sesquioxide, 108 Silica, 272 ; test for, 156 Silicates, 113, 272 Silicon, 272 Silky luster, 89 Silver, 183 ; test for, 145 Snow-crystals, 17 Soda, use of, 140 ; on charcoal, 144, 145 Sodium, 268 ; test for, 133 Soft minerals, 355 Solubility in acids, 154 Specific gravity, 79. 80, 357 balance, 81, 82 Spelter, 233 Sphenoid, 36 Spiegeleisen, 229 Splintery fracture, 74 Square prisms. 31, 32, 349 pyramids, 31. 82, 349 Stalactitic structure, 69 372 GENERAL INDEX. Stalactite, 250 Tetrabexabedron, 25 Stalagmite, 250 Test-paper, 126 Stellate structure, 67 Tin, 206 ; test for, 145, 146 Streak, 92 Titanium, 208 ; test for, 139 Striations, 52 Translucent, 93 Strike-figure, 305 Strontium, 266 ; test for, 133 Transparent. 92 Trapezohedron, 23 Sublimate, 142, 148 Triclinic system, 45 Subtrauslucent, 93 Trigonal prism, 317, 350 Sulphates, 112; test for, 146 Trisoctahedrons, 24 Sulphides, 109 Sulphur, 168 ; test for, 142, 146, Tuugstates, 112 Tungsten, 259 147, 148 Twin crystals, 57 Symbol, chemical, 100 Symmetry, defined, 27 Twinning axis, 57 plane, 58 planes of, 27, 35 Twinning, polysyntbetic. 59 System. Hexagonal 36 secondary, 60 Isometric, 22 Mouoclinic, 44 Uneven fracture, 74 Orthorhombic, 41 Un metal lie luster, 89 Rhombohedral, 39 Uranium, 210 ; tests for, 138, 139 Tetragonal, 31 Triclinic, 45 Vanadates, 112 Vesicular bead, 13o Tabular crystals. 352 Vitreous luster, 89, 359 Tautalales, 113 Tarnish, 91, 92 Water of crystallization, 116 Taste, 97 Water, test for. 152 Tellurium, 173 ; tests for, 134, 152 Waxy luster, 89 Tenacity. 77 Zinc, 233 ; test for, 143 Tetragonal system, 31 Zircouoid, 34 Tetrahedron, 29, 348 INDEX TO MINERAL SPECIES. Acadinlite, 337 Achroite, ixir Tourmaline, 317 Actinolite, 297 Adamantine spar, 240 Adular, Adularia, 287 Agaric mineral, 251 Agate, 279 Alabandite, 232 Alabaster, 257 Albite, 288 Alexandrite, 242 Allauite, 317 Almandine, Almandite, 302 Altaite, 110 Alum, Native, 244 Alumina, 239 Aluminite, 244 Aluminium carbonate, 244 fluoride, 242 fluo silicate, 320 hydrate, 241 oxide, 289, 240 phosphate, 243, 244 silicate, 323, 324 sulphate, 244 Alunite, 244 Amalgam, 187 Amazon-stone, 288 Amber, 170 Amblygonite, 244 Amethyst, 277 Oriental, 240 Amianthus, v. Serpentine Amphibole, 296 Aualcite, Analcime, 336 Anatase, v Octahedrite, 208 Audalusite, 323 Aiidrsdite. 302 Auglesite, 205 Anhydrite, 258 Ankerite, 261 Anorthite, 290 Anthracite, 170 Autiuiouite, v. Stibnite, 176 Antimony, Gray, v. Stibnite, 11 Native, 175 Antimony glance, 176 sulphide, 176 Apatite, 254 Apophyllite, 331 Aquamarine, 299 Aragonite, 252 Argentiferous galena, 199 Argentine, var. Calcite, 247 Argentite, 184 Arkansite, -car. Brookite, 209 Arragonite, v. Aragonite, 252 Arsenates, 175 Arsenic, Native, 173 Red, 174 Yellow, 174 White, 175 Arsenic oxide, 175 sulphide, 174 Arsenides, 175 Arsenolite, 175 Arseuopyrite, 215 Asbestus, 297, 328 Blue, 298 Asparagus-stone, 255 Asphaltum, 170 Atacamite, 197 AugU e, 294 Auriferous pyrites, 214 Auripigmentum, 174 Autuuite, 210 374 INDEX TO MINERAL SPECIES. Aventurine quartz, 278 Axinite, 325 Azurite, 196 Barite. 