Title: Field book of common rocks and minerals
for identifying the rocks and minerals of the United States and interpreting their origins and meanings
Author: Frederic Brewster Loomis
Photographer: Walter Everett Corbin
Release date: August 18, 2017 [eBook #55382]
Most recently updated: October 23, 2024
Language: English
Credits: Produced by Stephen Hutcheson, Dave Morgan and the Online
Distributed Proofreading Team at http://www.pgdp.net
For identifying the Rocks and Minerals of the United States and interpreting their Origins and Meanings
By
Frederic Brewster Loomis
Late Professor of Mineralogy and Geology
in Amherst College
With 47 Colored Specimens and over 100 other Illustrations from Photographs by W. E. Corbin and drawings by the Author
G. P. Putnam’s Sons
New York and London
FIELD BOOK
OF
COMMON ROCKS AND MINERALS
Copyright, 1923, 1948
by
Frederick Brewster Loomis
Twenty-sixth Impression
Revised 1948
All rights reserved. This book, or parts thereof, must not be reproduced in any form without permission.
Made in the United States of America
Dedicated
TO
MY MOTHER
WHO ENCOURAGED ME WHILE A BOY TO GATHER MINERALS, ROCKS AND FOSSILS.
Everyone, who is alert as he wanders about this world, wants to know what he is seeing and what it is all about. Here and there with the aid of capable guides a few have been introduced into the sphere of that wide and fascinating knowledge of Nature which has been so rapidly accumulated during this and the latter part of the last century. It is a full treasure house constantly being enriched, but unfortunately the few who have been initiated have soon acquired a technical language and habit, so that their knowledge and new acquisitions are communicated to but few. The public at large, not having the language nor an interpreter at hand, has come almost at once to a barrier which few have the time or patience to surmount.
Latterly it has become clear that the largest progress cannot be made if the knowledge of any branch of Science is confined to a few only. The most rapid advances have been made where many men are interested and enthusiastic. In no science should there be a difficult barrier between the amateur and the professional student. All Nature is equally open for everyone to study, and there should never be created obstacles as by the use of terminology not easily acquired by anyone. Of late these barriers have been in part broken down and competent students have written guides which anyone can follow, and soon begin to know the plants, trees, birds, insects, etc. So far no one has attempted to make the study of minerals and rocks so direct and simple that everyone can get a start. Most books on minerals, and practically all those on rocks are written for school courses, and to say the least chill any enthusiasm which is naturally aroused by the finding of interesting looking rocks or minerals.
The purpose of this book is first of all to provide a means of identifying minerals and rocks by such methods as are practical without elaborate equipment or previous training: and second to suggest the conditions under which the various minerals and rocks were formed, so that, at the first contact, one may get a conception of the events which have anteceded the mineral or rock which has been found. For this purpose keys have been worked out for determining the rocks and minerals by such obvious features as color, hardness, etc. Each mineral or rock is introduced by a summary of its characters, then the features by which it may be distinguished from any other similar mineral are given, after which its mode of origin and its meanings are considered. For those interested in the composition of the minerals, it is given in chemical symbols with each mineral. Most classifications of minerals are based on the composition, all the sulphides, carbonates, etc., being grouped together, but in this book, because the popular interest and commercial uses are primarily in the metal present, the minerals are grouped in each case about the chief metal, all the minerals of iron being grouped together, for instance.
A few minerals and rocks which are not strictly common have been included such as gems and meteorites; the gems because they are of intense interest to their owners and are often simply perfect examples of a fairly common mineral; and such forms as meteorites because it is important that, if one should run across one, it should be recognized, and so not lost to the world.
The book is freely illustrated, those minerals in which color is important for identification being illustrated in colors, and those which are black, or in which the color is not a determining factor, are shown in either photographic or outline figures.
