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. It has a hardness of 6, a specific gravity of 2.56-2.58, and a vitreous to pearly lustre. It can be colored white, gray, yellow or red; rarely green. Twin crystals are not uncommon. Orthoclase is a common constituent of many igneous rocks and is often found in huge masses in pegmatite veins. Orthoclase is used in the manufacture of porcelain and as a constituent of scouring powder. Adularia (from Adular) is essentially pure potassium silicate; when pearly and opalescent it 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 found in the trachytes of the Drachenfels, Germany.
Microcline (KAlSi3O8) is chemically the same as orthoclase, but belongs to the triclinic 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 unmistakeable. 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 anthorite 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.
|Name||% NaAlSi3O8||% CaAl2Si2O8|
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.
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.
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.
See also: list of minerals