means of indicators. Small recognizable crystals of the following minerals may be kept at hand as a set of indicators: gypsum (sp. gr. 2·32), colemanite (2·42), orthoclase (2·56), quartz (2·65), calcite (2·72), aragonite (2·93), rubellite (3·02), apatite (3·20), dioptase (3·32), &c. With a series of tubes containing mixtures of methylene iodide and benzene of different densities and suitable indicators, specific gravities may be rapidly and accurately determined. Values intermediate between those of the indicators may be estimated by a diffusion column of the liquid, or by noting the rate at which the benzene evaporates and the specific gravity of the liquid increases. For use with minerals of specific gravity greater than 3·33 various other heavy liquids have been suggested; the best being thallium silver nitrate (TlAg(NO3)2), which melts at 75° C. to a clear liquid with a density of 4·8, and is miscible with water.
e. Touch, Taste and Smell.—In their action on the senses of touch, taste and smell a few minerals possess distinctive characters. Talc is unctuous or soapy to the touch; tripolite and trachyte are respectively meagre and harsh. Some porous minerals (e.g. clays and hydrophane) adhere to the tongue. Gem-stones may often be distinguished from their glass imitation by the fact that they feel colder, since they are better conductors of heat. Bitumen and clays, when moistened, have a characteristic smell; pyrites and some other sulphides when rubbed emit a sulphurous odour. Minerals which are soluble in water have taste: e.g. saline (salt), alkaline (natron), bitter (epsomite), astringent (chalcanthite), &c.
3. Chemical Characters.
Chemical composition is the most important character of minerals, and on it all modern systems of classification are based. A mineral-species cannot, however, be defined by chemical composition alone, since many instances are known in which the same chemical element or compound is dimorphous or polymorphous (see Crystallography). Thus both the minerals diamond and graphite consist of the element carbon; both calcite and aragonite consist of calcium carbonate; and rutile, anatase and brookite consist of titanium dioxide. In such cases a knowledge of some other essential character, preferably the crystalline form, is necessary, before the mineral can be determined.,
All the known chemical elements have been found in minerals; and of many of them minerals are the only source. On the other hand, nitrogen, which is frequently present in organic substances, is rare in minerals; carbon has a wide distribution in mineral carbonates. It is estimated that the minerals of the earth's crust consist of about 47% by weight of oxygen, 27 of silicon and 8 of aluminium; silicates, and especially aluminosilicates, therefore predominate, these being the more important rock-forming minerals.
The chemical composition of minerals is determined by the ordinary methods of analytical chemistry. Since, however, minerals of different kinds usually occur intimately associated, it is often a matter of some difficulty to select a sufficiency of pure material for analysis. For this reason the exact composition and the empirical formulae of several minerals, particularly amongst the silicates, still remain doubtful. There are even cases on record in which the chemical composition and the crystalline form have been determined on different materials in the, belief that they were the same. Whenever possible, therefore, the chemical analysis should be made on small pure crystals which have been previously determined crystallographically. For the qualitative chemical examination of minerals, when only a small amount of material is available, the methods of blowpipe analysis and microchemical analysis are often convenient. (See G. J. Brush, Manual of Determinative Mineralogy, 16th ed., by S. L. Penfield, New York, 1903; H. Behrens, Manual of Microchemical Analysis, London, 1894.)
The principle of isomorphism (see Crystallography) is of the highest importance in mineralogy, and on it the classification of minerals largely depends. In some minerals (e.g. quartz) isomorphous or vicarious replacement is not known to occur; but in the majority of minerals one or other of the predominating elements (generally the base, rarely that of the acid radicle) may be isomorphously replaced by equivalent amounts of other chemically-related elements. In some isomorphous groups of minerals replacement takes place to only a limited extent, and the element which is partly replaced always predominates; while in other groups the replacement may be indefinite in extent, and between the ends of the series the different members may vary indefinitely in composition, with no sharp demarcation between species. Thus in the group of rhombohedral carbonates the different species are usually sharply defined. In well-formed crystals of calcite the calcium is replaced by only small amounts of magnesium, iron, lead, &c.; in chalybite, however, iron is often more largely replaced by calcium, magnesium, manganese, &c., and the “brown spars” are not always readily distinguishable. In the dimorphous group of orthorhombic carbonates isomorphous replacement is less frequent, and the different species (aragonite, cerussite, &c.) are quite sharply defined. In other groups of minerals, particularly amongst the silicates, isomorphous replacement of the basic elements is so general that the several members of the series vary almost indefinitely in chemical composition, and will scarcely be the same for any two specimens, though it may be reduced to the same type of formula. For example, the formula of all varieties of garnet may be expressed generally as R″ 3R‴2(SiO4)3, where R″= Ca, Mg, Fe, Mn, and R‴= Al, Fe, Mn, Cr, Ti. Tourmaline affords another good example. In the plagioclase felspars (see Plagioclase) we have an example of the isomorphous mixing of two end-members, albite (NaAlSi3O8) and anorthite (CaAl2(SiO4)2) in all proportions and with no sharp line between the several subspecies. In some other similar cases the end-members of the series are purely hypothetical: e.g. in the scapolite group (mixtures of Ca4A16Si6O25 and Na4Al3Si9O24Cl) and in the micas and chlorites. In such instances, where the formulae of the two end-members differ in type, “mass effect” may have some influence on the isomorphism.
In addition to isomorphous series, there are amongst minerals several instances of double salts, which contain the same constituents as the members of isomorphous series: e.g. dolomite (q.v.) and barytocalcite (q.v.).
The manner in which water enters into the composition of minerals is often difficult to determine. In some cases, e.g. in the zeolites (q.v.), it is readily expelled at a low temperature, even at the ordinary temperature over sulphuric acid, and may be reabsorbed from a moist atmosphere or replaced by some other substances: it is then regarded as “water of crystallization.” In other cases, when expelled only at a higher temperature, it is to be regarded as “water of constitution,” forming either a basic salt (e.g. malachite, Cu(OH)2CO3) or an acid salt (e.g. dioptase, H2CuSiO4, and mica, q.v.). When present as hydroxyl it is often isomorphously replaced by fluorine (e.g. topaz, [Al(F,OH)]2SiO4). Sometimes the water is partly water of crystallization and partly water of constitution.
As to the actual chemical constitution of minerals the little that is at present known is mainly speculative. Dimorphous minerals, which have the same empirical formula may be expected to differ in constitution; and experiments have been made, for example on pyrites and marcasite, with the object of discovering a difference, but the conclusions of various investigators are not in agreement. More promising results have been obtained (by F. W. Clarke and others) by the action of various reagents on silicates, particularly on the more readily decomposed zeolites, and several substitution-derivatives have been prepared.
Synthesis of Minerals.—The production of minerals by artificial means is a branch of chemical mineralogy which has been pursued with considerable success, especially by French chemists. Most minerals have been obtained artificially in a crystallized condition, and many related compounds, not as yet found in nature, have also been prepared. Crystals of artificially prepared minerals, though usually quite small in size, possess all the essential characters of natural crystals, differing from these only in origin. The following are the principles of some of the