In the introductory chapter there are explanations of the terms used in describing minerals, and of the systems in which they are grouped. A knowledge of the systems may not be a necessity, but it is a great help in determining minerals, and is very important in understanding why the individual minerals take the varied forms which are characteristic of them. These systems will be better understood after a few minerals have been gathered and examined.
It is hoped the book will help those who have already some knowledge of rocks and minerals, and especially that it will tempt many to begin an acquaintance with the rocks and minerals which are all about them, and are the foundation on which our material progress is built. Rocks and minerals have some advantages over most objects which are collected in that they neither require special preparation before they can be kept, nor do they deteriorate with time.
The author will appreciate corrections or suggestions as to better presentation of the material in this book.
F. B. L.
Amherst, Mass.
Why should one be interested in rocks and minerals? Because the whole world is made of rocks and minerals. They are the foundations on which we build. From them we draw all our metals, and the extent to which we utilize our minerals is a measure of the advance of our civilization. Fragments of rock are the soil from which, by way of the plants, we draw our food, and ultimately our life. The rocks make wild or gentle scenery, one at least of the sources of pleasure. Knowledge of rocks and minerals is then knowledge of fundamentals, of ultimate sources. Between finding the raw materials and their present uses there are usually many steps (so many that we forget that the beginning and end are united), as for instance in your watch. It is made of gold, brass, steel, agate, glass, and perhaps has luminous radium paint on the hands. It is a long way from finding and mining gold, chalcopyrite, hematite, carnotite, etc., through the raw materials, gold, copper, iron, etc., to the finished watch, but the minerals are the foundations of the watch; and it took centuries to find them and learn one by one how to use them, from the gold 10,000 years ago down to the radium within the last fifty years. Then too there is joy in going out into Nature’s wild and raw places, joy in being on the foundations of the earth, joy in the scenery, in the beauty of the minerals themselves.
But why collect the rocks and minerals? First because this is the way to know them. Both mineral and rocks require careful examination in order to see all those fine points by which they are distinguished. It is often necessary to compare one with another to get in mind the differences of form, color, streak, though with increasing familiarity these characteristics are recognized at first sight. It is the repeated examination which makes a rock tell the story of the country from which it came. Our first attempts to read the story give us only the most general facts. Nature’s book, written in the rocks, has to be read closely, often between the lines. Until we are used to the characters in which the words are written, we read slowly. When they look at Nature’s book, always open, most people do not read; for they do not know their letters. Every mineral is a letter, every rock a word, and we learn to read as we learn the minerals and rocks, and every time we go over them we get more facts coming out. The place where a rock or mineral occurs is of course the relation between them, and is involved in reading the story. No one today is a perfect reader. We are all learning to see more in the rocks day by day. So it is important to have the rocks and minerals where they can be handled and repeatedly examined, where we can turn to them in our leisure moments. Don’t stop when you have learned the name of a mineral or rock. You need more. See what it means. Secondly, minerals have beauties of form, color, and structure, and they do not fade. They will be as perfect in ten years as when found. We are all naturally crows, and love to gather the objects which interest us. It is not a bad habit, and only needs directing. Cultivate it. Have a hobby, and minerals and rocks are a good one; for they are like treasures in Heaven which “neither moth nor rust doth corrupt.” Not only will they give you pleasure, but they will be a constructive education, training the eye to see, and the mind to think straight. No one ever regretted the time and effort spent in collecting either minerals or rocks.
In order to make a collection valuable two or three rules must be observed. In the case of rocks, collect large enough samples so that they will be characteristic, and clear in their make-up. The standard size for rocks is 3 × 4 inches on top and one to two inches thick according to the nature of the rock. Tiny fragments do not give the character of the rock as well, and they are all the time getting into confusion. Every specimen should be labeled, with at least its name and the exact locality from which it came. Composition, structural features, associations, and classification may be added, the more the better; for each item adds to the information and interest of the specimen. One may make his own labels or have printed blanks, and may put as much care and art into the labels as desired, the more the better. One thing is very important and that is to have a number on the label with a corresponding one on the specimen, so that in case they should get separated, they may be readily brought together, even by one who is not familiar with the individual specimens. Lastly, give your collection as good a place as possible, either in drawers, boxes or in a case. The specimens are worth being kept in order and where they can be readily seen and compared. Nature is systematic, and there is a reason for the order in which rocks and minerals are taken up. It is desirable either that this order, or some one of the orders of Nature appear in the collection. In this book the metals are the basis of classification, all those minerals primarily related to one of the metals being grouped together.
