psilomelane

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psilomelane A hydrated manganese oxide with the possible formula Ba3(Mn2+Mn74+) O16(OH)4; sp. gr. 3.7–4.7; hardness 5–6; grey-black; sub-metallic or earthy lustre; massive, botryoidal; occurs as a precipitate in secondary manganese deposits and may be worked commercially for manganese.

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