Formula: 70-90%(Na,Ca)Al(Al,Si)Si2O8 10-30%CaAl2Si2O8
Origin of name: from Greek oligas = small and klan = to fracture
Crystal system: triclinic
Crystal class: -1
Twinning: commonly by albite, Carlsbad or pericline law
Unit cell: a = 8.16 Å, b = 12.88 Å, c = 7.11 Å, a = 93.4°, b = 116.3°, g = 90.3°
Colour: Colourless, white, gray, greenish, yellowish, brown, reddish
Diaphaneity:
Luster: vitreous
Habit: crystals tabular, commonly twinned on Carlsbad, albite or pericline laws, massive, cleavable, granular or compact
Hardness: 6 to 6.5
Specific gravity: 3.63 to 3.67
Cleavage: 3; {001} perfect, {010} nearly perfect, {110} imperfect
Tenacity: conchoidal to uneven, brittle
Streak: white
Synonyms/varieties: plagioclase [An 10-30], sunstone (iridescent gem variety)
Comments: intermediate member of the plagioclase series
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