Silica
Silica is silicon dioxide, which forms quartz, cristobalite, tridymite and other minerals and glasses.
Silica is the compound silicon dioxide, SiO2. It is the main ingredient of sand, flint and window glass, and it forms many minerals depending on temperature and pressure.
On Earth's surface the familiar form is quartz. At high temperatures, tridymite and cristobalite form, while at extreme pressures quartz turns into coesite and stishovite. Silica can also form glass when molten rock cools very quickly, as in obsidian or impact glass.
In meteorites, free silica is uncommon because most of the silicon is tied up in olivine, pyroxene and feldspar. Where it does appear, as in eucrites, mesosiderites or certain enstatite meteorites, it shows that the rock was rich in silicon. In impact craters, silica minerals are among the best evidence of a strike from space: shocked quartz and its high-pressure forms are proof that rock experienced pressures no volcano can achieve.
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Level: Explorer Ages 10 and up