Crystal Chemistry of Minerals with Spinel Modules and New Data on the Symmetry, Structure, and Cation Distribution Features in Tashelgite, CaMgFe2+Al9O16(OH)

Rerefinement of the crystal structure of the mineral tashelgite with the idealized formula CaMgFe²⁺Al₉O₁₆(OH) (orthorhombic Pbam space group; a = 17.1823(1), b = 23.5718(4), c = 5.6973(5) Å) is performed using the earlier obtained diffraction data. Fine crystal structure details and distribution features of cation positions are retrieved from precision Mössbauer and optical spectroscopy data. It is shown that the main motif and the pattern of cation distribution over octahedral and tetrahedral positions are preserved, despite the symmetry increase. However, due to a smaller number of independent positions, the distribution in the Pbam model becomes less ordered than in the Pc model. The appearance of tashelgite, having a complex structure based on spinel modules, agrees well with the presence of associated minerals such as spinel-like magnetite and hercynite, as well as hibonite, also containing spinel modules in its structure. A comparative crystal chemical and modular analysis of natural and synthetic compounds containing spinel modules is performed.