Feldspar crystals are either monoclinic or triclinic (see crystal), and all show clean cleavage planes in two directions. Orthoclase feldspars have cleavage planes that intersect at right angles; triclinic feldspars, including the plagioclase feldspars (e.g., albite, anorthite, and labradorite) and microcline, have cleavage planes that form slightly oblique angles. Pure feldspar is colorless and transparent but the mineral is commonly opaque and found in a variety of colors.
Orthoclase and microcline are called potassium or potash feldspars. They usually range from flesh color to brick red, although other colors are found, and are used in the making of porcelain and as a source of aluminum in making glass. Moonstone is usually a milky, bluish variety of orthoclase used as a gem, and a green variety of microcline known as amazonite, or Amazon stone, is used for ornamental purposes. The plagioclase feldspars are most commonly gray and occasionally red. A milky variety of plagioclase feldspar oligoclase also is used as the gem moonstone, and a reddish or golden variety that exhibits flashes of reddish color is used as the gem sunstone. Another form of feldspar, labradorite, exhibits a play of colors, which makes it valuable for decorative purposes.
The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2023, Columbia University Press. All rights reserved.
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