Formula: NaBeSi3O7(OH)
Origin of name: From Greek epi and didymos = twin
Crystal system: Orthorhombic
Crystal class: 2/m 2/m 2/m
Twinning: Common on {100}, simple, polysynthetic, or trillings
Unit cell: a = 12.66 Ǻ, b = 7.33 Ǻ, c = 13.61 Ǻ
Colour: White, colourless, gray, yellow, blue, violet
Diaphaneity:
Luster: Vitreous, pearly on cleavage
Habit: Crystals tabular; micaceous, coarsely crystalline; cryptocrystalline to finely crystalline; radial fibrous
Hardness: 6 to 7
Specific gravity: 2.55
Cleavage: 2; {001} perfect, {100} distinct
Tenacity: Brittle
Streak:
Synonyms/varieties:
Comments: Dimorphous with eudidymite
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