Plagioclase – Bandage – inflatable cervical collar
Plagioclase series members
The composition of a plagioclase feldspar is typically denoted by its overall fraction of anorthite (%An) or albite (%Ab), and readily determined by measuring the plagioclase crystal’s refractive index in crushed grain mounts, or its extinction angle in thin section under a polarizing microscope. The extinction angle is an optical characteristic and varies with the albite fraction (%Ab). There are several named plagioclase feldspars that fall between albite and anorthite in the series. The following table shows their compositions in terms of constituent anorthite and albite percentages.
Plagioclase minerals and their compositions
Name
% NaAlSi3O8(%Ab)
% CaAl2Si2O8(%An)
Albite
10090
010
Oligoclase
9070
1030
Andesine
7050
3050
Labradorite
5030
5070
Bytownite
3010
7090
Anorthite
100
90100
Albite
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
Anorthite was named by Rose in 1823 from the Greek meaning oblique, referring to its triclinic crystallization. Anorthite is a comparatively rare mineral but occurs in the basic plutonic rocks of some orogenic calc-alkaline suites.
The intermediate members of the plagioclase group are very similar to each other and normally cannot be distinguished except by their optical properties.
Oligoclase
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
Andesine is a characteristic mineral of rocks such as diorite which contain a moderate amount of silica and related volcanics such as andesite.
Labradorite
Labradorite displaying typical iridescent effect termed labradorescence. (unknown scale)
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 light refracting within the lamellae of the crystal. It is named after Labrador, where it is a constituent of the intrusive igneous rock anorthosite which is composed almost entirely of plagioclase. A variety of labradorite known as spectrolite is found in Finland.
Bytownite
Bytownite, named after the former name for Ottawa, Canada (Bytown), is a rare mineral occasionally found in more basic rocks.
See also
List of minerals
References
Webmineral
Wikimedia Commons has media related to: Plagioclase
Categories: Sodium minerals | Aluminium minerals | Calcium minerals | Tectosilicates
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