Formula: (Ca,Na)2(Fe,Fe,Ti)6(Si,Be,Al)6O20
Origin of name: After locality; Proterozoic Høgtuva Window, Norway
Crystal system: Triclinic
Crystal class: 1
Twinning:
Unit cell: a = 10.317, b = 10.724, c = 8.855, α = 105.77°, β = 96.21°, γ = 124.77°
Colour: Black
Diaphaneity: Opaque to subtranslucent
Luster: Subadamantine
Habit: Late-stage poikiloblastic metamorphic crystals prismatic to 6 x 40 mm
Hardness: 5.5
Specific gravity: 3.85
Cleavage: 3; Two at 55 degrees, imperfect, imperfect parting
Tenacity:
Streak: Dark green
Synonyms/varieties:
Comments: Insoluble in most common acids
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