Tremolite from the Precambrian of New York State, USA. (field of view: ~2 cm 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.

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 cummingtonite, a brownish to grayish to dark greenish amphibole ((Mg,Fe)7Si8O22(OH)2) that occurs in some metamorphic rocks and occasionally in some igneous rocks. It has a silky luster and usually forms radiating masses of acicular (needle-shaped) crystals or irregular masses of fibers.

The rock in which this cummingtonite occurs is a Paleozoic-aged metamorphic rock from Massachusetts. In addition to brownish-colored sprays of cummingtonite needles, the rock also has quartz, garnet, and magnetite.

consists of sprays of brownish cummingtonite amphibole needles, quartz, garnet, and magnetite.

Locality: unrecorded/undisclosed site at or near the town of West Chesterfield, western Hampshire County, western Massachusetts, USA

Photo gallery of cummingtonite: