 Occurrence
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| Measured by mass, silicon makes up 25.7% of the Earth's crust and is the second most abundant element on Earth, after oxygen. Pure silicon crystals are only occasionally found in nature; they can be found as inclusions with gold and in volcanic exhalations. Silicon is usually found in the form of silicon dioxide (also known as silica), and silicate.
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| Silica occurs in minerals consisting of (practically) pure silicon dioxide (SiO2) in different crystalline forms (quartz, chalcedony, opal). Sand, amethyst, agate, quartz, rock crystal, flint, jasper, and
opal are some of the forms in which silicon dioxide appears. (They are known as "lithogenic", as opposed to "biogenic", silicas.)
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| Silicon also occurs as silicates (various minerals containing silicon, oxygen and one or another metal), for example feldspar. These minerals occur in clay, sand and various types of rock such as granite
and sandstone. Asbestos, feldspar, clay, hornblende, and mica are a few of the many silicate minerals.
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| Silicon is a principal component of aerolites, which are a class of meteoroids, and also is a component of tektites, which
are a natural form of glass.
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