262 Barium carbonate, 265 sulphate, 2ti2 Baryta = Barium oxide, v. Ba- rium, 262 Burytes, v. Barite, 262 Barytocalcite, 265 Basjinite, 280 Bauxite, Beauxite, 241 Bell- metal ore, v. Stauuite, 206 Beryl, 298 Beryllium aluminate, 242 phosphate. 300 silicate, 298, 300 Beryllonite, 300 Bismuth, Native, 178 Biotite. 308 Bismuth sulphide, 178 Bismuthinite, 178 Bitter spar, v. Dolomite, 260 Bitumen, 170 Bituminous coal, 170 Black lead, 169 Black jack, 234 Blende, 234 Bloodstone. 280 Blue-John, 246 Blue- vitriol. 197 Bog iron ore, 223 Bog manganese, 231 Boracite, 262 Borax, 270 Boruite, 190 Bort, var. Diamond Bournouite, 200 Braunite, 231 Breithauptite, 228 Breunerite, 262 Brittle mica, 310 Brochantite, 197 Bronzite, 295 Brookite, 209 Brown coal, 170 hematite, 222 Brucite, 260 Cacholong, var. Opal Cadmium sulphide, 238 Cairngorm stone, 277 Calamiue, 237 Calc spar, v. Calcite, 247 Calc sinter, 251 Calcite, 247 Calcium arsenate, 259 boraie, 259 boro-silicate, 329 carbonate, 247, 252 fluoride, 245 phosphate, 254 silicate, 295 sulphate, 256, 258 tantalate, 259 titanate, 259 tungstate, 258 Cancriuite, 292 Capillary pyrites, t>. Millerite, 227 Carbon, 166 Carnelian, 279 Cassiterite. 207 Cat's-eye, 242, 278 Cat's gold, 306 Cat's silver, 306 Celestite, Celestine, 266 Cerargyrite, 185 Cerium phosphate, 271 Cerussite, 203 Clmbazite, 336 Chalcanthite, 197 Chalcedony, 278 Chalcocite, 190 Chalcopyrite, 191 Chalk, tar. Calcite, 247 Chalybite, t. Siderite, 223 Chert, 280 Chiastolite, 55, 323 Childrenite, 225 Chlor-apatite, 255 Chlorite Group, 310 Chloritoid, 310 Chlorophaue, 247 Chondrodite, 326 Chromic iron, 221 Chromite, 221 Chrysoberyl, 242, 299 Cl.rysocolla, 197 Chrysolite, 312 Chrysoprase, 280 Chrysotile, 328 Cinnabar, 187 Cinnamon-stone, 301 INDEX TO MINERAL SPECIES. 375 Clay, 239 Diaspore, 240 Clinuchlore, 511 Diopside, 293 Coal, 170 Dioptase, 197 Cobalt bloom, 228 arsenate, 238 Dipyre. 315 Dislheue, v. Cyanite, 324 arsenide, 228 Dog-tooth Spar, 249 sulphide. 228 Dolomite, 260 Cobaltine, Cobaltite, 228 Dry-bone, 237 Coccolite 294 Colestiue, v. Celestite Eisenkiesel, v. Quartz, 62 Coluinbite. 224 Eisenrose, 62 Copper, Emerald, 197 Elteolite, 292 Gray, 193 Emerald, 299 Native, 189 Emerald copper, 197 Purple, 191 Emery, 239 Red, 194 Eustatite, 295 Yellow, 191 Eosphorite, 225 Copper arseuate. 