The ericssonite group of Fe3+ disilicate minerals

The ericssonite group contains two Fe3þ disilicates with the ericssonite structure-type arrangement: ericssonite, BaMn2Fe3þ(Si2O7)O(OH), and ferroericssonite, BaFe2+2Fe3+(Si2O7)O(OH) (Memorandum 78-SM/17). Ericssonite has two polytypes, of monoclinic symmetry and orthorhombic symmetry. The crystal chemistry of the ericssonite-group minerals and bafertisite- and lamprophyllite-group minerals (seidozerite supergroup) is compared. Ericssonite and ferroericssonite do not contain the combination of features that would allow them to belong to any group of the seidozerite supergroup and hence require a completely separate group.