In collecting minerals, the size of the specimens can not be so regularly followed, but it should be followed when collecting non-crystalline minerals, and when possible. Crystals however are chosen from a variety of points of view, as perfection of form, color, examples of cleavage, twinning, etc.; so that in many cases smaller or larger examples must appear in the collection. It is always desirable that as many variations of a rock or mineral as possible should appear in the collection, and in many cases examples of the matrix from which the crystals came. When crystals are tiny, it is well to place them in vials, that they may not be lost.
Where shall we start in making a collection? Near home. Get the local minerals and rocks first, and then range as widely as possible. The best places are bare and exposed rocks, especially where fresh and un-weathered surfaces are available. Quarries and where there has been blasting along roads offer fine opportunities. Fissures and cavities in the rocks are especially likely to have fine crystals, and in all localities continued search will reveal a surprising number of different minerals. The greatest variety occur in metamorphic rocks, or where igneous rocks come in contact with other rocks, but even the sedimentary rocks have a goodly range of minerals. All through the glaciated regions of the northern United States lie scattered boulders brought from afar, which will yield a surprising number of minerals and variety of rocks.
One may start with a very simple equipment, a geologist’s or stone mason’s hammer which can be obtained at any hardware store, being sufficient for field work. Rocks should be broken, so as to show fresh surfaces and to get below the disintegrating effects of weathering. At home one should have a streak plate (a piece of unglazed porcelain), a set of hardness minerals (see page 20), and a small bottle each of hydrochloric and nitric acid. A pocket lens is useful in order to see more clearly the form of small minerals. These things can be purchased of any Naturalist’s Supply Co., like Ward’s Natural Science Establishment, P.O. 24, Beachwood Sta., Rochester, N. Y., or the Kny-Scheerer Corp., 483 First Ave., New York City. Success depends upon a quick eye, and persistent hunting. When traveling, opportunities are offered at frequent intervals to see and get new specimens.
Be sure and see the meaning in each rock and mineral. The history of the country is revealed in its rocks and minerals. Note whether the rocks are horizontal or folded, whether they change character from place to place, or vertically. In going over a piece of country you may locate an ancient mountain system now leveled, by noting a series of metamorphic rocks, with a central core of granite, the roots of former mountains. Don’t be afraid to draw conclusions from what you see. Later, when the opportunity offers, look up the region in the geological folio, bulletin, or map of that section, and check up your findings. These geological folios and bulletins, of which there is one for nearly every region, are a great help to collectors in suggesting where to look for various rocks and minerals. Write to the Director of the U. S. Geological Survey, Washington, D. C., for a catalogue of the publications of the United States Survey, or find out from him what are the maps or folios for the region in which you are interested. These U. S. publications cost but little. When opportunity presents itself, visit other collections. In them you will see some of the minerals or rocks which have puzzled you, and there is nothing quite so satisfactory as seeing the rocks or minerals themselves. No description can always be so convincing. Then too you will get suggestions as to localities that you can visit.
As your collection grows, if you find you have special interest in one or another branch of the field, you can get books giving more details in that line; and at the back of this book will be found a list of such books.
All we know of the earth by direct observation is confined to less than four miles depth; though by projecting downward the layers of rock that come to the surface, we may fairly assume a knowledge of the structure down to six or eight miles depth. This outer portion is often referred to as the “crust of the earth,” but the idea that the deeper portions are molten is no longer held. This outer portion is made of rocks, and a rock may be defined as, a mass of material, loose or solid, which makes up an integral part of the earth, as granite, limestone, or sand. The rocks (except glassy igneous ones) are aggregates of one or more minerals; either in their original form like the quartz, feldspar and mica of granite, or in a secondary grouping, resulting from the units having been dislodged from their primary position and regrouped a second time, as in sandstone or clay.
