Cyanotrichite
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Cyanotrichite is a hydrous copper aluminium sulfate mineral with formula Cu₄Al₂[(OH)₁₂|SO₄]·2H₂O, also known as lettsomite. Cyanotrichite forms velvety radial acicular crystal aggregates of extremely fine fibers. Wikipedia
Formula (repeating unit): Cu4Al2·2H2O
IMA symbol: Cya
Luster: Silky
Mohs scale hardness: 1–3
Optical properties: Biaxial (+)