Forsterite olivine (each mass is ~10 to 15 mm across)

A mineral is a naturally-occurring, solid, inorganic, crystalline substance having a fairly definite chemical composition and having fairly definite physical properties. At its simplest, a mineral is a naturally-occurring solid chemical. Currently, there are over 5100 named and described minerals - about 200 of them are common and about 20 of them are very common. Mineral classification is based on anion chemistry. Major categories of minerals are: elements, sulfides, oxides, halides, carbonates, sulfates, phosphates, and silicates.

The silicates are the most abundant and chemically complex group of minerals. All silicates have silica as the basis for their chemistry. "Silica" refers to SiO2 chemistry. The fundamental molecular unit of silica is one small silicon atom surrounded by four large oxygen atoms in the shape of a triangular pyramid - this is the silica tetrahedron - SiO4. Each oxygen atom is shared by two silicon atoms, so only half of the four oxygens "belong" to each silicon. The resulting formula for silica is thus SiO2, not SiO4.

The simplest & most abundant silicate mineral in the Earth's crust is quartz (SiO2). All other silicates have silica + impurities. Many silicates have a significant percentage of aluminum (the aluminosilicates).

Olivine is a common mineral in many ultramafic igneous rocks. It is magnesium iron silicate - (Mg,Fe)2SiO4. “Olivine” actually refers to a continuous spectrum of chemistries between pure magnesium silicate, Mg2SiO4 and pure iron silicate, Fe2SiO4. Magnesium-rich olivine (forsterite) is more common than iron-rich olivine (fayalite). Olivine is typically greenish-colored. It has a hardness of 7, a conchoidal fracture, a glassy, nonmetallic luster, and a white streak.

Photo gallery of olivine: