Colloid Geochemistry of Silica

Abstract

Silica, SiO2, is common in nature and occurs as seven distinct minerals, of which five show crystalline structures (quartz, tridimite, cristobalite, coesite, and stishovite) and two are amorphous (opal-A and lechatelierite). Lechatelierite is a silica glass and is very rare. Coesite and stishovite are also very rare and are found only in meteoritic craters, where they have been formed from quartz because of the high pressure resulting from the meteoritic impact. Opal is obtained by deposition from aqueous silica solutions at low temperatures. It is deposited by thermal waters associated with igneous activity and is also secreted by sponges, radiolaria, and diatoms. The most common form of silica is quartz.