Popular Science Monthly/Volume 32/April 1888/The Present Status of Mineralogy
By Professor F. W. CLARKE.
IN the study of any branch of science it is well to pause occasionally, that we may look about us, see where we are, what we are doing, and what we had better do. For that which distinguishes science from empirical knowledge is its unity of purpose, its coherence, and its definite relation of part to part; and these features develop best when attention is temporarily withdrawn from the details of special research. As a science growls, and increases in complexity, the individual worker must confine himself more and more to particular investigations; these, to him, assume undue importance, and their higher significance as part of a broad general field is ignored or lost. The petty details are essential; but, incoördinated, they make not science, but chaos. The scattered bricks are good material, but they must be brought together into one symmetrical structure.
These remarks are particularly true of a concrete science like mineralogy. Here we have a branch of knowledge which rests upon the observation of material facts; and which, hitherto, has owed little to abstract reasoning. Ii has grown up, partly as a "natural" science, partly as an outlying division of chemistry; and hypothesis has had little to do with its upbuilding. The mineralogist collects, observes, describes, and classifies species as he finds them, determines their mode of occurrence, chemical composition, and physical properties; and then, too often, considers his work finished, except as regards the gathering of more data of like kind, with possible refinements of method. The relations and bearing of mineralogy toward other sciences have been, with rare exceptions, slighted; and a general theory of its nature and purpose has hardly been considered at all.
Of late years, however, an improvement has been noticeable. New lines of investigation are open, new modes of thought are recognized, and philosophical treatment is in order. In mineralogy, as in most other sciences, a central stem of growth is perceivable, around which the facts are grouping themselves to obvious advantage, Lithology (which mainly deals with the association of minerals), the study of pseudomorphs and alterations, and the synthesis of many mineral species, all furnish lines of evidence which converge toward certain general conceptions dominating the entire subject, and linking it intimately with other divisions of scientific thought. The discovery of new species is no longer the main object of the mineralogist, who is learning to look upon the correlation of known minerals as much more important.
In the study of minerals as such, apart from their geological relations, two main avenues of research are followed: First, the physical method, which is now mainly devoted to morphological considerations; and, secondly, the chemical method, which discusses composition. Philosophically regarded, the former method is subordinate to the latter, for physical properties, including form, are undoubtedly functions of chemical structure, which alone is fundamental and determining. A mineral species is best denoted as a definite chemical substance occurring in the crust of the earth; and its integrity depends upon the sharpness with which its constitution can be made out. In any given case, the claims of a species to recognition depend upon definiteness of composition, together with, in less degree, definiteness of form. The latter consideration, however, is only approximately general; for in some cases it can not be determined, and some minerals are amorphous; whereas the former test applies without exception. Crystalline form is but one property of a substance—more important to the mineralogist, doubtless, than any other physical condition, yet governed ultimately by chemical determinations. The nature of the substance is the one fundamental fact in the description of any mineral species.
It is not always easy, however, to prove definiteness of composition. A mineral may, to all outward appearance, be uniform in texture, and yet, seen in thin section under the microscope, it may be found to contain several different things. The microscope, therefore, with its adjunct the polariscope, is an important weapon in the hands of the mineralogist. By its aid he determines the mechanical purity of a given specimen; and upon examination under polarized light he can tell something as to its crystalline system, even though distinct crystals as such may not be visible. Few minerals are wholly free from mechanical admixtures, which complicate analysis, and vitiate the conclusions drawn from analytical results. Proof of homogeneity is an essential datum in the establishment of a mineral species. Many a supposed species has been overthrown by the microscope.
