The composition of a plagioclase feldspar is typically denoted by its overall fraction of anorthite or albite, 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. 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.
Anorthite was named by Gustav Rose in 1823 from the Ancient Greek meaning oblique, referring to its triclinic crystallization. Anorthite is a comparatively rare mineral but occurs in the basic plutonic rocks of some orogeniccalc-alkaline suites.
Albite is named from the Latinalbus, 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 pegmatitedikes, often as the variety Cleavelandite with rarer minerals like tourmaline and beryl.
Intermediate members
The intermediate members of the plagioclase group are very similar to each other and normally cannot be distinguished except by their optical properties. The specific gravity in each member increases 0.02 per 10% increase in anorthite.
Bytownite, named after the former name for Ottawa, Ontario, Canada, is a rare mineral occasionally found in more basic rocks.
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 rockanorthosite which is composed almost entirely of plagioclase. A variety of labradorite known as spectrolite is found in Finland.
Andesine is a characteristic mineral of rocks such as diorite which contain a moderate amount of silica and related volcanics such as andesite.
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 with flakes of hematite.
