The birnessite group includes: birnessite: (Na,Ca,K)0.6(Mn4+,Mn3+)2O4·1.5H2O ranciéite: (Ca,Mn2+)0.2(Mn4+,Mn3+)O2.0.6H2O takanelite: (Mn2+,Ca)2x(Mn4+)1-xO2.0.7H2O Back to Minerals