The crystal structure of baliczunicite, Bi2O(SO4)2, a new natural bismuth oxide sulfate

The crystal structure of baliczunicite, Bi2O(SO4)(2), a new mineral species from the 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) over bar, with a = 6.7386(3), b = 11.1844(5), c = 14.1754(7) angstrom, alpha = 80.082(2), beta = 88.462(2)degrees, gamma = 89.517(2)degrees, V = 1052.01(8) angstrom(3) and Z = 6. The crystal structure consists of six independent Bi sites, six S sites and 27 O sites of which three are oxo oxygen atoms not bonded to sulfur. Bismuth and S atoms are arranged close to a eutectic pattern parallel to the (100) plane. 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 at T > 535 degrees C. 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 baliczunicite are clusters of five Bi atoms which form nearly planar trapezoidal Bi-5 groups with oxo oxygens located in the centres of the three Bi-3 triangles, which form the trapezoids. The trapezoidal Bi5O39+ ions are joined along [100] with SO42- groups by means of strong bismuth-sulfate oxygen bonds, forming infinite [100] rods with composition Bi5O3(SO4)(5)(-). One sixth of the Bi atoms do not participate in trapezoids, but form, with additional SO42- groups, rows of composition BiSO4+, also parallel to [100]. [Bi5O3(SO4)(5)(-)] rods form infinite layers parallel to (010) with [BiSO4+] 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 baliczunicite shows greater variability than its high-temperature polymorph which has only two types of the Bi coordination spheres present in baliczunicite.