The Feldspar Group

Written by Administrator

Tuesday, 13 November 2007

The Feldspar Group The feldspars are an important group of rock-forming minerals and are widely distributed over the earth. Chemically, they are silicates of alumi¬num, and another metal, which may be potassium, sodium, or calcium. The milky-white shimmering moonstone so often met with in jewelry is the best known of the feldspar gems. It belongs to the orthoclase variety and is a potassium feldspar with a general formula KAlSi308. Chief sources of moonstone are the gem gravels of Ceylon and Burma. A yellow transparent orthoclase form is also found in the island of Madagascar. Yet another potassium feldspar with the same formula is the green opaque microline variety known as amazonite, found chiefly in Colorado and in the Ural Mountains. It is somewhat similar in appearance to poor quality turquoise. The sodium and calcium varieties fall into the group known as plagioclase feldspars. Only two of these are of interest in jewelry: the gray variety, labradorite, which shows a metallic play of color at its cleavage surfaces, and a reddish spangled variety known as sunstone. The latter seems to derive its red glow from the light reflection of crystals of an iron mineral within its structure. All feldspar gems have a hardness approximating 6 and are mostly used in cheaper jewelry. In industry, feldspar minerals are of importance in the manufacture of porcelain and are a component of glazes on earthenware, enameled brick, and many other objects that are glazed before use.

Last Updated ( Tuesday, 13 November 2007 )