Formula: Ca2Al(Al,Si)O7
Origin of name: For A. Gehlen, German chemist
Crystal system: Tetragonal
Crystal class: -42m
Twinning:
Unit cell:
Colour: Colourless, grayish green to brown, yellowish
Diaphaneity: Transparent to translucent
Luster: Vitreous to resinous
Habit: Crystals short prismatic; massive, granular
Hardness: 5 to 6
Specific gravity: 3.04
Cleavage: 2; {001} distinct, {110} indistinct
Tenacity:
Streak: White to grayish
Synonyms/varieties:
Comments: Series with åkermanite
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