AM_Olmsteadite Formula: (Nb.59 Ta.41) Fe2 K P2 O12 H4 Name: Olmsteadite Unit cell: 7.512 10.0 6.492 90.0 90.0 90.0 , V=487.68 Space group: PB21M (26)

Moore P B , Araki T , Kampf A R , Steele I M , American Mineralogist , 61 (1976) p.5-11, Olmstaedite, K2Fe2[Fe2(Nb,Ta)2O4(H2O)4(PO4)4], a new species, its crystal, structure and relation to vauxite and montgomeryite