Pyrolusite
Common manganese mineral, manganese dioxide (MnO2), that constitutes an important ore. Always formed under highly oxidizing conditions, it forms light-gray to black, metallic, moderately heavy coatings, crusts, or fibres that are alteration products of other manganese ores (e.g., rhodochrosite); bog, lake, or shallow marine products; or deposits left by circulating waters.
posted by TiredKnife @ 5:38 AM
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