SummaryA manganese pan near Birness contains grains of an optically uniaxial negative mineral near (Na0·7Ca0·3)Mn7O14·2·8H2O, giving an X-ray powder pattern similar to that of synthetic materials described as ‘manganous manganite’ and δ-MnO2. Material giving a similar pattern has been described from a natural occurrence in Canada, but no mineral name was assigned; the name birnessite is now proposed. The mineral is probably formed by air-oxidation of manganous oxides under alkaline conditions.

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