Determination of the tegengrenite superstructure: another case of tetrahedral Mn3+ in spinel-type minerals?

摘要

The crystal structure of the spinel-related, Sb mineral tegengrenite from the Filipstad district, Varmland, Sweden, has been solved in the space group R3 [alpha = 16.0285(9), c = 14.8144(8) angstrom, V = 3296.1(3) angstrom(3) , Z = 42] and refined up to R = 0.0484 for 3589 reflections with F-o > 4 sigma(F-o). Tegengrenite exhibits a rhombohedrally distorted spinel-type structure with cations occupying 1/8 of the tetrahedral (T) and 1/2 of the octahedral (M) interstices of a nearly regular cubic close-packing of oxygen atoms. Due to the cation ordering, which leads to a complex superstructure with a unit-cell volume of 21/4 that of a common spinel, the M and T sites of the spinel-type structure split into ten and six independent sites, respectively. Chemical composition determined by electron microprobe led to the empirical formula Mg1.26Mn0.852+Zn0.04 Mn0.193+Al0.01Si0.12Ti0.03Sb0.505+O4, on the assumption that no vacancies occur in the mineral (Sigma(cat) = 3.00 and Sigma(charge) = 8.00 per formula unit). Crystal-chemical considerations indicate that octahedra are occupied mainly by Mg, Mn3+ and Sb5+ (+ Al, + Ti), whereas tetrahedra are filled mainly by Mn2+ and Mg (+Zn) with Si ordered in a specific site. However, the structure refinement shows a low site-scattering factor for one octahedral site, implying substantial vacancies and a larger overall mean Mn valence than stated above. Long mean distances also suggest some Mn2+ on octahedral sites. Together, these observations necessitate the presence of substantial Mn3+ on tetrahedral sites.