Molecular versus layered structure in arsenic sulphide minerals: the case of duranusite, As4S.

Most of the natural and synthetic compounds belonging to the binary As-S system exhibit a structure based on the packing of discrete cage-like, covalently bonded As4Sn, whereas there are only few examples of layered structures. Here we report the structure of a new kind of covalent crystal, the mineral duranusite As4S, which shows a layered structure related to that observed in one of the natural polymorph of arsenic (arsenolamprite). Duranusite exhibits an orthorhombic structure [a = 3.611(5), b = 6.755(8), c = 10.10(1): Å, alpha = beta = gamma = 90°, Pmna space group], with 8 As and 2 S atoms per unit cell, consisting of an alternated stacking along the [010] axis of two kinds of corrugated layers which match the As2 and As2S composition, respectively. As-As bond distances range from 2.440 to 2.567 Å, while the As-S bond is 2.299 Å. The shortest contact between adjacent layers is 3.101 Å, well below the sum of van der Walls radii. The crystal structure determination of duranusite expands the number of the known covalent layered structures of As sulphides suggesting that in minerals and stable compounds the As4Sn molecules are limited to those having n = 3, 4 and 5.