Formula: (Mn,Fe,Mg)2Al4Si2O10(OH)4
Origin of name: after locality near Ottrez, Belgium
Crystal system: monoclinic and triclinic
Crystal class: m, 2/m and -1 or 1
Twinning: polysynthetic
Unit cell:
Colour: pistachio green
Diaphaneity:
Luster: vitreous to adamantine on cleavage
Habit: flakes, polysynthetic twinning
Hardness: 6 to 7
Specific gravity: 3.52
Cleavage: 1; {001} perfect
Tenacity:
Streak: greenish, grayish
Synonyms/varieties:
Comments: both monoclinic and triclinic forms exists
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