Synopses: Mitridatite has a 17·53, b 19·35, c 11·25 Å, β 95·92°, space group A2/a (Moore, 1974) and is closely related to robertsite and arseniosiderite. Its structure (the atomic co-ordinates of which are given) is based on a compact sheet, Fe93+O6(PO4)912−, with pseudotrigonal symmetry (fig. 1), formed from a ring of nine FeO6 octahedra with a PO4 tetrahedron at the centre, linked by two further PO4 tetrahedra in the plane of the sheet and by six more PO4 tetrahedra above and below the plane of the sheet (not shown on fig. 1); Ca ions and H2O molecules lie between these sheets, the Ca in a CaO5(H2O)2 polyhedron, and three water molecules not linked to any metal complete the asymmetric unit Ca6(H2O)6Fe93+O6(PO4)9·3H2O.
Betpakdalite and melkovite, which have unit cells dimensionally similar to that of mitridatite, may be related structurally. Other structures, known or hypothetical, based on the Fe9O6 ring are shortly discussed.