Page:EB1911 - Volume 18.djvu/534

 not therefore depend on the nature of the material, but only on the conditions which prevailed during its growth. (See e.g. and .)

In the following enumeration of the more salient characters of minerals it is to be noted that many of the terms used for non-essential characters are purely descriptive and have no exact definition; on the other hand, essential characters can be expressed numerically and are therefore perfectly definite. 1. Morphological Characters.

a. Crystalline Form.—This most important character of minerals can, of course, be determined only when the material available is in the form of crystals (i.e. crystallized), which is not always the case. Massive aggregates of crystalline material are of much more frequent occurrence; when small fragments or thin sections of such material are transparent, the crystalline symmetry may be determined, within certain limits, by the help of the optical characters (see below). External crystalline form must not, however, be considered alone apart from all other characters, for crystals of substances quite different chemically, e.g. silver iodide, zinc oxide and zinc sulphide, are sometimes almost identical in crystalline form; further, in groups of isomorphously related minerals the degree of symmetry will usually be the same and the angles vary only slightly, and unless the crystals are perfectly developed and suitable for exact goniometric measurement no crystallographic distinction can be made between two such species.

All the six systems of crystals and most of the thirty-two symmetry-classes are represented amongst minerals (see ). Crystals of the same mineral-species may differ very widely in general form or habit; e.g. crystals of (q.v.) may be rhombohedral, prismatic, scalenohedral or tabular in habit. Other descriptive terms of the habit of crystals are pyramidal, acicular or needle-shaped (from the Lat. acicula, a needle), capillary or hair-like (from the Lat. capillus, hair), &c.; and these peculiarities of habit may sometimes be characteristic of certain minerals. Sometimes also there are characteristic kinds of groupings of crystals: thus parallel, divergent or radiating (e.g. scolecite), rosette-shaped (e.g. haematite—Eisenrosen), reticulated (e.g. rutile), or matted. The faces of natural crystals may be smooth, rough, striated, curved or drusy, i.e. studded with small crystal faces and angles.

b. State of Aggregation: Structure.—According to the particular state of aggregation of a number of imperfectly developed crystals, which have grown together, various kinds of structure may be presented even by the same mineral species. The descriptive terms applied to these structures are almost self-explanatory: thus the structure may be granular (e.g. marble), fibrous (asbestos), radio-fibrous or stellated (wavellite), columnar (beryl), laminar or lamellar (talc), bladed (cyanite), &c., according to the relative shape and sizes of the individual crystals composing the aggregate. 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); many which are apparently amorphous are really microcrystalline (e.g. turquoise). The term massive is often used loosely for a crystalline mineral not showing crystal-faces. Crystal-aggregates often assume more or less accidental and imitative external forms to which the following descriptive terms are applied: dendritic or arborescent (e.g. copper, pyrolusite), mossy (copper), leafy (gold), wiry or filiform (silver), capillary (millerite), coralloidal (aragonite), globular (aragonite, with concentric structure; wavellite, with radiated structure), mamillary or with breast-like protuberances (arsenic), nodular (malachite), warty (menilite), botryoidal or resembling a bunch of grapes (from , a bunch of grapes) (dolomite), reniform or kidney-shaped (menilite), amygdaloidal or almond-shaped (agate), stalactitic (calcite, chalcedony), &c.

2. Physical Characters.

a. 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 refractive indices, strength of the double refraction, optic axial angle, extinction angles on certain faces, &c., are characters capable of exact measurement and numerical expression, and are constant for each mineral-species. (See .)

In their “diaphaneity,” or degree of transparency, minerals differ very widely even in the same species. Some, such as metals and most metallic sulphides are always opaque; while others may vary in different specimens from perfect transparency to perfect opacity (in the latter case, however, minute fragments will, as a rule, still be transparent). A good example of this is afforded by the varieties of quartz: rock-crystal is water-clear, chalcedony is translucent, and jasper opaque.

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. Crystals of fluor-spar of an emerald-green, purple, golden-yellow, bright pink or other colour are at first sight very different in appearance, and yet the difference is due solely to the presence of traces of colouring matters so small in amount that their exact nature is difficult or impossible to determine. The value of diamond, corundum and other gemstones depends largely on these accidental differences in colour. Such substances, which are essentially colourless and owe their colour to the presence of colouring matter as an impurity, are said to be “allochromatic”: any colour they may possess is nonessential. In some other substances, known as “idiochromatic,” the colour is a definite and essential character; for example, the yellow colour of gold, the red of cinnabar, &c.; but even here, owing to differences in the state of aggregation and the presence of various impurities, they may be wide variations in colour. Colour is thus a character of little determinative value, especially in minerals which are allochromatic; but it is sometimes a useful guide when taken in conjunction with other characters. An elaborate list of colour-names for descriptive use was drawn up by A. G. Werner in 1774.

An important character of transparent crystals is that of unequal absorption in different directions; so that light will, as a rule, be differently coloured according to the direction in which it has travelled through the crystal: this is known as dichroism or pleochroism (see ). Certain minerals (e.g. zircon, almandine and those containing cerium) when examined with a spectroscope by transmitted light exhibit characteristic absorption spectra.

The colours of minerals may also be due to the interference of rays of white light at the surfaces of thin crevices or minute inclusions, either tabular or fibrous in form, in the mineral; for example, the play of colours of opal; the change of colours of labradorite; the bands of rainbow colours (Newton’s rings) seen along cleavage cracks and irregular internal fractures (e.g. in quartz); the iridescent tarnish due to a superficial film of a, decomposition product (e.g. “peacock copper ore”); or the bluish opalescence of moon-stone and cat’s-eye.

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;