But a mineral may be apparently homogeneous, and yet indefinite chemically; for, apart from mere impurities which are unrecognizable by physical means, there are modes of admixture even more difficult to determine. Two distinct compounds may crystallize together in varying proportions, so as to yield definite forms which are, to all physical tests, perfectly homogeneous. Such mixtures of "isomorphous" substances are almost infinite in number; they are among the commonest occurrences in Nature; and they complicate the mineralogic problem enormously. Theoretically, a species is easily defined; practically, the definition is most troublesome. Oftentimes all the members of an isomorphous group are regarded as one species, in which certain analogous elements are said to "replace" each other. Iron and alumina are thus mutually replaceable; so are the oxides of the magnesia group; so also are sodium and potassium. But this usage, though common and sanctioned by weighty authorities, is not rigidly scientific. It is allowable conventionally, but only so long as we do not lose sight of what it really means. The so-called replacement is in reality a phenomenon of mixture between isomorphous salts of allied metals or acids, which salts are the true, definite species. For example, garnet varies in composition in just this peculiar way, and six or more compounds, all different but similar, are represented in it. Sometimes we find one of these compounds nearly pure; but oftener two or more exist in a given crystal. Garnet, therefore, is not one species, but a group, and should be so treated. A mixture is a mixture, whether visibly so or not, and has no title to specific naming. On a systematic basis the current policy needs modification; for it varies too widely and can not be universally applied. Seeking to evade one set of difficulties, it creates new ones.
In consequence of the tendency toward mixture among species, and of the wide-spread fashion of regarding the crystal as the mineralogical unit, there has grown up a general belief that minerals are in most cases very complex chemically. Some species, undoubtedly, are quite simple, like quartz, fluor-spar, or calcite; but others, especially among the silicates, appear to be most complicated, and even variable, in composition. This complexity, which is in great part due to the influences already mentioned, is perhaps apparent rather than real. Mixtures, whether crystalline or mechanical, can hardly be given either simple or definite chemical formulæ. The true individual units are probably not very complex, for their modes of origin favor simplicity. A complex molecule is likely to be unstable—the more complex, the more unstable; while minerals seem to be generated under conditions adverse to instability. Some have been deposited from solutions in which many reactions were possible; others originate under conditions of high temperature; and in either case only the more stable compounds are likely to be formed. Simplicity of chemical structure is therefore to be presumed; and the reverse should be the exception rather than the rule. Great complexity may sometimes exist; but in most cases it may be traced to the commingling of isomorphous bodies, or to impurities which have been overlooked.
Suppose, now, that for a given mineral the true chemical composition has been made out and expressed in a definite formula. All errors due to interminglings of other bodies may have been eliminated, and yet something still remains to be done before we can truly understand the nature of the substance. Here we must draw from the stock of conceptions furnished to science by organic chemistry. Two or more compounds, identical in percentage composition, may be widely different in other respects. Among organic compounds this fact is one of the commonplaces, but among minerals it is rarer and less easily explained. For example, calcite and aragonite, differing in crystalline form and in physical properties, are alike in composition, both being simply carbonate of lime. The differences lie within the molecule, and arise from the fact that the atoms are differently grouped or arranged. Partly from physical evidence, and partly from ultimate composition, the organic chemist infers the number of atoms in an organic molecule, and by a study of the changes which a substance can undergo he draws conclusions as to the position of these atoms with reference to each other. These conclusions are expressed in terms of chemical structure. By reasoning, too special for review here, he accounts for the differences between two "isomeric" compounds, and by means of "structural formulæ" he symbolizes the relations of each to the other. Can similar reasoning be applied to mineral species?
It is easy enough to devise structural formulæ, even with all the limitations which chemical science imposes. Given a certain number of atoms, built up into a molecule, and we can represent them as arranged in a variety of ways. But, to have value, that way must be chosen which shall represent the relations of the substance under consideration to other substances, and which shall, therefore, fulfill a definite scientific purpose in the interpretation of known facts. Formulæ so devised are of great utility; they shed much light upon the changes which bodies undergo, and upon their possible modes of generation; and this they do independently of all speculative considerations as to their ultimate meaning. The simpler chemical formulæ express composition only; the structural formulæ indicate function also. The latter, equally with the former, is essential to the discussion of our fundamental problem, namely, to determine the nature of the substances with which we deal.
