A manganese pan near Birness contains grains of an optically uniaxial negative mineral near (Na_0·7Ca_0·3)Mn₇O_14·2·8H₂O, 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.