Page:EB1911 - Volume 18.djvu/535

 limonite, yellow; and chalybite, white. 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. For example, the usual adamantine lustre of diamond is not exhibited by the compact aggregate known as carbonado; while earthy masses of any mineral will be devoid of lustre. Descriptive terms applied to the kinds of lustre are: metallic (e.g. pyrites), adamantine (diamond), vitreous (quartz), resinous (pyromorphite), greasy (elaeolite), waxy (chalcedony), pearly (talc, heulandite and other minerals with a perfect cleavage), silky (satin-spar), &c. The degrees of intensity of lustre are described as splendent, shining, glistening, glimmering and dull, and depend usually on the smoothness of the crystal-faces.

The phenomena of (q.v.),  (q.v.) and (q.v.) are strikingly exhibited by some minerals. (See, , &c.)

b. Magnetic, Electrical and Thermal Characters.—These, as far as related to crystalline form, are discussed under (q.v.). Magnetite (“lode-stone”) is the only mineral which is strongly magnetic with polarity; a few others, such as pyrrhotite and native platinum, possess this character to a much less degree. Many minerals are, however, attracted by the pole of a strong electro-magnet, while a few (diamagnetic) are repelled. 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. Some minerals—for example, salt, sylvite and blende—are highly diathermanous, i.e. transparent for heat-rays.

The specific heat and melting point of minerals are essential characters capable of exact measurement and numerical expression, but they are not often made use of. Different minerals differ widely in their “fusibility”: the following scale of fusibility was proposed by F. von Kobell:— The melting points given above in parentheses were determined by J. Joly. Stibnite readily fuses to a globule in a candle-flame, while quartz is infusible even on the thinnest edges before the ordinary blowpipe.

c. Characters depending on Cohesion.—Some minerals (e.g. a sheet of mica) are highly elastic, springing back to their original shape after being bent. Others (e.g. talc) may be readily bent, but do not return to their original form when released; these are said to be pliable or flexible. Sectile minerals (e.g. chlorargyrite) may be cut with a knife without being fractured: related characters are malleability (e.g. argentite) and ductility (e.g. silver). The tenacity, or degree of frangibility of different minerals varies widely: they may be brittle, tough, soft or friable. The fractured surface produced when a mineral is broken is called the “fracture,” and the kind of fracture is often of determinative value; descriptive terms are: conchoidal (e.g. quartz, which may often be recognized by its glassy conchoidal fracture), sub-conchoidal, uneven, even, splintery (e.g. jade), hackly or with short sharp points (e.g. copper), &c.

In many cases when a crystallized mineral is broken it separates in certain definite directions along plane surfaces. This property of “cleavage” (see ) is an important essential character of minerals, and one which is often of considerable assistance in their recognition. For example, Calcite, with its three directions of perfect cleavage parallel to the faces of a rhombohedron, may always be readily distinguished from aragonite or quartz; or again, the perfect cubical cleavage of galena renders this mineral always easy of recognition.

“Hardness,” or the resistance which a substance offers to being scratched by a harder body, is an important character of minerals, and being a test readily applied it is frequently made use of. It must, however, be remembered that the hardness of an incoherent or earthy aggregate of small crystals will be very different from that of a single crystal. A comparative “scale of hardness” was devised by F. Mohs in 1820 for the purpose of giving a numerical statement of the hardness of minerals.

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 3. 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; the powder should be wiped off and the surface examined with a lens to see if a scratch has really been produced or only powder rubbed off the corner of the mineral with which the scratching was attempted. 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. = 6) and copper-pyrites (H. = 3), apatite (H. = 5) and beryl (H. = 7), 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. (See .)

d. Specific Gravity.—The density or specific gravity of minerals is an essential character of considerable determinative value. In minerals of constant composition it has a definite value, but in isomorphous groups it varies with the composition: it also, of course, varies with the purity of the material. It is a character which has the advantage of numerical expression: minerals range in specific gravity from 1·01 for copalite to 22·84 for iridium. The exact determination of the specific gravity of minerals is therefore a matter of some importance. Three methods are in common use, viz. hydrostatic weighing, the pycnometer, and the use of heavy liquids. The first two methods are only applicable when a weighable amount of pure material can be selected or picked out; this is, however, generally a laborious operation, since impurities are often present and usually several species of minerals are closely associated, and in selecting material it is often necessary to determine some other character to make certain that only one kind is being selected. For exact determinations the pycnometer method is usually to be recommended, using for material the pure fragments which have been selected for quantitative chemical analysis. With a single pure crystal or a faceted gem-stone the method of hydrostatic weighing is usually applicable, providing the stone is not too small. The most ready method, however, is that afforded by the use of a heavy liquid, and the most convenient liquid for this purpose is methylene iodide. This is a clear, mobile liquid with a specific gravity of 3·33, and by the addition of benzene, drop by drop, the specific gravity may be reduced to any desired amount. With such a liquid the specific gravity of the minutest fragment, the purity of which has previously been scrutinized under the microscope, may be rapidly determined. The liquid is diluted with benzene until the fragment just remains suspended, neither floating nor sinking; the specific gravity of the fragment will then be the same as that of the liquid, and the latter may be determined by hydrostatic weighing or, more conveniently, by