373 (3) from chemical replacement, the substitution of a smaller or larger quantity of a coloured isomorphous ingredient.- As illustration of the first, silica, colourless in rock-crystal, has been found of almost every tint, due frequently to volatile hydrocar bons which are dissipated by heat. Fluorite also, found of almost every shade of every colour, may possibly be to a certain extent referred here. Quartz, felspar, and calcite are often coloured accidentally by imbedded layers of foreign "inclusions," or by " spangling endo- morphs." These are mechanically mixed, so far as regards their presence in a structure of different and non-assimilable chemical composition, but crystallographically arranged. They either mark the lines of interrupted or intermittent growth; or, in the case of cndomorphs, the axial positions of the minute intruding foreign crystals lie in one plane, or in the same sets of planes. As an instance of colours introduced through definite chemical replacement, calcite may be cited. Carbonate of lime is colourless ; if a portion of this be replaced by carbonate of magnesia there is a certain amount of pearly opacity ; if by carbonate of manganese, of a pink tinge ; if by carbonate of iron, of yellow, which may be increased through oxygen absorption and "weathering" to an ochre tint, and ultimately to a dark brown. Sulphuret of zinc, chemically white, and mineralogically trans parent, may, through metallic substitution, be found of almost all tints of yellow, orange, brown, and black. Again, hornblende, augite, and garnet, silicates, which in their purest states of tremo- lite, malacolite, and water garnet are colourless, acquire green, brown, red, and black tints from the assimilation of other metallic silicates. Hence it would appear that a very advanced practical know ledge of the subject is necessary to enable us to avail ourselves of the information which is to be derived from this external feature. The accidentally coloured minerals sometimes present two or more colours or tints, even in a single crystal, very remarkable examples occurring in fluor-spar, apatite, sapphire, amethyst, tour maline, and cyanite. This is still more common in compound minerals, on which the colours are variously arranged in points, streaks, clouds, veins, stripes, bands, or in brecciated and ruin-like forms. Some minerals again change their colour from exposure to light, the air, or damp. Then either the surface alone is affected or "tarnished," and appears covered as with a thin film, producing in some minerals, as silver and arsenic, only one colour ; in others, as chalcopyrite, haematite, bismuth, stibine, and anthracite, various or iridescent hues, when they are said to have a pavonine lustre. Or occasionally the change pervades the whole mineral, the colour either becoming paler, or disappearing, as in chrysoprase and rose-quartz, or becoming darker, as in brown spar, siderite, and rhodonite. In a f ew minerals a complete change of colour takes place, as in heterosite, and in the chloroplueite of the Western Isles of Scotland, which, on exposure for a few hours, passes from a transparent yellow-green to black. These mutations are generally connected with some chemical or physical change. The tarnished colours sometimes only appear on certain faces of a crystal belonging to a peculiar form. Thus a crystal of copper pyrites (like fig. 89) has one face F free from tarnish ; the faces b and c, close to F, dark blue ; the remainder of c, first violet, and then, close to P, gold-yellow. Some crystalline minerals exhibit in certain directions a very lively play or change of colours from reflected light. It is well seen in many various hues on the cleavage-planes of labradorite, and seems produced by a multitude of very thin quadrangular pores, interposed in the mineral, like minute parallel laminae. On the cleavage-plane; of hypersthene it appears copper-red, and is occasioned by similar pores, or by numerous small brown or black lamiiue of some foreign substance interposed in a parallel position between the planes of the hypersthene. The chatoyant or changing colours of the sun-stone arise from scales of haematite similarly inter posed, and that of avanturine from scales of mica. The play of colour in the noble opal seems to be produced very nearly in the same manner as that in the labradorite. A similar opalescence is seen in certain minerals when cut in particular forms. In the sapphire, cut hemispherically over the chief axis, it appears like a star with six rays; in garnet it shows four rays; in certain varieties of chryso- beryl and of adularia it has a bluish tint; and it is also very remark able in the cat s-eye variety of quartz. Iridescence often arises from very fine fissures, producing semicircular arches of prismatic tints, which, like the colours of thin plates in general, are referred to the interference of light. Streak. Streak. This name is applied to the appearance and the colour of the line or furrow produced in minerals by drawing the