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.