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Mica
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Micas are a group of silicate minerals whose outstanding physical characteristic is that individual mica crystals can easily be split into extremely thin elastic plates. This characteristic is described as perfect basal cleavage. Wikipedia
Category: Phyllosilicates
Cleavage: Almost perfect
Diagnostic features: cleavage
Formula (repeating unit): AB2–3(X, Si)4O10(O, F, OH)2
Fracture: flaky
Luster: pearly, vitreous
Mohs scale hardness: 2.5–4 (lepidolite); 2.5–3 (biotite); 2.5–3 (phlogopite); 2–2.5 (muscovite)