The crystal structure of grumiplucite: its OD character and structural relationships

Abstract

A new single-crystal X-ray diffraction study of grumiplucite, HgBi2S4, from Levigliani mine, Apuan Alps, Tuscany, Italy, has allowed the refinement of its crystal structure, confirming its identity with the synthetic analogue. Its crystal structure was refined up to R1 = 0.116 in the space group C2/m, with a 14.1690(16), b 4.0508(4), c 13.9751(14) A, β 118.292(6)°, V 706.29(13) A3. The crystal structure is formed by (001) layers, built up by Bi2S4 rods running along b, connected through Hg2+ ions in the [100] direction. Each rod is built up by chains of edge-sharing square-pyramidal BiS5 polyhedra, paired through the apical Bi–S bonds. Adjacent layers are connected through weaker Bi–S bonds, in agreement with the good {001} cleavage of grumiplucite. Grumiplucite has an order–disorder (OD) character and can be described as formed by equivalent layers having a layer symmetry C 1 2/m 1, with translation vectors a = 14.161, b = 4.051, c0 = 6.18 A, β 90°. Applying the OD theory, two Maximum Degree of Order (MDO) polytypes were derived. MDO1 polytype is orthorhombic, with space group Ccm21, with a 14.16, b 4.05, c 12.36 A, whereas the MDO2 polytype, corresponding to the refined structure, is monoclinic, space group C2/m, with a 14.16, b 4.05, c 14.2 A, β 119.9°, in satisfying agreement with experimental data. Grumiplucite is a member of the pavonite homologous series and is structurally related to kudriavite (Cd,Pb)Bi2S4, and some synthetic compounds. In addition, appealing structural relationships occur also with livingstonite, HgSb4S8.