edge of a hard-tempered knife or file along their surface, or to the stain obtained by rubbing a soft mineral on such a substance as paper or porcelain. Taken along with the hardness, which may to a certain extent be determined by the same operation, it is one of the most valuable tests which we possess. The furrow may be lustrous or it may be dull. Powder or splinters may lie along its course, or a still adherent ridge may have been merely rolled over. The furrow and the powder may each be possessed of colour, though such may not be distinguishable in the mineral, or may have a colour quite different from that of the mineral. Three illustrations of the usefulness of this test may suffice. Argentiferous gold, chalcopyrite, and pyrite, differing immensely in value, may readily be mistaken for each other. The knife, when drawn along the surface of the first, sticks in it, ruts up an adhering ridge, and leaves a shining streak of the same colour as the specimen. When drawn along the second it ruts up a trench covered with a dusty powder, which when rubbed on paper or in the hand is greenish yellow. When drawn along the third it has no effect, as pyrite is harder than the knife. Psilomelane, haematite, and limonite all occur in black, glossy, stalactitic forms, and have all been termed "black haematite." There is here also great difference in the value. The knife makes little impression on psilomelane, but leaves a blue lustrous line ; it makes a blood-red line in haematite, and a rich ochre-yellow in limonite. Graphite and molybdenite both crystallize in hexagonal plates, both occur in the same rocks, both have a grey-black colour and a brilliant metallic lustre, both stain the hands or paper ; the streak of the first best seen on paper is black, tending to blue ; that of the last is greenish. Rough porcelain is the best material for determining the streak of soft minerals. Diaphaneity. Minerals, and even different specimens of Trans- the same species, vary much in this quality. Some mission transmit so much light that small objects can be clearly of ll & } *" seen, or letters read, when placed behind them ; such are named transparent. They are semitransparent when the object is seen only dimly, as through a cloud, and translu cent when the light that passes through is so broken that the form of the object can be no longer discerned ; some minerals are only thus translucent on the thinnest edges. Others transmit no light, and are named opaque. Refraction. It has already been mentioned that most Double crystals all, in fact, except those of the cubical system refrac- exhibit the phenomena of double refraction. For a t * on- general explanation of these phenomena the reader is referred to LIGHT, vol. xv. p. 609 sq. The direction in which there is no double refraction is named Optic the optic axis of the crystal, sometimes, less happily, the axis of axis, double refraction. Now in certain minerals it is found that there is only one direction with this property, whereas in others there are two such directions ; and they have in consequence been divided into uniaxal and binaxal. To the former belong all crystals of the tetragonal and hexagonal systems, to the latter all those of the other three systems. In the former the optic axis coincides with or is parallel to the crystallographic chief axis. In some uniaxal crystals the index of refraction for the extraordinary ray is greater than for the ordinary ray ; and in others it is smaller. According as it is greater or less they are said to have positive (attractive) or negative (repulsive) double refraction. Quartz is an example of the former, the index of refraction, accord ing to Mains, being for = 1 5484, for E = 1 5582; calc-spar of the latter, the index of being =1-6543, that of E 1 4833. The index of E is in both cases taken at its maximum. It should be observed that the optic axes are not single lines, but directions parallel to a line, passing through every part of the crystal. It is also important to remark that this property divides crystals into three precise groups : the cubic, with single refrac tion ; the tetragonal and hexagonal, with double refraction, and uniaxal ; those of the other three systems, also double, but binaxal. These properties are therefore of the greatest use in determining the system to which a mineral belongs. Polarization. Intimately connected with this property Polariza- is that of the polarization of light, which affords an easier ti 011 - means of determining mineralogical characteristics than the direct study of double refraction. For the elements of this subject see LIGHT, vol. xv. p. 611 sq. While a consideration of the optic axes enabled us merely to arrange the systems of crystallization in three groups, the phenomena of polarization not only bear out a further subdivision of the whole into the above six systems, but disclose, in many cases, phenomena markedly special to individual species. The optical consideration
of these phenomena enables us to fix three directions at