Abstract

The crystal structure of comancheite, Hg552+N243(OH,NH2)4(Cl,Br)34, orthorhombic, space group Pnnm, a = 18.414(5), b = 21.328(6), c = 6.6976(19)Å, V = 2630(2)Å3, Z = 1, was solved by direct methods and refined to an R1 index of 4.3% based on 4160 unique observed reflections. In the structure of comancheite, there are nine crystallographically distinct Hg2+ cations, each of which is coordinated by two N3− anions to form near-linear N3−–Hg2+–N3− groups. Four other crystallographically distinct Hg2+ cations are coordinated by a mixture of N3−, O2−, (OH) and (NH2) anions, and there is a small amount of [Hg–Hg]2+ dimer. In addition, there are eight crystallographically distinct halogen sites, three of which are completely occupied by Cl, and five of which are occupied by both Cl and Br. The principal anion, N3−, shows a strong preference for tetrahedral coordination by Hg2+, which results in a strongly bonded three-dimensional {–Hg2+–N3−–} framework. This framework is both interrupted and contains large interstices that incorporate additional Hg2+ cations, a very small amount of [Hg+–Hg+]2+ dimer and additional anion species, O2−, (OH) and (NH2), that coordinate Hg2+.

Comancheite was described originally as an Hg-oxide mineral. The major change in chemical composition indicated by the present work was approved by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association (Voting Proposal 13-B). The presence of N provides some analytical challenges, particularly in the presence of Hg. New bond-valence parameters were derived for Hg2+–N3− bonds [RoN3−) = 1.95] using well refined Hg2+ structures, and this allows discrimination between Hg2+–O2− and Hg2+–N3− bonds based on the valence-sum rule. Comparison of the Raman spectra of several Hg-bearing minerals shows that peaks in the range 500–700 cm−1 are characteristic of Hg2+–N3− stretching vibrations whereas peaks in the range 350–500 cm−1 are characteristic of Hg2+–O2− stretching vibrations; Hg2+–O2− and Hg2+–N3− bonds may be discriminated on this basis.

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