Arfvedsonite
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Arfvedsonite or soda hornblende is a sodium amphibole mineral with composition: [Na][Na₂][(Fe²⁺)₄Fe³⁺][(OH)₂|Si₈O₂₂]. It crystallizes in the monoclinic prismatic crystal system and typically occurs as greenish black to bluish grey fibrous to... Wikipedia
Color: Black, deep green on thin edges
Cleavage: Perfect on
Crystal class: Prismatic (2/m); (same H-M symbol)
Crystal habit: Fibrous, radial prismatic aggregates
Crystal system: Monoclinic
Dana classification: 66.1.3c.9
Formula (repeating unit): Na