Plagioclase feldspar ranges in composition from calcium aluminum silicate (anorthite) to sodium aluminum silicate (albite). Plagioclase is common in igneous rocks such as . Plagioclase is an alkalai feldspar that is one of several feldspar minerals, including sanidine, orthoclase, and microcline. 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 a northite.
Pronunciation: PLAJ•ee•oh•klase FELD•spar (accent on capitalized syllable)
Color: ranges from white (albite) to gray (anorthite)
Luster: vitreous
Diaphaneity: translucent, but may appear to be opaque in hand specimen
Hardness: 6
Specific gravity: 2.62 (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. Plagioclase is the most common feldspar, and occurs in igneous, metamorphic, and as (uncommon) detrital grains in sedimentary rocks. Pl agioclase feldspar may be mistaken for orthoclase feldspars, but orthoclase typically varies in color from white to pink and lacks the fine striations that plagioclase felds pars have.
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/
Return to the Minerals
page.
