Mineralogy

Mineralogy is the science of minerals. A mineral may be defined as a natural, homogeneous, inorganic substance, this definition excluding the artificial compounds of the laboratory, heterogeueous substances (such as are many rocks), and organic substances (such as pearl, amber,'or coal). Most minerals, though containing inconstant impurities, have a definite chemical composition, expressible in a formula; and, though often, occurring in indefinite ungeometrical shapes, have also a definite crystalline form. Chemical analysis and crystallography (q.v.) thus afford the chief means of identifying minerals; but, in addition to form and composition, minerals have other distinctive characters, optical, thermal, electrical, magnetic, aggregational, etc. Among the irregular or indeterminate forms of minerals are the nodular, with irregularly rounded surfaces, as in flint (q.v.); the mammillary or botryoidal, with spheroidal prominences, as in malachite (q.v.) and kidney iron-ore; the stalactitic, or icicle-like cylindric masses, as in calcite (q.v.); and the dendritic, tree-like, or mossy, as in pyrolusite. Cleavage is an important character closely related to crystalline form. [Crystal.] The chief optical characters of minerals are transparency, refraction, polarisation, lustre, colour, streak, and phosphorescence (q.v.). Transparence, or diaphaneity, the power of transmitting light, is of five degrees - transparent, transmitting distinct outlines; sub-transparent, when they are indistinct; translucent, transmitting light only; sub-translucent, or translucent when very thin; and opaque. Refraction (q.v.) is also closely related to crystalline form, and polarisation (q.v.) and lustre (q.v.) depend mainly upon refraction. Colour sometimes presents so wide a range within the limits of one mineral species as to be of little discriminative value; but streak, the colour of the mineral when abraded, is more useful. Among other optical characters often very distinctive are dichroism (q.v.) and fluorescence (q.v.). Thermal, electric, and magnetic conductivity are not much employed by the mineialogist in diagnosis; but are connected with the crystalline system. The pyro-electric polarity of crystals of tourmaline, topaz, boracite, and other hemihedral forms, which causes them to reverse their electric character as they are heated, is important; and some minerals, such as lodestone (q.v.), are notably magnetic. Fusibility is measured by a scale of comparison drawn up by Von Kobell, in which antimonite, fusible in a candle-fiame, is 1; natrolite, slightly so, 2; almandine-garnet, requiring a blow-pipe, 3; actinolite, only fusible in thin splinters, 4; orthoclase, fusible with difficulty, 5; and bronzite, very infusible, is 6. The aggregational characters of minerals include (1) their molecular rigidity, whether gaseous, liquid, or solid; (2) their tenacity, including sectility or capability of being cut with a knife, malleability or capability of being beaten into foil, ductility or capability of being drawn into wire, flexibility, the property of bending, elasticity, that of springing back again when bent, and brittleness; (3) their fracture; and (4) their hardness (q.v.). The surfaces of fracture, whether conohoidal, or shell-like, as in flint, splintery, as in chert, or hackly, as in cast-iron, are sometimes characteristic. Other characters of minerals are their specific gravity (q.v.), which is generally compared to that of water at 60° Fahr., or 4° C, touch, taste, and odour. The soapy or greasy touch of many magnesian minerals, especially hydrous silicates, is characteristic. [Soapstone.] Taste is necessarily confined to soluble, odour to volatile minerals. The chief tastes are named as saline, in common salt; alkaline, in soda; cooling, in nitre; astringent, in the vitriols; sweetish astringent, in alum; and bitter, in Epsom salts (q.v.). Among the chief odours are the fcetid smell of sulphuretted hydrogen given off by some limestone [Stinkstein], and the argillaceous, or earthy smell of clays and serpentine when moistened.

Of these characters, chemical composition is mainly employed in the classification of minerals. The several thousand species of minerals which have been described, the vast majority of which are rare substances of no commercial or even geological importance, are grouped into five main divisions. These are (l)native elements, subdivided into the metals and the non-metals, (2) sulphides, arsenides, etc., (3) chlorides and fluorides, (4) oxides, and (5) (by far the largest division) oxygen salts. This last is subdivided into some seven classes : (1) carbonates, (2) silicates, (3) tungstates, etc., (4) sulphates and chromates, (5) borates, (6) nitrates, and (7) phosphates, arseniates, etc.