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

Abstract

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 As 4 S n , whereas there are only few examples of layered structures. Here we report the crystal structure of duranusite, As 4 S, which exhibits a new kind of layered structure related to that observed in one of the natural polymorphs of arsenic (arsenolamprite). Duranusite crystallizes in the Pmna space group [ a = 3.611(5), b = 6.755(8), c = 10.10(1)A, α = β = γ = 90°], with 8 As and 2 S atoms per unit cell; the structure consists of an alternated stacking along the [010] axis of two kinds of corrugated layers which match the As 2 and As 2 S composition, respectively. The As–As bond distances range from 2.440 to 2.567A, while the As–S bond is 2.299 A. The shortest contact between adjacent layers is 3.101A, well below the sum of van der Walls radii. The crystal-structure determination of duranusite expands the number of the known layered structures of As sulphides suggesting that, in minerals and stable compounds, the As 4 S n molecules are limited to those having n = 3, 4 and 5.