Olivine

Olivine is magnesium-iron silicate, (Mg,Fe)₂SiO₄, a common mineral in the basalt and other mafic igneous rocks that make up the oceanic crust. It is found in all compositions between pure magnesium silicate (forsterite) and pure iron silicate (fayalite), and olivine composition is often given as a number from zero to 100, where 100 is pure fayalite. Forsterite is white and fayalite is dark brown, but olivine is usually green, like this grain embedded in Hawaiian basalt magnified about 10 times. Olivine has a minor use as an abrasive in sandblasting. As a gemstone, olivine is called peridot.

Olivine prefers to live deep in the upper mantle, where it is the most common mineral. It never occurs in the same rock with quartz. It is very unhappy at the Earth's surface and breaks down fairly rapidly (geologically speaking) under surface weathering. This olivine grain was swept to the surface in an eruption of Kilauea. If olivine-bearing rocks are kept in the shallow crust, olivine readily takes up water and metamorphoses into serpentine.

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