Orthoclase feldspar is a potassium aluminum silicate, and is commonly called "potassium feldspar" or simply "K-spar," given that the chemical symbol for potassium is "K." Orthoclase is common in igneous rocks such as granite, granodiorite and syenite, as well as in crack-filling igneous vein material (pegmatite). Orthoclase is an alkalai feldspar that is one of several feldspar minerals, including sanidine, orthoclase, microcline and plagioclase.
Pronunciation: OR•thoe•klase FELD•spar (accent on capitalized syllable)
Color: ranges from white to gray to light pink
Luster: vitreous
Diaphaneity: translucent, but may appear to be opaque in hand specimen
Hardness: 6
Specific gravity: 2.55
Cleavage/fracture: 2 directions approximately at right angles.
Other distinguishing properties: Feldspar crystals commonly look like a rectangle or an elongated hexagon. The pink crystals in granites and rhyolites are generally orthoclase feldspar grains. Orthoclase feldspar may be mistaken for plagioclase feldspars, but plagioclase typically contains fine parallel striae (look like thin scratches) on one of the cleavage faces, and plagioclase varies in color from white to dark gray.
Klein, C., and Hurlbut, C.S., Jr., 1999, Manual of Mineralogy (after James D. Dana) [21st edition, revised]: New York, John Wiley & Sons, 682 p.
Los Angeles County Museum of Natural History, The Photo Atlas of Minerals: nhm.org/pam/
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