Silicate of sodium and aluminium to silicate of calcium and aluminium. This is an isornorphous series of minerals, of the feldspar group, consisting of solid solutions of one or other component in variable proportions. Depending on the amount of each component, the mineral may be called albite (the mainly sodium end-member), oligoclase, andesine, labradorite, bytownite, or anorthite (the mainly calcium end-mernber). They are very widely distributed in rocks.
Crystal system Triclinic.
Appearance Plagioclase feldspars occur as crystals of prismatic, often tabular habit, with fine striations (twinning planes or cleavage surfaces); also as crystalline masses.
The crystal form is hardly ever developed in rocks. They can be transparent, or (rarely) almost fully opaque to semiopaque. The color is often whitish or grayish white, but can be yellowish, pale green, or even pink. It sometimes displays iridescence on a dark gray ground. The varieties used as gems are mainly albite, which is mostly or wholly transparent, and labradorite, which is iridescent.
Physical properties It has a hardness of about 6-6.5.
The density steadily increases from about 2.62 g / i:m3 for albite to about 2.?6 g/cma for anorthite. The refractive indices also increase. from no 1.525, ny 1.536 for albite to no 1.5?6, fty 1.588 for anorthite.
Genesis These minerals crystallize in nearly all magmatic rocks, both intrusive and extrusive. in many metamorphic rocks. and in pegmatites.
Occurrence They are very common and distributed worldwide. Large crystals are found in pegmatites in Norway, the Soviet Union, and the United States.