Formula: Al6(Al,Ta)[(Si,As,Sb)O4]3(BO3)(O,OH)3
Origin of name: Named for Harold Edward Holt (1908-1967), Prime minister of Australia (1966-1967).
Crystal system: Orthorhombic
Crystal class: 2/m 2/m 2/m
Twinning:
Unit cell: a = 4.6914, b = 11.896, c = 20.383
Colour: Pale brownish white to brown, greenish
Diaphaneity: Semitransparent
Luster: Vitreous to resinous or dull
Habit: Crystals prismatic pseudohexagonal, or acicular; as pebbles
Hardness: 8.5
Specific gravity: 3.90
Cleavage: 1; {010} distinct
Tenacity:
Streak: Light buff
Synonyms/varieties:
Comments: Fluoresces dull orange under SW and bright yellow under LW
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