The crystal structure of balićžunićite, Bi2O(SO4)2, a new natural bismuth oxide sulphate

The crystal structure of balićžunićite, Bi2O(SO4)2, a new mineral species from "La Fossa" crater of Vulcano (Aeolian Islands, Italy), was solved from single-crystal X-ray-diffraction data and refined to R = 0.0507. The structure is triclinic, space group P-1, with lattice parameters a 6.7386(3), b 11.1844(5), c 14.1754(7) Å, α 80.082(2){degree sign}, β 88.462(2){degree sign}, γ 89.517(2){degree sign}, V 1052.01(8) Å3 and Z = 6. The crystal structure consists of 6 independent Bi sites, 6 S sites and 27 O sites of which 3 are oxo oxygen atoms not bonded to sulphur. Bi and S atoms are arranged close to an eutectic pattern parallel to the (100) crystal planes. The planes are stacked atom on atom such that Bi always overlays S and vice versa. This structural feature is shared with the known structure of the high temperature polymorph of the same compound, stable over 535oC. However, the sequences of Bi and S atoms in the two structures are different and so are the arrangements of oxygen atoms. Characteristic building blocks in the structure of balićžunićite are clusters of five Bi atoms which form nearly planar trapezoidal Bi5 groups with oxo oxygens located in the centres of the three Bi3 triangles which form the trapezoids. The trapezoidal Bi5O39+ ions are joined along [100] with SO42- groups by means of strong bismuth-sulphate oxygen bonds, forming infinite [100] rods with composition Bi5O3(SO4)51-. One sixths of Bi atoms do not participate in trapezoids, but form with additional SO42- groups rows of composition BiSO41+, also parallel to [100]. [Bi5O3(SO4)51-] rods form infinite layers parallel to the crystal plane (010) with [BiSO41+] rows located on the irregular surface of contact between adjacent layers. Bi atoms occur in four different coordination types, all showing the stereochemical influence of the Bi3+ lone electron pair. In this respect the crystal structure of balićžunićite shows greater variability than its high temperature polymorph which has only two types of Bi coordinations present in balićžunićite.