Leonard James Spencer

“ The “colour” of minerals is the character which first arrests attention; but being a character which may vary almost indefinitely in one and the same kind of mineral, it affords a typical example of a non-essential character. Thus, fluor-spar and quartz, when in well-formed and 'chemically pure crystals, are quite colourless and transparent; but it would be easy to collect a series of each of these minerals in which almost every shade of colour is represented. ”

“ These minerals, arbitrarily selected for standards, are successively harder from talc the softest, to diamond the hardest of all minerals: a piece of talc is readily scratched by gypsum, and so on throughout the scale. A mineral which is capable of scratching calcite and itself be as easily scratched by fluor-spar is said to have a hardness of 31/2. Some care is required to avoid error in the determination of hardness: it is best to select a smooth crystal-face, cleavage-surface or fracture on which to rub a sharp corner of the scratching mineral ”

“ 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. ”

“ Other sedimentary rocks consist of minerals deposited from solution either by chemical or organic agencies, from sea-water, lakes or springs: e.g. the calcite of limestones, deposits of bog-iron-ore (limonite) , gypsum, rocksalt, &c.
Minerals Segregated in Veins and Rock-cavities.—Water percolating through rock-masses takes up mineral matter in solution, and the solutions so formed may further react on the minerals composing the rocks. ”

“ 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.” ”

“ With a little practice a fair idea of the hardness of a mineral may be obtained with the use of a knife or file, which will scratch all minerals with a hardness of 6 or less. Thus iron-pyrites (H. = 61/2) and copper-pyrites (H. = 31/2) , apatite (H. = 5) and beryl (H. = 71/2) , or gem-stones and their paste imitations may be readily distinguished by this test. Talc and gypsum can be readily scratched with the finger-nail.
Planes of parting, etching figures, pressure- and percussion-figures are sometimes characters of importance in describing and distinguishing minerals. ”

“ 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 ”

“ 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. ”

“ The true colour of a mineral is best revealed by its “streak,” i.e. the colour of its powder. This is obtained by scratching the mineral, or by crushing a fragment of it on a sheet of white paper, or rubbing it upon unglazed porcelain. The streak of allochromatic minerals is white, while that of idiochromatic minerals is coloured and is often of determinative value. Ores of iron may, for example, generally be distinguished by their streaks: that of magnetite being black; haematite, blood-red ”

“ Most minerals with a metallic lustre are good conductors of heat and electricity; others are bad conductors. For example, graphite is a good conductor, while diamond is a bad conductor. Non-conductors of electricity become electrified by friction, some positively (e.g. quartz and topaz) , others negatively (e.g. sulphur and amber) . The length of time during which different gem-stones retain their charge of frictional electricity was made use of by R. J. Haüy as a determinative character. For the pyro-electrical and thermo-electrical characters of crystals see Crystallography. ”

“ 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. ”

“ The streak of a mineral may be either shining (e.g. argentite) or dull.
Another character depending on light is that of lustre, which is often very characteristic in certain minerals, though it may be considerably modified by the state of aggregation. ”

“ In gneisses and crystalline schists, quartz, felspar, mica, talc, amphibole, &c. are important constituents.
Minerals of Sedimentary Rocks.—By the weathering and disintegration of igneous and metamorphic rocks the various minerals set free and the products of decomposition of others supply the material of sedimentary rocks; thus sandstones consist largely of quartz, shales of kaolin and other clay minerals. Those minerals (e.g. gem-stones and gold) which resist the action of weathering processes are found as water-worn pebbles and grains in detrital deposits. ”

“ Optical Characters.—The action of crystallized matter on transmitted light is a character of the highest importance in mineralogy. Even when the substance is opaque in large masses, it may be sufficiently transparent when in small splinters or in thin sections for the determination of the optical characters. ”

“ 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. ”

“ Pseudomorphs.—Crystals which have been formed under one. set of conditions of temperature and pressure and in the presence of certain solutions, will in many 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. ”

“ 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 ”

“ A good example of the “pneumatolytic” action produced by the vapours from a mass of granitic magma is afforded by veins of tin-ore, in which the ore (cassiterite) is associated with minerals containing boron and fluorine, such as topaz, tourmaline. lepidolite, fluor-apatite and fluor-spar. The production of such minerals may be accounted for by assuming the presence of stannic fluoride in the vapours, which by reacting on water vapour would deposit cassiterite with the liberation of hydrofluoric acid, and this would again react on other minerals. ”

“ When the constituent crystals are invisible to the unaided eye the material is described as compact; incoherent aggregates are powdery or earthy. Minerals which are really amorphous, i.e. without any crystalline structure, are comparatively few in number (e.g. opal) ”

“ Finally, there is the economic side, dealing with the mining and application of useful minerals, the extraction of metals from their ores, and the uses of minerals for building, decoration and jewelry. ”