197 Epidote, 316 carbonate, 195, 196 Epsom Salt, Epsoruite, 262 chloride, 197 Erubescite, 190 glauce, 190 Erythrite, 228 iiickel, 227 Essonite, v. Hessouite, 301 oxide, 194 Euclase, 299 phosphate, 197 Eulytiue, Euiytite, 303 pyrites, 191 silicate, 197 Fahlerz, v. Tetrahedrite, 198 sulphate, 197 False galena, 234 sulphide. 190, 191 lead, 234 vitriol, 197 topaz, 277 Copper ore, Red, 194 Feldspar Group, 284 Yellow, 190 Fibrolite, v. Sillimauite, 324 Copperas, 226 Cordierite, 322 Fire-opal, 283 Fleches d 'amour, 209 Corundum, 239 Flint, 280 Crocidolite, 298 Flos ferri, 253 Crocoite, Crocoisite, 202 Fluor-apatite, 255 Cross-stone, 325 Fluorite, 245 Cryolite, 242 Fluor spar, 245 Cuprite, 194 Frankliuite, 221, 236 Cyanite, 324 Gahnite, 236 Danaite, 228 Galena, Galenite, 198 Danalite, 303 Garnet, 300 Dan bu rite, 321 Garnierite, 228 Datholite, Datolite, 329 Genthite, 228 Dawsonite, 244 Gey se rite, 283 Deraantoid, 302 Gibbsite, 241 Derbyshire spar, v. Fluorite, 245 Glauber salt, 270 Descloizite. 202 Gluuberite, 270 Desmiue, 337 Glaucodot, 228 Diallage, 294 Glimmer, v. Mica, 303 Diamond, 166 Gmeiinite, 337 376 INDEX TO MINERAL SPECIES. Goethite. 223 Gold, 179 Iron, arsenate, 225 arsenide, 215 Gold telluride, 179 GOthite, 223 carbonate, 223 columbate, 224 Graphic tellurium, 181 Graphite, 168 Gray antimony, 176 copper, 193 uiobate, 224 oxide, 217, 219, 222, 228 phosphate, 225 sulphate, 225 Greenockite, 238 sulphide, 212, 213, 215 Grenat, v. Garnet, 300 tantalate, 224 Grossularite, 301 Iron pyrites, 213, 215 Guano, 255 White. 215 Gypsum, 256 Jade, 297 Halite, 268 Jadeite, 297 Harmotome, 337 Jamesonite, 200 Hauerite, 232 Jasper, 280 Hausmannile, 231 Jasper-opal, 283 Haydenite, 337 Job's tears, 312 Heavy spar, 262 Heliotrope, 280 Kaolin. Kaolinite, 239 Helvite, 303 Hematite, 217 King's yellow, 174 Kyaiiite, v. Cyanite, 324 Brown, 222 Herderite, 300 Labradorite, 291 Hessouite, 301 Labrador feldspar, 291 Heulandite, 338 Lapis-lazuli, 291 Hidden ite, 295 Laumonite, Laumoutite, 33 Hornblende, 297 Lazulite, 243 Horn silver, 185 Lead, Argentiferous, 199 Hornstoue, 280 Black, 169 Horse-flesh ore, 191 Native, 198 Humite, 326 Lead carbonate, 203 Hyalite, 283 chromate, 202 Hyalopl.ane. 285 Hydraulic limestone, 252 Hypersthene, 295 molybdate, 202 phosphate, 200 sulphate, 205 sulphide, 198 Ice, 172 telluride, 110 Ice-stone, 242 Lepidolite 309 Iceland spar, 249 Lepidomelane, 308 Ichtliyophthalmite, 832 Idocrase, 315 Leucite, 291 Libethenite, 197 Ilmeuite, 222 Lime = Calcium oxide, 244 Indicolite, var. Tourmaline, 317 Limestone, 250, 251 Infusorial earth, 283 Limonite, 222 lolite, 322 Linnoeite, 228 Iridosmine, 183 Lithia mica, 309 Iron, Magnetic. 219 tourmaline, 319 Meteoric. 211 Lithiophilite, 225 Native, 211 Love's arrows, 209 Olitfist (hematite), 217 Lumachelle, 250 INDEX TO MINERAL SPECIES. 