At the present moment mineralogy is just entering upon this higher field of chemical study. Some mineralogists are vaguely distrustful of the new structural conceptions; some are indifferent to them; others are openly opposed. The latter, however, mostly belong to a school of thought which, chemically speaking, is obsolescent; and by refusing to accept the later notation they bar the doors of progress against themselves. They discover details, but they develop no principles. A reasonable distrust of novelty, however, is always legitimate; and the question may fairly be raised whether the methods of reasoning which are valid in organic chemistry can safely be applied to mineralogic research. The organic chemist deals-with compounds for which the starting-points are simple and well known; in many cases he can determine molecular weights with ease; and his material is so plastic that it can be altered, built up, or transferred in readily traceable ways. Every step in his processes can be followed, and his results may be checked from many sides. Minerals, on the other hand, are hard and stubborn; they form slowly and change with difficulty; they can not be handled as systematically as their organic analogues, and the evidence concerning their chemical structure is therefore less complete and convincing. Still, the case is not quite hopeless, and much positive work may be done.
Just at this point the main lines of mineralogic investigation seem to converge toward the central stem of growth. Leaving out of account mere questions of descriptive detail, the raw material of scientific thought, we may consider three great divisions of study which touch the problem of chemical structure. First of all, we have the branch of associative mineralogy. Minerals do not occur together at random, in all conceivable groupings, but only in accordance with definite laws which are now subjects for investigation. We can not clearly formulate these laws as yet, but we are learning much about them empirically; so that in many cases, upon finding one species, we instinctively look for certain others, which we are quite sure must exist with or near it. Some minerals are found in granite veins, some in volcanic rocks, and some only in ore-bodies, and each one may be evidence for its neighbors. The chief work of the lithologist is in a limited portion of this field; for he considers the minerals which are aggregated into rock-masses, which latter represent definite and frequently recurring associations exhibited upon a large scale. The very classification of the rocks is based upon their mineralogical characteristics. Lithology, however, takes into account only a small minority of known species.
Now each well-established group of mineral associates indicates something relative to their origin. It represents the collective conditions under which they came into existence, and points distinctly toward the chemical reactions which formed them. If we study any one locality closely, we shall discover some details of curious significance. Some minerals occur enveloped by, inclosed in, or implanted upon others; some line cavities, and some represent incompleted processes. We see clearly that one was formed before or after another; we trace out the left-over material which was last deposited; we find secondary growths built up from more primitive substances. Throughout we gather evidence bearing upon the life-history of each mineral, and this may be directly correlated with the conception of chemical structure. When we can determine the conditions under which a compound can be formed, we shall have made a long step forward in understanding the nature of the substance.
The second of our three lines of investigation is closely allied to the first, and, indeed, overlaps it somewhat. It is the study of alterations. A mineral has not only an origin and growth, but also a process of decay, during which its material, disintegrated, is made over into new forms. It is very common to find a crystal with its nucleus unchanged, and its surface transformed into some other species. Some of these alterations are easy to understand; as when, by oxidation, a cube of iron pyrites becomes a cube of the brown hydroxide, limonite; or when an arseniate or sulphate is derived from an arsenide or sulphide. Other changes, however, are less simple, such as the transformation of topaz into mica, or of corundum into margarite; but all of them tell something as to the nature of the substance altered, and help to elucidate the problems of structure and function. An alteration product is the record of a chemical reaction, which may be traced and reasoned about; and the evidence which it offers is quite analogous to that used by the organic chemist to determine the structure of a carbon compound. In the latter case alteration products—that is, derivatives—are produced artificially; in the former the mineralogist finds them ready formed in Nature. Unfortunately, however, such alteration products are not attractive specimens; and the ordinary collector throws them aside as worthless. An altered crystal has lost its perfection and beauty, and is variable only for what it signifies. But, from a scientific point of view, its value is real and considerable, if only it be studied thoroughly, apart from superficial appearances, and without jumping at conclusions. Here, again, the microscope and the chemical analysis are necessary coadjutors.
