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A triclinic mineral, (Na,Ca)[(Si,Al)AlSi2 O8 ] ; plagioclase
series of the feldspar group; has NaSi (albite) 10 to 30 mol % and CaAl
(anorthite) 90 to 70 mol %; pseudomonoclinic with prismatic cleavage and
characteristic polysynthetic twinning commonly visible on cleavage traces;
white; may be chatoyant; a common rock-forming mineral in igneous and
metamorphic rocks of intermediate to high silica content; forms the entire
mass in some anorthosites; less commonly a vein mineral.
Source:
Dictionary of Mining, Mineral, and Related Terms
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