Monday, September 20, 2010

Rock Forming Minerals

The bulk of the rocks of the Earth’s crust consist of silicates. They have a plethora of names, compositions and complex structures, but the most important ones can be grouped into families.

                These are the most abundant of all minerals at the Earth’s surface: spiral chains of silicate tetrahedral linked at the corners often transparent and crystalline. Acid rocks are at least ten percent quartz.

                These make up to 50-60 percent of the mass of all igneous rocks: 3D frame work of silicate tetrahedral with aluminum and varying amount of potassium, sodium, calcium and aluminium. There are two families of feldspars: orthoclase feldspars have varying amounts of aluminum and potassium; plagioclase feldspars have varying amounts of sodium and calcium.

This dense, ultra basic family of minerals is common in the Earth’s mantle and igneous rocks originating from a deep source: dense, close packed silicate tetrahedra containing magnesium and iron; grassy in appearance.

                 This is a family of chain silicates in igneous rocks containing magnesium (in the case of enstatite), calcium and magnesium (in diopside) or iron and aluminium (in augite).

                These have double chains of silicates containing iron and magnesium. Many amphiboles are described as fellomagnesium and one of the best known examples ins hornblende.

                Mica consists of layered silicates that cleave easily into flakes or sheets, and include muscovite (white mica) and biotite (dark mica); a common constituents of gneiss and granite.