Feldspar is a family of alumino-silicate minerals that constitute the most common minerals in Earth's crust. 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.
Plagioclase feldspar ranges in composition from calcium aluminum silicate (anorthite) to sodium aluminum silicate (albite). Plagioclase is common in igneous rocks such as gabbro and diorite. There are several types of plagioclase feldspar which differ from one another based on variations in relative calcium and sodium content, including albite, oligoclase, andesine, labradorite, bytownite and anorthite.
Pronunciation: FELD•spar (accent on capitalized syllable)
Color: orthoclase ranges from white to gray to light pink, while plagioclase ranges from white (albite) to gray (anorthite)
Luster: vitreous
Diaphaneity: translucent, but may appear to be opaque in hand specimen
Hardness: 6
Specific gravity: 2.55 (orthoclase), 2.62 (albite) to 2.76 (anorthite)
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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