Quartz

Quartz, which is the most abundant silica mineral and which occurs in most igneous and practically all metamorphic and sedimentary rocks, is nearly pure silicon dioxide (SiO2). It was also been found in some lunar rocks and meteorites. The name of quartz is believed originated in the early 1500s from the Saxon word querklufterz (cross vein ore), which was corrupted to queretz and then to quartz. Quartz was well known to the ancient, who called it crystal or rock crystal.

Quartz is colorless and transparent when pure. Its hardness is 7 on the Mohs scale, and its specific gravity is 2.651. Quartz has no cleavage and fails by brittle fracture, the fracture surfaces have vitreous luster. Quartz is diamagnetic but does not conduct electricity. Piezoelectricity makes quartz valuable in pressure gauges, electronic frequency control devices, and radio equipment.

Crystallography
Quartz occurs in a wide range of crystal sizes, from single crystal weighing many tons to cryptocrytalline varieties whose crystalinity may be seen only with the aid of an electron microscope. Quartz crystallizes in the trigonal transpezophedral class of the rhombohedral subsystem of hexagonal symmetry.

Varieties
Although coarsely crystalline quartz occurs in colorless or white (milky) masses, colored varieties are numerous and popular. Amethyst (violet); smoky quartz, cairrgorm, or morion (black), citrine (yellow), and rose quartz are common and arise by the incorporation of a tiny fraction of elements that substitute for silicon atoms, such as iron, aluminum, manganese and titanium.

Very fine grained and cryptocrystalline varieties of quartz are numerous. Collectively called chalcedonic quartz, these varieties form slowly from evaporating or cooling solutions as crusts and fillings of veings and open spaces. When color bending is conspicuous, the variety is called agate. Agate with numerous flat bands of white, black or dark brown is called onyx. Translucent red or brown chalcedonic quartz colored by iron oxides, green varieties colored by chlorite, amphiboles, or nickel minerals, and mottled moss agates are used as semiprecious stones. Bloodstones is a green variety is often colored by chemical processes.

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