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Quartz: Minerals
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Quartz is a mineral which is put to many uses. Several of the varieties are cut into gems and ornaments, balance weights, pivot supports for delicate instruments, agate mortars, &c.; or used for engraving, for instance, cameos and the elaborately carved crystal vases of ancient and medieval times. Clear transparent rock-crystal is used for optical purposes and spectacle lenses. Fused quartz has recently been used for the construction of lenses and laboratory vessels, or it may be drawn out into the finest elastic fibres and used for suspending mirrors, &c., in physical apparatus. For striking fire, flint is used even to the present day. Buhrstone, a cellular variety of chalcedonic quartz from the Tertiary strata of the Paris basin, is largely used for millstones.
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Quartz is one of the most common minerals on Earth. The crystal forms as a six-sided prism with pointed ends. The ends look like six-sided pyramids. The sides of the crystal are marked with crosswise striations (ridges). It occurs in all geological environments, and makes up most of the dust in air.
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Quartz crystallizes in the rhombohedral system. The size of the crystals varies from specimens weighing a metric ton to minute particles that sparkle in rock surfaces. Quartz is ... common in massive forms, which contain particles ranging in size from coarse-grained to cryptocrystalline (grains invisible to the naked eye but observable under a microscope). The mineral has a hardness of 7 and specific gravity of 2.65. The luster in some specimens is vitreous; in others it is greasy or splendent (shining glossily). Some specimens are transparent; others are translucent.
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Quartz is by far the most abundant of the polymorphic forms of silica and the most widespread and abundant mineral, making up 12% of the earth's crust by volume. It occurs in igneous, metamorphic and sedimentary rocks as well as hydrothermal veins, metasomatic and hot spring deposits. Quartz crystals are found as a common gangue material in veins of many minerals and in cavities in granite porphyries and pegmatites.
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Quartz is found in most geologic settings, but it most typically forms sedimentary rocks like sandstone and is the defining mineral of igneous rocks like granite. In the granite below, the pink mineral is feldspar and the quartz grains appear gray. When rocks like this crystallize deep underground, quartz is generally the last mineral to form and it usually has no room to form crystals. But in pegmatites quartz can sometimes form very large crystals, as long as a meter.
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The complexity involved in this one mineral is so great that entire books just about Quartz have been written. The information provided here just briefly touches upon all the details of this mineral.
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