Plagioclase Feldspars

Category  :  feldspar, tectosilicate
Hardness  :  6-6.5
Gravity  :  2.62
Reflection  :  1.54 / 1.53
                    1.59 / 1.58
Color  :  whitish or greyish white,yellowish, plane green or even pink.
Chemical formula  :  NaAlSi3O8-CaAl2Si2O8
Crystal system  :  triclinic
Major varieties  :  Albite Moonstone, Labradorite

Plagioclase is a major constituent mineral in the Earth's crust, and is consequently an important diagnostic tool in petrology for identifying the composition, origin and evolution of igneous rocks. Plagioclase is also a major constituent of rock in the highlands of the Earth's moon.

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.

Albite Moonstone

It is typically misty, semi transparent or semi opaque with a pale, shimmering reflection, less well defined that in chatoyant stones. Curved pieces are also cut as necklace beads or pendants. The composition is midway between that of orthoclase and that of albite. It is almost always cut in cabochon.

Labradorite

This is sodium rich plagioclase feldspar which displays a particular type of iridescence on a dark ground. It has the presence of very fine platelets of different compositions and minute inclusions of ilmenite, rutile and perhaps, magnetite, which cause diffraction. It is cut into gems, or small, not too convex, polished plaques for setting.


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