Cryptomelane

oxide

supergene

Formula: K(Mn4+7Mn3+)O16
Multiple oxide, cryptomelane group
Specific gravity: 4.3
Hardness: 5 to 6½
Streak: Brownish black
Colour: Grey to black (Dana)
Environments:

Pegmatites
Hydrothermal environments

Cryptomelane is the commonest of the hard, black, fine-grained manganese oxides formerly called psilomelane. It is a low-temperature(R&M88.3.279). supergene mineral(JRS 8(1).1-9), widespread in oxidised manganese deposits as open-space fillings or replacing primary manganese-bearing minerals. It is commonly replaced by other secondary manganese minerals, and associated with pyrolusite, nsutite, braunite, chalcophanite, manganite and other manganese oxides (HOM).
At Burdell Gill, Caldbeck Fells, Cumbria, England, there is some evidence that cryptomelane has pseudomorphed a rhombohedral mineral, possibly ankerite or siderite (JRS8(1).1-9).

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