cases be unstable under another set of conditions. The crystals
may then be corroded or even completely redissolved, or the
substance may undergo a chemical or physical change and give
rise to the formation of other minerals which are stable under
the new conditions. The results of such changes and alterations
of minerals are very frequently to be observed in nature, and
several instances have already been cited in the preceding
section. A good example of the secondary products which may
result by the decomposition of a mineral is afforded by pyrites
(FeS2), of which two types of alteration may be distinguished.
By oxidation in the presence of pure water it gives rise to ferrous
sulphate (melanterite), free sulphur and sulphuric acid; the
melanterite by further alteration gives various basic ferric
sulphates (copiapite, &c.); and the sulphuric acid by acting
on surrounding rocks (limestone, clay, &c.) gives rise to the
formation of gypsum, aluminite and other sulphates. By
the action of water containing oxygen and calcium carbonate
in solution, pyrites suffers another kind of alteration:
the sulphur is carried away in solution as gypsum and the iron is
left behind as a ferric hydroxide (limonite) which preserves
the original form of the crystals. We have then a pseudomorph
(from ψευδής, false and μορφή, form) of limonite
after pyrites; i.e. limonite with the external form of a crystal of
pyrites.
Pseudomorphs are frequently met with in nature, and they are of considerable importance in studying the changes which minerals undergo. Several kinds of pseudomorphs are to be distinguished. When the alteration has involved no change in chemical composition of the material, but only in the internal crystalline structure and physical properties, the altered crystal is called a “paramorph.” For example, crystals of aragonite are often altered to a confused granular aggregate of crystalline individuals of calcite, the change being accompanied by an increase in specific gravity but without change in external form: such a change may be effected artificially by simply heating a crystal of aragonite. Other examples of paramorphs are rutile with the form of anatase, and hornblende with the form of augite. An “epimorph” results from the encrustation of one mineral by another; the first may be afterwards partly or wholly dissolved out, leaving the second as a hollow shell (e.g. chalybite after fluor-spar). As instances of pseudomorphs in which there has been some chemical change the following may be cited: by the gain of chemical constituents, e.g. malachite after cuprite; by the loss of material, e.g. native copper after cuprite; or by an interchange of constituents, e.g. galena after pyromorphite and limonite after pyrites. In other cases there may be no evident chemical relationship between the two minerals, as, for example, in pseudomorphs of native copper after aragonite or quartz after calcite. Different minerals may also take the form of various organic remains.
III.—Nomenclature and Classification of Minerals.
A mineral species, or simple mineral, is completely defined by the statement of its chemical composition and crystalline form. When we are dealing with a definite chemical compound the limitation of species is easy enough; thus corundum, cassiterite, galena, blende, &c. are quite sharply defined mineral species. But with isomorphous mixtures the division into species, or into sub-species and varieties, must be to a certain extent arbitrary, there being no sharp lines of demarcation in many isomorphous groups of minerals. Thus in the mineral tourmaline the chemical composition varies indefinitely between wide limits, but no corresponding difference can be traced in the crystalline form or in the external characters save colour and specific gravity. Some authors have therefore questioned the advisability of separating minerals into species each with distinctive names, and they have attempted to devise chemical names for the different kinds of minerals. Owing, however, to the frequency of polymorphism and isomorphism amongst mineral substances such a system presents many practical difficulties. Thus the three modifications of titanium dioxide are more simply and conveniently referred to as rutile, anatase and brookite, while to give a purely chemical designation to such a mineral as tourmaline would be quite impracticable. Further, later investigations often show that such chemical names require revision, and hence confusion may arise.
The practice of giving distinct names to different kinds of minerals dates from very early times (e.g. diamond). The common termination ite (originally itis or ites) was adopted by the Greeks and Romans for the names of stones, the names themselves indicating some character, constituent, or use of the stone, or the locality at which it was found. For example, haematite, because of the blood-red colour; siderite, containing iron; alabaster (originally alabastritis), a stone from which a vessel called an alabastron was cut; magnetite, from the locality Magnesia. The custom of naming minerals after persons is of modern origin; e.g. prehnite, biotite, haüyne, zoisite. Unfortunately there is a lack in uniformity in the termination of mineral names, many long-established names being without the termination ite, e.g. beryl, blende, felspar, garnet, gypsum, quartz, zircon, &c. The termination ine is also often used, e.g. nepheline, olivine, serpentine, tourmaline, &c.; and many others were introduced by R. J. Haüy without much reason, e.g. anatase, dioptase, epidote, analcime, sphene, &c. (see A. H. Chester, A Dictionary of the Names of Minerals, New York, 1896).
The number of known mineral species differs, of course, according to different authors; roughly there may be said to be about a thousand. The total number of mineral names (apart from chemical names), many of them being applied to trivial varieties or given in error, amount to about 5000.
Minerals may be classified in different ways to suit different purposes; thus they may be classified according to their uses, modes of occurrence, system of crystallization, &c. The earlier systematic classifications, being based solely on the external characters of minerals, were on natural history principles and too artificial to be of any value. J. J. Berzelius, in 1815, was the first to propose a purely chemical system of classification: his primary divisions depended on the basic (electro-positive) element and the sub-divisions on the acid (electro-negative) element. Such a method of classification, though still in use for metallic ores, must be quite arbitrary or give rise to much duplication; since, apart from isomorphous replacement, many minerals contain more than one metal. The systematic classifications in use at the present day are modifications in detail of the crystallo-chemical system published by G. Rose in 1852. Here there are four main divisions, viz. elements; sulphides, arsenides, &c.; halogen compounds; and oxygen compounds: the last, and largest, division is subdivided into oxides and according to the acid (carbonates, silicates, sulphates and chromates, phosphates and arsenates, &c.); in each section isomorphous minerals are grouped together. The classifications adopted by different authors differ much in detail, especially in the large section of the silicates, which presents many difficulties and for which no satisfactory classification has yet been devised.
As an example of a systematic classification of minerals the following may be given. Except in a few details it is the classification of Dana’s System of Mineralogy (6th ed., 1892). Only those minerals which are described under their respective headings in these volumes are included: the list therefore serves, at the same time, as an enumeration of the more common and important species and varieties of minerals, and as a system of classification it is necessarily incomplete. Species belonging to the same isomorphous group are bracketed together: varieties are given in parentheses after the species to which they belong. The chemical composition of each species is given by the formula; and the crystal-system by the initial letters C (cubic), T (tetragonal), O (orthorhombic), M (monoclinic), A (anorthic), H (hexagonal) and R (rhombohedral): when the crystal class is definitely known to be some other than the holosymmetric this is indicated by a number corresponding to those used in the article Crystallography, e.g. C2 for the tetrahedral class of the cubic system.
