Biogenic Silica

Biogenic silica (BSi), also referred to as opal, biogenic opal, or amorphous opaline silica, forms one of the most widespread biogenic minerals. Silica is an amorphous metal oxide formed by complex inorganic polymerization processes. This is opposed to the other major biogenic minerals, comprising carbonate and phosphate, which occur in nature as crystalline iono-covalent solids (e.g. salts) whose precipitation is dictated by solubility equilibria. Chemically, BSi is hydrated silica (SiO2ยทnH2O), which is essential to many plants and animals.

Read more about Biogenic Silica:  Silica in Marine Environments, Opaline Silica On Mars

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Biogenic Silica - Opaline Silica On Mars
... of Mars, the Mars Exploration Rover Spirit inadvertently discovered opaline silica ... Later analysis showed that the silica was evidence for hydrothermal conditions ...