37? Lydian stone, 280 Milller's glass, 283 Made, 323 Native antimony, 175 Magnesia = Magnesium oxide, arsenic, 173 259 bismuth, 177 Magnesite, 260 Magnetic iron ore, 219 copper, 189 gold, 179 Magnetic pyrites, 212 iron, 211 Magnetite, 219 lead, 198 Malachite, Blue. v. Azurite, 196 mercury, 187 Green, 195 platinum, 181 Alalacolite, v. Diopside, 293 silver, 183 Manganese carbonate, 232 sulphur, 170 oxide, 229, 230, 231 Natrolite, 335 silicate, 231 Natron, 270 sulphide, 232 Needle zeolite, 335 Manganite, 230 Marble, 250, 262 Nepheline, Nephelite, 291 Nephrite, 29? Verd -antique, 328 Niccolite, 227 Marcnsite. 215 Nickel antimonide, 228 Margarite, 310 arsenide, 227 Marialite, 315 silicate. 228 Meerschaum, 329 sulphide, 227 Mt-ionite, 315 Melanile, 302 Ocher. Brown 222 Meianterite, 226 lied, 218 Menaccanite, v. Ilmenite, 222 Octahedrite, 208 Mercury. Native, 186 Oliiroclase, 290 Mercury chloride, 187 Olivenite, 197 sulphide, 187 Olivine, :!12 Mesitine, Mesitite, 262 Onyx, 279 Mica Group, 303 Opal, 282 Microcline, 288 Ophiolite, 328 ilicrolite, 259, 271 Oraugite, 314 Milk opal, 283 Orpiment, 174 quartz, 277 Onhoclase, 285 tfillerite. 227 Miuietite, 201 Oriental amethyst, ruby, sap- phire, topaz, 240 Mineral coal, 170 Ottrelite. 310 oil, 170 Ouvaroviie, 302 wax, 170 Ozocerite, 170 Mirabilite, 270 lispickel. 216 Pachuolite, 243 Molybdenite, 178 Molybdenum sulphide, 178 Monazite, 271 Peacock copper, 92, 190 Pearl-mica, . Margarite, 310 Pearl-spar = Cryst. dolomite, Moonstone, 287 260 Moss-agate, 279 Pectolite, 333 Mountain cork, 297 Pencil-stone. 327 leather, 297 Penninite, 311 Muscovite, 304 Pentlandite, 227 Muscovy glass, 304 Peridot, v. Chrysolite, 312 378 INDEX TO MINERAL SPECIES. Perofskite, Perovskite, 259 Petroleum, 170 Pharmacolite, 259 Pharmacosiderite, 225 Phucoliie, 337 Phenacue, 300 Pbillipsite. 338 Phlogopite, 308 Finite, 307 Pitchblende. 210 Plagioclase, 289 Plasma, var. Quartz, 273 Plaster of Paris, 257 Platinum, Native, 181 Plumbago, 168 Plumose mica, 306 Polianite, 230 Polydymite, 227 Potassium chloride, 270 Potter's ore, 200 Prase, 280 Precious garnet, 302 opal, 282 serpentine, 328 Prehnite, 331 Proustite, 185 Psilomelaue, 231 Pucherite, 112 Purple copper ore, 190 Pycuite, v. Topaz, 320 Pyrargyrite, 185 Pyrile, 213 Pyrites, Arsenical, 215 Auriferous, 214 Capillary, . Millerite, Cockscomb, 215 Copper, 191 Iron, 213 Magnetic, 212 Spear, 215 White iron, 215 Pyrochlore, 271 Pyrolusite, 229 Pyromorphite, 200 Pyrope, 302 Pyrophyllite, 324 Pyroxene, 292 Pyrrhotite, 212 Quartz, 273 Quicksilver. 