One line of research yet remains to be considered. The two already disposed of deal with material as gathered in the field; the third is an affair of the laboratory. Of late years many mineralogists have been actively at work upon the synthesis of mineral?, building up their crystals by artificial means, and reproducing in a rapid way the slower processes of Nature. Many species have thus been formed in well-defined condition; and other compounds, different from but closely analogous to well-known minerals, have also been produced. Every year there are great advances in this field of work, and every step which is taken is in the direction of the main problem. Sometimes results are attained by methods unlike those which grew the native crystal; but even then new light is shed upon its nature, and we know more of its possible modes of genesis. Some experiments also bear upon the subject of alterations; and definite alteration products are artificially obtained. So far, little has been done toward generalizing upon this class of observations; but, as the facts accumulate, new relations will appear and reasoning must follow. Each experiment suggests new experiments; each discovery points toward others, and the connecting theories will grow up around the conception of chemical structure. It is the only conception yet clearly recognized which is general enough to cover the whole field.
It has already been argued that the physical study of minerals is subordinate to their chemical investigation, for the reason that all properties depend upon composition. Physical researches, nevertheless, have great value in mineralogy, and a paper under the caption of this essay would be wretchedly incomplete if it failed to consider them. Physical data, moreover, aid in the discussion of chemical structure, and point out analogies of weighty significance. Specific gravity, for instance, is always an important datum in the study of a species; and the ratio between it and the molecular weight of a compound tells us something of the condensation which the elementary material has undergone in combining. One eminent mineralogist is now using this ratio as a basis for mineral classification, especially among the silicates; and his results are likely to emphasize the conclusions drawn from quite different sources. This method of study, however, presupposes a knowledge of true chemical composition. With the latter it means much; alone it signifies little.
Upon the thermal and electrical properties of minerals comparatively little has been done; and that little has slight reference to mineralogy in general. The optical constants, on the other hand, are elaborately studied by mineralogists, on account of their direct relations to crystalline form. Indeed, optical and crystallographic work is a dominant feature of modern mineralogical investigation, although a great part of it never rises above the plane of mere descriptive detail. In its higher aspect it deals with the internal molecular structure of crystals, and so furnishes data which may some day be connected with the broader general conceptions of the chemical field of research. The question of how the atoms are grouped has a mechanical as well as a chemical side; and some time it will be systematically attacked from both directions. At present we only see the future possibility of so handling the physical evidence; but the expectation is philosophically just. To-day a knowledge of crystalline form is mainly useful in the identification of minerals; for by it we may determine a species without destroying the specimen; but its deeper potential significance is none the less apparent. Along these lines we may safely prophesy progress, which can only end in the complete correlation of all mineralogic facts, and therefore in the solution of the fundamental problem.
Looked at from the descriptive side alone, mineralogy is a small affair. Only about a thousand species are known, and one large volume may fairly cover the field. It is when we consider the mineral as a growth—as a body having a past and a future—that broad treatment of the subject becomes possible. The geologist, dealing with phenomena of the grandest character, sees at a first glance little that is attractive in mineralogy. He forgets that mineral species make his alphabet, and that upon their properties the properties of rock-masses must depend. He can not safely generalize upon the one without knowing something of the other. He can not understand the chemical changes occurring in the earth's crust, if he ignores the separate units and the reactions of which they are capable. The very genesis of many rocks must depend upon the conditions under which their individual units can concurrently exist, and the latter must be known before the larger question can be adequately handled. Mineralogy gives to the geologist the weightiest of evidence. To the chemist also it is something more than debtor. It gives him, ready made, whole groups of compounds which else would be difficultly attainable, and these are the starting-points for many lines of research. The true character of each science is best seen in the interaction of all the sciences. Each in its way is both servant and master; not one can stand wholly alone.