Feldspar (from the German Feld, field, and Spat, a rock that does not contain ore) is the name of an important group of rock-forming minerals which make up perhaps as much as 60% of the Earth's crust. They crystallize from magma in both intrusive and extrusive rocks; they occur as compact minerals, as veins, and are developed in many types of metamorphic rock. They may also be found in many types of sedimentary rock.
This group of minerals consists of three silicates: a potassium-aluminium silicate (the orthoclase feldspars), a sodium-aluminium silicate, and a calcium-aluminium silicate (the plagioclase feldspars) and their isomorphous mixtures.
Orthoclase (KAlSi3O8) is named based on the Greek for "straight fracture," because its two cleavages are at right angles to each other. Orthoclase crystallizes in the monoclinic crystal system. It has a hardness of 6, a specific gravity of 2.56-2.58, and a vitreous to pearly luster. It can be colored white, gray, yellow or red; rarely green. Twin crystals are not uncommon. Orthoclase is a common constituent of most granites and other felsic igneous rocks and is often found in huge crystals and masses in pegmatite veins. Orthoclase is used in the manufacture of porcelain and as a constituent of scouring powder. Adularia (from Adular) is found in low temperature hydrothermal deposits. When pearly and opalescent orthoclase is called moonstone and is used in jewelry. These opalescent varieties are known to be an intergrowth of orthoclase and albite. A glassy kind of orthoclase, called sanidine, is typical of felsic volcanic rocks and is found in the trachytes of the Drachenfels, Germany.
Microcline (KAlSi3O8) is chemically the same as orthoclase, but belongs to the triclinic crystal system, the prism angle being slightly less than right angles; hence the name "microcline" from the Greek "small slope." Microcline is identical to orthoclase in all physical properties and can be distinguished only by optical examination; under a polarizing microscope microcline exhibits a minute multiple twinning which results from a grating-like structure that is unmistakable. It is probable that much orthoclase is actually microcline. Amazon stone, or amazonite, is a beautiful green variety of microcline. It is not found anywhere in the Amazon basin, however, Spanish explorers who named it apparently confused it with another green mineral from that region.
A soda microcline named anorthoclase is known, which is an isomorphous mixture of KAlSi3O8 and NaAlSi3O8, the sodium-aluminium silicate being in larger proportion.
The sodium feldspar albite (NaAlSi3O8) and the calcium feldspar anorthite (CaAl2Si2O8) form an isomorphous series from pure albite at one end and pure anorthite at the other, the molecules being completely miscible with each other. The members of this series are known as the soda-lime (or lime-soda) feldspars, and as a group are called the plagioclase feldspars (from the Greek meaning "oblique fracture," in reference to the two not-quite-right-angle cleavages). Always present are striations, fine parallel lines, resulting from minute multiple twinning which is never seen on orthoclase or microcline.
More or less arbitrarily, four intermediate plagioclase feldspars are recognized between albite and anorthite.
Albite is named from the Latin albus, in reference to its unusually pure white color. It is a relatively common and important rock-making mineral associated with the more acid rock types and in pegmatite dikes, often with rarer minerals like tourmaline and beryl.
Anorthite was named by Rose in 1823 from the Greek meaning oblique, referring to its triclinic crystallization. Anorthite is characeristic of the basic igneous rocks such as gabbro and basalt.
The intermediate members of the plagioclase group are very similar to each other and cannot be distinguished except by optical means.
Oligoclase is common in granite, syenite, diorite and gneiss. It is a frequent associate of orthoclase. The name oligoclase is derived from the Greek for little and fracture, in reference to the fact that its cleavage angle differs significantly from 90°. Sunstone is mainly oligoclase (sometimes albite) with flakes of hematite.
Andesine is a characteristic mineral of rocks such as diorite which contain a moderate amount of silica and related extrusives such as andesite.
Labradorite is the characteristic feldspar of the more basic rock types such as diorite, gabbro, andesite or basalt and is usually associated with one of the pyroxenes or amphiboles. Labradorite frequently shows an iridescent display of colors due to minute inclusions of another mineral. It is named after Labrador, where it is a constituent of anorthosite.
Bytownite, named after the former name for Ottawa, Canada (Bytown), is a rare mineral occasionally found in more basic rocks.