Birnessite

Birnessite (Na_0.3Ca_0.1K_0.1Mn⁴⁺Mn³⁺O₄·1.5(H₂O)) is an mineral oxide of manganese along with calcium, potassium and sodium. It has a dark brown to black color with a submetallic luster. It is also very soft, with a hardness of 1.5. Birnessite is formed by precipitation in lakes, oceans and groundwater and is a major component of desert varnish and deep sea manganese nodules.

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It was first described in 1956 and named for an occurrence in Birness, Aberdeenshire, Scotland.

References

• Mindat with location data
• Webmineral data
• Mineral Data Publishing - PDF