Quartz

On the Earth’s surface, quartz (or, more properly, alpha-quartz) is by far the most common of the crystalline forms of silicon dioxide (SiO₂), constituting 12 % of the crust by volume. Among the igneous rocks, quartz is abundant within granites, granodiorites, pegmatites, and rhyolites. The weathering and erosion of these rocks tend to concentrate quartz as sedimentary particles because of its low solubility (~10 ppm) and its mechanical durability (with a Mohs hardness of 7 out of 10). As a result, quartz is the major constituent of highly mature arenitic sandstones. Quartz also can form in sedimentary environments through the chemical precipitation of dissolved silica. The dissolution of volcanic clays or of organosiliceous tests (as are found in diatoms, radiolaria, and sponges) produces a hydrous silica gel that will transform to quartz over thousands of years through diagenetic dehydration at temperatures up to 300 °C and pressures up to 2 kb. Metamorphism of siliceous sedimentary...