Since the rocks are aggregates of minerals, it is best to take up the minerals first. A mineral may be defined as a natural inorganic substance of definite chemical composition. It is usually solid, generally has crystalline structure, and may or may not be bounded by crystal faces. A crystal is a mineral, bounded by symmetrically grouped faces, which have definite relationships to a set of imaginary lines called axes. There are between 1100 and 1200 minerals, of which 30 are so frequently present, and so dominant in making up the rocks, that they are termed rock-forming minerals. About 150 more occur frequently enough so that they can be termed common minerals, and one may expect to find a fairly large proportion of them. Some of these are abundant in one part of the country and rare in others, but this book is written to cover the United States, and so all those which have a fair abundance are included, though some will only be found in the west and others mostly in the east. Then there are some more minerals which are really rare, but which are cherished because of their beauty of color, and are used as gems. These are mentioned, and many of the gems are simply clear and beautiful examples of minerals, which in dark or cloudy forms are much more common. If one finds any of these rare minerals which are not mentioned in this book, he must turn to one of the larger mineralogies mentioned in the literature list to determine them.
A crystal is a mass of molecules, all of the same composition. A molecule in its turn is made up of atoms, and each atom is a unit mass of an element. Thus the calcite molecule is made up of one unit or atom of calcium, one of carbon, and three of oxygen (CaCO₃). These atoms are held together by an attraction, and make a molecule, and for the study of minerals the molecule is the unit. The mineral, calcite, is a mass of molecules all like the one above, and each molecule so small as to be invisible even with the aid of the most powerful microscope. When calcite is in crystal form, the molecules, like ranks of soldiers, are arranged each in its place, each at a definite distance from the other. While each molecule may vibrate or wiggle within certain limits it does not leave its place. (The comparison with soldiers is a good one for the molecules of one layer, but it must be remembered that in a crystal there are also like spacings and ranks up and down as well.) As long as the molecules remain in fixed ranks, up and down, forward and back, and sideways, the crystal is perfect. Calcite may be heated until it melts and becomes liquid. Then the molecules leave their definite arrangement and move about in all sorts of directions, like the soldiers after ranks have broken. So long as the molecules are thus free to move about but keep together, the substance is a liquid. There are cases when the molecules in this disorder take fixed positions without falling into ranks. Such minerals are non-crystalline and usually appear glassy. If still greater heat is applied to the mineral in liquid form, a point is reached (the vapor point), above which the molecules go flying away from each (like soldiers in a panic), each seeking to get as far from the other as possible, so only a container will prevent their dissipation. When in this condition a mineral is gaseous. When cooled, the reverse order obtains. The molecules of gas gather into a miscellaneous mob or liquid: and if this is further cooled (but not too suddenly), they fall into ranks and make a crystal. This may be illustrated with water. When above 212° F. it is steam (molecules wildly dissipated); when between 212° and 32° it is water (molecules close to each other, but milling like a herd of cattle); and when below 32° it is ice, the molecules ranged in perfect order, rank on rank.
With all the possible forms that crystals can and do take, there are six systems of arrangement. First there is the case where ranks, files, and vertical rows are all equal, and now to be scientific, instead of talking about ranks, files, etc., we use the term axes to express these ideas; the files or arrangements from front to back, being called the a axis, the ranks, or side to side arrangement the b axis, and the vertical arrangement the c axis. (See Plate 1.) These axes are imaginary lines, but they represent real forces.
