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Crystal structure of potassic-mangani-leakeite from the Wessels Mine, Kalahari manganese field, South Africa

Rosa Micaela Danisi and Thomas Armbruster
Crystal structure of potassic-mangani-leakeite from the Wessels Mine, Kalahari manganese field, South Africa
European Journal of Mineralogy (October 2016) 29 (1): 143-148

Abstract

New crystallographic data for holotype potassic-mangani-leakeite, with the ideal composition KNa (sub 2) (Mg (sub 2) Mn (super 3+) (sub 2) Li)[Si (sub 8) O (sub 22) ](OH) (sub 2) , are presented. This end-member of the amphibole supergroup was formerly reported as "kornite" by Armbruster et al. (1993a) with the empirical formula (K (sub 0.65) Na (sub 0.31) )(Na (sub 1.79) Li (sub 0.21) )(Mg (sub 2.13) Mn (super 3+) (sub 1.43) Fe (super 3+) (sub 0.52) Li (sub 0.91) )Si (sub 8) O (sub 22) (OH) (sub 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 C2/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, beta = 105.275(3) degrees , V = 909.0(3) A (super 3) , yielded a refined composition of (K (sub 0.62) Na (sub 0.38) )Na (sub 2) (Mg (sub 1.94) (Mn,Fe) (super 2+) (sub 0.36) (Mn,Fe) (super 3+) (sub 1.85) Li (sub 0.85) )[Si (sub 8) O (sub 22) ](OH) (sub 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.


ISSN: 0935-1221
EISSN: 1617-4011
Serial Title: European Journal of Mineralogy
Serial Volume: 29
Serial Issue: 1
Title: Crystal structure of potassic-mangani-leakeite from the Wessels Mine, Kalahari manganese field, South Africa
Affiliation: University of Bern, Institute of Geological Sciences, Mineralogical Crystallography, Bern, Switzerland
Pages: 143-148
Published: 20161017
Text Language: English
Publisher: Schweizerbart'sche Verlagsbuchhandlung (Naegele u. Obermiller), Stuttgart, Germany
References: 20
Accession Number: 2016-094594
Categories: Mineralogy of silicates
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 4 tables
S27°11'60" - S27°11'60", E22°58'00" - E22°58'00"
Country of Publication: Germany
Secondary Affiliation: GeoRef, Copyright 2022, American Geosciences Institute. Abstract, copyright, Schweizerbart'sche Verlagsbuchhandlung. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 201646
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