Feldspar is the most common rock-forming mineral (about 60% of the earth’s crust) (Kauffman and Van Dyk, 1994). The mineral name feldspar is derived from the German words feld + spar. The word "feld" is "field" in German and "spar" is a term for light colored minerals that break with a smooth surface. Feldspar minerals are usually white or very light in color, have a hardness of 6 on the Mohs’ Scale of Hardness and perfect to good cleavage (plane of breakage) in two directions.

Feldspar is a common name that applies to a group of minerals with a general chemical formula of x Al(Al,Si)3O8, where x can be sodium (Na) and/or calcium (Ca) and/or potassium (K). (Specific compositional varieties (i.e. albite, microcline…) are discussed under Mineral Descriptions).

For some applications, the ore has to undergo processing in order to remove some accessory minerals (quartz, mica, etc.) while in other cases, their presence is advantageous. The two main properties that make feldspar useful for industry are its alkali and aluminum content. Main applications include the ceramic and glass industries.

Occurrences & mineralogy

Feldspar is by far the most abundant group of minerals in the earth's crust, forming about 60% of terrestrial rocks. Feldspar minerals are essential components in igneous, metamorphic and sedimentary rocks, to such an extent that the classification of a number of rocks is based on feldspar content. The crystalline structure of feldspars consists of an infinite network of SiO2 and AlO4 tetrahedra. They usually crystallise in the monoclinic or triclinic system.

The mineralogical composition of most feldspars can be expressed in terms of the ternary system Orthoclase (KAlSi3O8), Albite (NaAlSi3O8) and Anorthite (CaAl2Si2O8).

The minerals whose composition is comprised between Albite and Anorthite are known as the plagioclase feldspars, while those comprised between Albite and Orthoclase are called the alkali feldspars. This latter category is of particular interest in terms of industrial use of feldspars.

Mineralogy

Feldspar minerals can be subdivided into two groups - plagioclase feldspars, and potassium feldspars. Plagioclase feldspars are sodium / calcium aluminum silicates. The plagioclase feldspar series is:
•         albite (moonstone) (sodium rich),
•         oligoclase,
•         andesine,
•         labradorite, and
•         anorthite (calcium rich).
Potassium feldspars are potassium aluminum silicates. The most common K (potassium) feldspars are microcline (amazonite, moonstone) and orthoclase