The crystal structure of namuwite, a mineral with Zn in tetrahedral and octahedral coordination, and its relationship to the synthetic basic zinc sulfates

Abstract

The crystal structure of namuwite, (Zn,Cu)4S04(OHk4H20, trigonal, a = 8.331(6), C = 10.54(1) A, V = 634(1) A3, Pj, Z = 2, has been solved by direct methods and refined to an R index of 7.9% using MoKa X-ray data. The fundamental building unit of the structure is a gently modulated sheet of edge-sharing (Zn,Cu) octahedra parallel to (001), with one vacancy for every six occupied octahedral sites. The sheets are decorated on both sides by S04 and [Zn(OHMH20)]Itetrahedra; the latter are located above and below the vacant octahedral sites. Adjacent tetrahedral-octahedral-tetrahedral (T -0T) layers are held together by hydrogen bonds to interstitial H20 molecules. In synthetic Zn4S04(OH)6.4H20 successive octahedral layers are displaced by Ih[a + b]. In namuwite, this is not the case, which suggests that the presence of Cu in the octahedral sites prevents this from happening. Namuwite undergoes a reversible transition, probably to the pentahydrate phase, at approximately room temperature.