Quartz

The characters as given above apply more particularly to crys tals of quartz, but in the various massive and compact varieties the material may be quite different in general appearance. Thus in the microcrystalline chalcedony the lustre is waxy, the fracture fibrous to even, and the external form botryoidal or stalactitic : flint and chert are compact and have a splintery fracture : jasper is a compact variety intermixed with much iron oxide and clay and has a dull and even fracture. Further, these varieties may be of almost any colour, whereas transparent crystals have only a limited range of colour, being either colourless (rock-crystal), violet (amethyst), brown (smoky quartz) or yellow (citrine).

Occurrence.

Quartz occurs as a primary and essential con stituent of igneous rocks of acidic composition such as granite, quartz-porphyry and rhyolite, being embedded in these either as irregularly shaped masses or as porphyritic crystals. In pegmatite (graphic granite) and granophyre it often forms a regular inter growth with felspar. It is also a common constituent, as irregular grains, in many gneisses and crystalline schists, a quartz-schist being composed largely of quartz. By the weathering of silicates, silica passes into solution and quartz is deposited as a secondary product in the cavities of basic igneous rocks, and in fact in the crevices and along the joints of rocks of almost all kinds. Exten sive veins of quartz are especially frequent in schistose rocks.

Vein-quartz, often of economic importance as a matrix of gold, may, however, in some cases have been of igneous origin. In min eral veins and lodes crystallized quartz is usually the most abun dant gangue mineral; the crystals are often arranged perpendicular to the walls of the lode, giving rise to a "comby" structure. In limestones of various kinds it occurs as nodules and bands of chert and flint, being in this case of organic origin. Quartz being a mineral very resistant to weathering agencies, it forms the bulk of sands and sandstones; and when the sand grains are cemented together by a later deposit of secondary quartz a rock known as quartzite results. Pseudomorphous quartz, i.e., quartz replac ing other minerals, is of frequent occurrence, and as a petrifying material replacing organic remains it is often met with. As a de posit from hot springs, quartz is much less common than opal. Crystals of quartz may be readily prepared artificially by a num ber of methods; for example, by heating glass or gelatinous silica with water under pressure.

For particulars respecting the special characters, modes of oc currence and localities of the more important varieties of quartz, reference may be made to the following articles : AGATE, AME THYST, AVENTURINE, BLOODSTONE, CAIRNGORM, CARNELIAN, CAT'S-EYE, CHALCEDONY, CHRYSOPRASE, FLINT, JASPER, MOCHA STONE, ONYX, SARD AND SARDONYX. For other forms of silica see