Structural and vibrational properties of arsenic sulfides: alacranite (As8S9).

Alacranite, As(8)S(9), has been studied by a combined approach based on micro-Raman measurements and ab initio molecular dynamics simulations, with the Car-Parrinello method. The structure of this arsenic sulfide mineral consists of an ordered packing of As(4)S(4) and As(4)S(5) cagelike molecules, with a topology closely resembling that found in the β-As(4)S(4). The presence in the crystal structure of molecular clusters with permanent dipole moment induces stronger intermolecular interactions than those observed in other arsenic sulfides, making the adoption of ab initio computational methods particularly important for a complete characterization of the structural and spectroscopic properties.

Reinvestigation of the crystal structure of lautite, CuAsS.

The crystal structure of the mineral lautite (copper arsenic sulfide), CuAsS, previously described as either centrosymmetric [Pnma; Marumo & Nowacki (1964 ▶). Schweiz. Miner.

Superspace-symmetry determination and multidimensional refinement of the incommensurately modulated structure of natural fresnoite.

The structure of natural fresnoite, Ba2TiSi2O8, from the sanbornite deposits of eastern Fresno County, California, has been solved and refined as an incommensurate structure in five-dimensional superspace. The structure is tetragonal, superspace group P4bm(alpha, alpha, 1/2)(-alpha, alpha, 1/2)0gg, cell parameters a=8.5353 (6), c=10.

Twinning in natural melilite simulating a fivefold superstructure.

Additional reflections seemingly leading to a tetragonal fivefold supercell have been observed in a number of melilites. A careful examination of the collected intensity data reveals that this feature is due to the coexistence of two twin domains related by a (120) twin plane. Reflections of the first domain (I) with h(b)(2) + k(b)(2) = 5n overlap those of the second domain (II) when both the following additional conditions are verified: -h(b) + 2k(b) = 5n and 2h(b) + k(b) = 5n.