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The crystal structure of the new mineral aravaite Ba2Ca18(SiO4)(6)(PO4)(3)(CO3)F3O [R (3) over barm, a = 7.12550 (11), c = 66.2902 (13) angstrom, V = 2914.81 (8) angstrom(3), Z = 3] was solved from single-crystal diffraction data, collected using synchrotron radiation at the X06DA beamline of the Swiss Light Source. The unit cell of this modular mineral contains six layers of {Ba(PO4)(1.5)(CO3)(0.5)}(3.5-) (T-layer), three triple antiperovskite layers (tAP) {(F2OCa12)(SiO4)(4)}(4+), and three single antiperovskite layers (sAP) {(FCa6)(SiO4)(2)}(3+). The structure refinement confirms a model with a layer sequence of T-sAP-T-tAP as an average structure of this mineral. However, one-dimensional diffuse scattering observed parallel to c* implies imperfections in the stacking sequence of the average structure. Qualitative modelling of disorder confirms that the alternating sequence of T-sAP and T-tAP blocks is disturbed. The blocks occurring in this new mineral are known from other so-called hexagonal intercalated antiperovskite structures: T-sAP (stracherite and zadovite group), T-tAP (ariegilatite and nabimusaite group).