Feldspars moonstone

The feldspars are widely distributed and comprise almost two-thirds of all igneous rocks. Orthoclase and albite (NaAlSisOg) are feldspars in which one-fourth of the silicon atoms are replaced by aluminum and anorthite (CaAl2Si20g) and has one-half of the silicon atoms replaced by aluminum. Because the ionic radius of Na+ (0.095 nm) and Ca" " (0.1 nm) are the same, solid solutions are often formed between albite and anorthite. Good stones of albite and orthoclase are known as moonstones. 

Feldspars are the most abundant minerals in the earth s crust, accounting for about 60% of all igneous rocks. They are derivatives of silica in which about one-half or one-quarter of the silicon atoms have been replaced by aluminum. Feldspar is used in the manufacture of certain types of glass and pottery. Some feldspar crystals, such as moonstone (white perthilte), Amazon stone (green microcline), and multicolored labradorite, are used as gem stones and in architectural decorations. Some are used as a coating and filler in the production of paper. 

Opalescence is a pearly or milky internal reflection, most common in white or light-colored minerals such as some opal or moonstone (albite or ortho-clase feldspar). This is also known as adularescence. Like iridescence and play of color, these properties are not consistent within any natural material, and can only be used to describe individual specimens. Some synthetic materials have been created specifically to display these properties and so always show them. 

Moonstone Glass. A type of opal glass (q.v.) resembling moonstone (a whitish, pearly feldspar). 

Orthodase ) (common feldspar) KAlSijOg (Figure M66) Monoclinic. Grains are tabular and have nearparallel orientation. Blunted prisms A reddish, cream-yellow, or gray mineral. If green, it is called amazonstone or amazonite. If opalescent, it is called moonstone... 

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