When the axes are all equal and at right angles to each other, a crystal is said to be in the isometric system. The cube is the basal form and each side is known as a face. The ends of the axes come to the middle of the cube faces. The essential feature of this system is that whatever happens to one axis must happen to all, which is another way of saying that all the axes are equal. If we think of the cube as having the corners cut off, we would have a new face on each of the eight corners, in addition to the six cube faces. Then if each of these new faces were enlarged until they met and obliterated the cube faces, an eight-sided figure, the octahedron, would result. In this the axes would ran to the corners. Another modification of the cube would be to bevel each of its twelve edges, making twelve new faces in addition to the six cube faces. If we think of these new faces being developed until they meet and obliterate the cube faces, there will result a twelve-sided figure, the dodecahedron. And the 24 edges of the dodecahedron could be beveled to make a 24-sided figure, and so on. Of course in Nature the corners are not cut, nor the edges beveled, but as a result of the interaction of the forces expressed by the axes and the distribution of the molecules, the molecules arrange themselves in a cube, octahedron, dodecahedron or combination of these basal forms.
Crystals are formed in liquids as they cool or evaporate and can no longer hold the minerals in solution. Crystals start about a center or nucleus, and molecule by molecule, the orderly arrangement is increased and the crystal grows, there being no size which is characteristic. If free in the liquid the crystal grows perfectly on all sides, but if crystals are growing side by side, there comes a time when they interfere with each other. Then the free faces continue to grow and the orderly internal arrangement is maintained, though externally there is interference.
In the second or tetragonal system one axis (the c axis) is different from the other two, but all three are still at right angles with each other. This is saying scientifically that the lines of force are greater or less in one direction than in the other two, but they act at right angles to each other. The a and the b axes are equal and anything that happens to one of these two must happen to the other, but need not happen to the c axis. Thinking of the molecules that arrange themselves under this system of forces, it is clear that the simplest form will be a square prism, i.e., front to back, and from side to side the numbers of molecules will be equal, but up and down there will be a greater or lesser number. If the eight corners of this prism were cut, and these corner faces increased in size until they met, the resulting octahedron would be longer (or shorter) from top to bottom than from side to side or front to back, but the measurement from front to back would be equal to the one from side to side. In this system we may have the vertical edges of the prism beveled, and not have to bevel the horizontal ones, or we may bevel the horizontal edges and not the vertical ones. There is no dodecahedron in this system or in any other system than the isometric. The forms in this tetrahedral system are really a combination of the four sides of the square prism with such modifications as equally affect them all, with two ends which may be flat, or pyramidal, or modified pyramidal faces.
The third system has all three axes unequal, but all three are still at right angles with each other. This is saying that the lines of force in the crystals are all at right angles to each other but of unequal value. The faces in this case are all in pairs. What happens at one end of an axis must happen at the opposite end, but does not need to happen at the ends of any of the other axes. We are dealing with pairs of faces (one at either end of an axis), and if three such pairs are combined in the simplest manner, the resulting figure will be a rectangular prism. If we cut the eight corners of this prism and enlarge the faces until they meet, the result is an octahedron, in which the distance from top to bottom, from side to side, or from front to back is not the same in any two cases. (See Plate 2.) In this system if a face is made by beveling one edge of the prism there must be a corresponding face on the edge diagonally opposite, but there does not have to be one on any of the other edges. However if a corner is cut, that face affects all the axes and so all the corners must be cut. A great many crystals occur in this system, and some of them which are prismatic in shape may give trouble, for it is not uncommon for the vertical edges of the prism to be so beveled, that two of the original prism faces are obliterated, and the two remaining faces added to the four new faces make a six-sided prism, which at first glance seems to belong to the hexagonal system. (See Plate 3, fig. 3.) Close examination however will show that, instead of all the prism faces being alike, as would be necessary for the hexagonal system, they are really in pairs, and one pair at least will be distinguished in some way, such as being striated, pitted, or duller.
