THE CRYSTAL STRUCTURE OF BAKHCHISARAITSEVITE, [Na2(H2O)2] , A NEW MINERAL SPECIES OF HYDROTHERMAL ORIGIN FROM THE KOVDOR PHOSCORITE-CARBONATITE COMPLEX, RUSSIA

The crystal structure of a new mineral species, bakhchisaraitsevite [Na 2 (H 2 O) 2 ]{(Mg,Fe) 5 (H 2 O) 5 (PO 4 ) 4 }, a 8.3086(8), b 12.906(1), c 17.486(2) A, β 102.01(1)°, V 1834.0(1) A 3 , space group P 2 1 / c , Z = 4, ρ calc = 2.499 g/cm 3 , has been determined [automated single-crystal diffractometer, Mo K α, graphite monochromator, T = 193 K, 21597 reflections, R = 0.033 for 3527 unique reflections with I ≥ 2σ( I ), wR 2 = 0.072]. It can be described as an open framework of Mg(Fe) octahedra and PO 4 tetrahedra. Extremely contorted chains of edge-sharing Mg(Fe) octahedra form layers parallel to the ab plane. The unit cell of bakhchisaraitsevite contains two layers of this kind. In the c direction, these layers are also joined by dimers of Mg(Fe) octahedra having common edges. The PO 4 tetrahedra consolidate the structure, sharing the majority of vertices with octahedra. Na atoms and H 2 O molecules are distributed in the channels of an open framework. The main feature of the crystal structure, layers of octahedra parallel to the ab plane, explains the flattened habit and the perfect cleavage of the crystals. The crystal-chemical formula of the mineral, [Na 2 (H 2 O) 2 ]{(Mg 4.5 Fe 0.5 )(H 2 O) 5 (PO 4 ) 4 }, where curly brackets enclose the framework component and square brackets show the contents of the channels, reflects the different functions of the H 2 O molecules in the structure. Bakhchisaraitsevite as a possible biomineral can be expected to play a role in accumulating phosphorus in recent sulfidic sediments with the participation of bacteria. We describe the close structural relations between bakhchisaraitsevite and rimkorolgite.