Formula: KAlSi3O8
Origin of name: from Greek orthos = right and klas = to fracture
Crystal system: monoclinic
Crystal class: 2/m
Twinning: common by Carlsbad, Baveno or Manebach law
Unit cell: a = 8.56 Å, b = 12.98 Å, c = 7.21 Å, b = 116.0°
Colour: colourless, white, gray, yellow, reddish, greenish
Diaphaneity:
Luster: vitreous to pearly
Habit: crystals short prismatic, blocky, may have orthorhombic or tetragonal aspect, massive, cleavable to granular, twinned
Hardness: 6 to 6.5
Specific gravity: 2.55 to 2.63
Cleavage: 6; {001} and {010} perfect, {100}, {110}, {-110} and {-201} partings
Tenacity: conchoidal to uneven, brittle
Streak: white
Synonyms/varieties: adularia (wedge-shaped variety), K-feldspar, potassium feldspar, potash feldspar, moonstone (adularescent gem variety)
Comments: trimorphous with microcline and sanidine, series with celsian and hyalophane
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