Crystal structure of potassic-mangani-leakeite from the Wessels Mine, Kalahari Manganese Field, South Africa

Abstract

New crystallographic data for holotype potassic-mangani-leakeite, with the ideal composition KNa2(Mg2Mn3+2Li)[Si8O22](OH)2, are presented. This end-member of the amphibole supergroup was formerly reported as “kornite” by Armbruster et al. (1993a) with the empirical formula (K0.65Na0.31)(Na1.79Li0.21)(Mg2.13Mn3+1.43Fe3+0.52Li0.91)Si8O22(OH)2. X-ray crystal-structure refinements of four crystals taken from the holotype material (Wessels Mine, Kalahari Manganese Field, South Africa) showed that this amphibole composition has the standard space group C 2/ m , contrary to the original report. Differences in chemical composition (Mg vs . Li) were found based on scattering power determined by structure refinement. The crystals tested consistently show compositions significantly different from that reported in the original mineral description, especially as far as the Li content is concerned (0.45–0.85 vs . 1.11 apfu). The new data suggest that the Li content in the original paper is not characteristic of the studied single crystals and may have been overestimated. Three of the investigated crystals, represented by crystal no. 1, a = 9.9603(17), b = 17.830(3), c = 5.3056(9) A, β = 105.275(3)°, V = 909.0(3) A3, yielded a refined composition of (K0.62Na0.38)Na2(Mg1.94(Mn,Fe)2+0.36(Mn,Fe)3+1.85Li0.85)[Si8O22](OH)2 when using the simplified model without vacancies at the (K, Na)-bearing A site, M (1) and M (2) filled with Mg and (Mn,Fe), and M (3) occupied by Li and Mn. The range of Li at M (3) was bracketed by two models either with (0.64 Li + 0.36 Mg) or with (0.85 Li and 0.15 Mn) having the same scattering power. The choice of the above formula was made considering the agreement between observed and calculated 〈 M (3)–O〉 bond lengths. On the same grounds, the one crystal with the lowest Li content of 0.46 apfu belongs to the compositional space of eckermannite. Small concentrations of A group vacancies cannot be excluded.