Aravaite, Ba2Ca18(SiO4)6(PO4)3(CO3)F3O: modular structure and disorder of a new mineral with single and triple antiperovskite layers

The crystal structure of the new mineral aravaite Ba2Ca18(SiO4)6(PO4)3(CO3)F3O [R{bar 3}m, a = 7.12550 (11), c = 66.2902 (13) A, V = 2914.81 (8) A3, 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).