Formula: CaMg(AsO4)F
Origin of name: for Daniel Tilas (1712-1772), Swedish mining engineer
Crystal system: Monoclinic
Crystal class: 2/m
Twinning: common on {001} as contact twins
Unit cell: a = 6.681-6.701, b = 8.950-8.944, c = 7.553-7.573, b = 120°58'-121°14'
Colour: Milky white, gray to violet-gray, rose-red, apple-green to olive-green
Diaphaneity: Translucent
Luster: Resinous
Habit: Crystals complex, may be elongated and flattened, to 6 mm, twinned, massive, veinlets
Hardness: 5
Specific gravity: 3.77
Cleavage: 4; {101} good, {133}, {102} and {011} parting
Tenacity: Brittle
Streak: White
Synonyms/varieties:
Comments: Piezoelectric, may fluoresce pale yellow, pale pinkish orange, dark red under SW-UV
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