Formula: KAl2(Si3B)O10(OH,F)2
Origin of name: From boron and muscovite
Crystal system: Monoclinic
Crystal class: 2/m
Twinning:
Unit cell: -1M: a = 5.077 Å, b = 8.775 Å, c = 10.061 Å, b = 101.31°, -2M1: a = 5.075 Å, b = 8.794 Å, c = 19.815 Å, b = 95.59°
Colour: Buff to white or pale cream
Diaphaneity: Nearly opaque
Luster: Porcelaneous
Habit: Grains < 3-4 µm forming hard, porcelaneous coatings to 1 cm thick
Hardness: 2.5 to 3
Specific gravity: 2.81
Cleavage: 2; {100} perfect, {010} poor parting
Tenacity: Subconchoidal fracture
Streak: White
Synonyms/varieties:
Comments: 2 polytypes -1M and 2M1, micagroup. Found as coating on other minerals on the floor of a pegmatite pocket, hydrothermally deposited after rupture of the pocket
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