Formula: (Sc,Y)2Si2O7
Origin of name: For Olaus Thortveit, Norwegian engineer who discovered the mineral
Crystal system: Monoclinic
Crystal class: 2/m
Twinning: Common, twin axis perpendicular {110}, composition plane {110}, rarely polysynthetically
Unit cell: a = 6.587, b = 8.547, c = 4.695, β = 102.65°
Colour: Grayish green to black
Diaphaneity: Semitransparent
Luster: Vitreous to subadamantine
Habit: Crystals typically prismatic along [110], somewhat pointed, distorted
Hardness: 6 to 7
Specific gravity: 3.27-3.58
Cleavage: 2; {110} distinct, {001} parting, prismatic
Tenacity: Very brittle, uneven to conchoidal fracture
Streak: Pale grayish green
Synonyms/varieties:
Comments: Found in granitic pegmatite rocks
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