186 Realgar, 174 Red copper ore, 194 hematite, 217 iron ore, 217 ocher, 218 silver ore, 185 zinc ore, 236 Resiu, Mineral, 170 Rhodoohrosite, 232 Rhodonite, 231 Ripidolite, 311 Itock crystal, 277 meal, 251 milk, 251 salt, 268 Rose quartz, 278 Rubelliie, 319 Ruby, Balas, 241, Oriental, 240 Spinel, 241 Ruby silver, 185 Rutile, 208 Sahlite, Salite, 293 Salt, Common, 268 Samarskite, 224 Sanidine, 287 Sapphire, 240 Sard, 279 Sardonyx, 280 Satin -spar, 250, 257 Scapolite Group, 314 Scheelite, 258 Scolecite, Seolezite, 336 Scorodite, 225 Selenite, 256 Sepiolite, 329 Serpentine, 327 Siderite, 223 Silex, v Quartz, 278 Silica, 272 Siliceous sinter, 283 Silicined wood, 281 Sillimanite, 324 Silver, 183 Horn, 185 Native, 183 Ruby, 185 Silver chloride, 185 sulph-antimonite, 185 sulph-arsenite, 185 sulphide, 184 INDEX TO MINERAL SPECIES. 379 Silver glance, 184 Siuter, Siliceous, 283 Smalt, 229 Smaltite, 228 Suiaragdite, var. Amphibole, 296 Smithsonite, 237 Smoky quartz, 277 Soapstone. 3:27 Soda = Sodium oxide, v. So- dium, 268 Soda niter, 270 Sodalite, 291 Sodium borate, 270 carbonate, 270 chloride, 268 nitrate, 270 sulphate, 270 Spathic iron, 223 Spear pyrites, 215 Specular iron, 217 Sperrylite, 183 Spessartite, 302 Sphalerite. 233 Spheue, 322 Spiegeleisen, 229 Spiuel, 241 Spodumene, 295 Stalactite, 250 Stalagmite, 250 Stannite, 206 Staurolite, Staurotide, 325 Steatite, 327 Stibnite, 176 Stilbite, 337 Stream-tin, 207 Strontianite, 267 Strontium carbonate, 267 sulphate, 266 Sulphur, Native, 170 Sylvanite, 181 Sylvine, Sylvite, 270 Tabular spar, 296 Talc, 326 Tantalite, 224 Tellurium, Graphic, 181 Native, 173 Tennantite, 194 Tenorite, 111 Tetrahedrile, 193 Thenardite, 270 Thomsenolite, 243 Thomsonite, 335 Thorite, 314 Tiger-eye, 278, 298 Tin ore, Tin stone, 207 oxide, 207 pyrites, v. Stannite, 206 Titanic iron, 222 Titauite, 322 Titanium oxide, 208 Topaz, 320 False, 277 Topazolite, 302 Torbernite, Torberite, 210 Touchstone, 280 Tourmaline, 317 Tremolite, 297 Tridymite, 272 Triphylite, 225 Triplite, 232 Trona. 270 Troostite, 236 Turgite, 223 Turnerite, 271 Turquois, 243 Ulexite, 259 Ultramarine, 291 Uraninite, 210 Uranium, 210 phosphate, 210 mica, 210 Uvarovite, 302 Vanadiuite, 201 Verd-antique, 328 Vermiculite, 309 Vermilion, Native, 187 Vesuvianite, 315 Vitriol, Blue, 197 Viviauite, 225 Wad, 231 Water, 172 Wavellite, 243 Wernerite, 314 White-lead ore, 203 Willemite, 236 Witherite, 265 Wolfram, Wolframite, 225 Wollasionite, 295 Wood opal, 283 380 INDEX TO MINEIIAL SPECIES. Wood-tin, 207 Wulfenite, 203 Xenotime, 271 Yttrium phosphate, 271 Zeolites, 333 Ziuc blende, 234 carbonate, 237 Zinc ore, Red, 236 oxide, 236 silicate, 236, 237 sulphide, 233 spinel, 236 Zincite. 236 Zircon, 312 Zirconium silicate, 313 Zoisite, 317 Zuuyite, 303 THE LIBRARY UNIVERSITY OF CALIFORNIA Santa Barbara THIS BOOK IS DUE ON THE LAST DATE STAMPED BELOW. RETURNED MAYJJU982 ~> 1 71QOC 50m-5,'64( E5474 8 8)9482 3 120 UC SOUTHERN REGIONAL LIBRARY F AA 000 941 057 2 \ ,P3> ,