Formula: Al4(Ta,Nb)3(O,OH,F)14
Origin of name: for E. Simpson, Australian mineralogist
Crystal system: hexagonal
Crystal class: 3
Twinning:
Unit cell:
Colour: colourless, pale yellow, cream to light brown
Diaphaneity: transparent
Luster: vitreous to adamantine
Habit: crystals tabular or short prismatic, sometimes bipyramidal, crystalline masses
Hardness: 7 to 7.5
Specific gravity: 6.70
Cleavage: none
Tenacity:
Streak:
Synonyms/varieties:
Comments: fluoresces pale yellow, bluish white or yellowish blue
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