The fourth system has all the axes unequal, the a axis and the b axis at right angles to each other, but the c axis is inclined to the a axis, meeting it at some other than a right angle. The monoclinic system is like the orthorhombic system except that it leans, or is askew, in one direction. The result is that the faces at the ends of the b axis are rhombohedral, while the others are rectangular. As in the foregoing system, the faces are in pairs at opposite ends of the axes; and as in the orthorhombic system, a face may occur on one edge and only have to be repeated on the edge diagonally opposite. The simplest form in this system will be made by combining the three pairs of faces at the opposite ends of the axes, which gives a prism, which is rectangular in cross section, but leans backward (or forward) if placed on end. As in all the systems, if a corner is cut, all must be cut; and if these corner faces are extended to meet each other, an octahedron results, in which, as in the prism, no two axes are equal. If this octahedron is properly orientated (i.e. with the a and b axes horizontal), it will lean forward or backward. Many minerals belong to this system; and, as in the orthorhombic system, it is not uncommon to have the vertical edges so beveled that two of the prism faces are obliterated, and the remaining two prism faces with the four new faces make a six-sided prism, which seems hexagonal. (See plate 3, figure 3.) However, such a pseudo-hexagonal prism may be recognized by at least one pair of the faces having distinguishing marks (striæ, pits, or dullness), instead of all being just alike.
The fifth or triclinic system has all the axes unequal, and no two of them intersect at right angles. As in the two preceding systems the faces occur in pairs at the opposite ends of the axes. This is the most difficult system in which to orientate a crystal, but fortunately only a few crystals occur in this system, such as the feldspars.
Lastly there is a group of crystals which have four axes, one vertical, and three in the horizontal plane which intersect each other at angles of 60°, all these three being equal to each other, but different from the vertical axis. The simplest form in this system is the six-sided prism. If one corner of this prism is cut all must be, and if these corner faces are extended to meet each other, a double-six-sided pyramid results. In this system if one of the vertical edges of the prism is beveled, all must be, but the horizontal edges need not be; or the horizontal edges may be beveled and the vertical ones not. The ends as they are related to the c axis may be developed independently of the prism, and so the prism may be simply truncated by a flat end, or have pyramids on either end.
In this system it is quite common to have forms which result from the development of each alternate face of either the prism or the double pyramid. In the case of the prism, if every alternate face is developed (and the others omitted) a three-sided prism results, as in tourmaline. In the case of the double pyramid if the three alternate faces above are united with the three alternate faces below, a six-sided figure is formed, which is known as the rhombohedron, as all the faces are rhombohedral in out-line and all equal. These forms in which only half the faces are developed are known as hemihedral forms. The same sort of thing may happen in the isometric system in the case of the octahedron, and also in the case of the octahedron of other systems. When half the faces of the octahedron are developed, two above unite with two below and make a four-sided figure, known as a tetrahedron. (See plate 10.) While tetrahedrons may occur in any of the first five systems they are not common outside the isometric system.
Another modification of the simple forms which will be met occasionally is twinning. By this is meant two crystals growing together as though placed side by side on some one of the faces, and then revolved until the two axes which would normally be parallel are at some definite angle with each other, 60°, or 180° which is commoner. The surface of contact between the two crystals is called the composition face, and as no more material can be added on that face the crystals continue to grow developing the other faces, and we find faces in contact with each other which should be at the opposite end or other side of the crystals. This contact of faces which should not come in contact, and the presence of reentrant angles are indications of twinning. In some minerals the twinning may be repeated time and again, and if the twinning is on one of the end faces a branching structure results, as in frost and snow crystals, or the multiple twinning may be of crystals growing side by side when the final form will approximate a series of thin sheets placed side by side as in some feldspars. The peculiar forms characteristic of individual minerals are taken up under the respective minerals.
Other important properties of minerals are hardness, cleavage, specific gravity, streak, luster, and color.
Hardness may be defined as the mineral’s resistance to abrasion or scratching. It is measured by comparing a mineral with Moh’s scale, a set of ten minerals arranged in the order of increasing hardness, as follows: