Actinolite from California, USA.

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.

Amphibole is a group of silicate minerals. The garden-variety type of amphibole is hornblende. Other common varieties include tremolite and actinolite. The sample shown above is actinolite, a greenish-colored amphibole having the formula Ca2(Mg,Fe)5Si8O22(OH)2 - calcium magnesium iron hydroxysilicate. Actinolite forms a continuous chemical spectrum with whitish-colored tremolite. Unlike tremolite (see elsewhere in this photo album), actinolite has a significant iron component, which causes the dark coloration. Actinolite often forms radiating clusters of elongated blades.

Photo gallery of actinolite: