The crystal chemistry of oxo-mangani-leakeite and mangano-mangani-ungarettiite from the Hoskins mine and their impossible solid-solution: An XRD and FTIR study

New chemical (EMP, SIMS) and structural data are reported for a suite of crystals of oxo-mangani-leakeite and mangano-mangani-ungarettiite from their common type locality, the Hoskins mine (New South Wales, Australia). Notwithstanding the low OH content, FTIR analysis of selected samples has provided considerable information on short-range order in these Mn3+-rich amphiboles, and shows that Li is associated with occupied A sites and is linked to the oxo-component at the O(3) site. Comparative analysis of all available data allows us to (1) further improve our understanding of the crystal-chemistry of these very peculiar compositions of the oxo-amphibole group, and (2) calculate reliable site-populations. The proposed limited compositional variability has been confirmed. The two amphiboles have completely different arrangements of CR3+ cations. In oxo-mangani-leakeites, those CR3+ cations related to the oxo-component occur at the M(1) site, whereas those CR3+ cations related to the leakeite charge-arrangement occur at the M(2) site. In mangano-mangani-ungarettiite, all CR3+ cations order at the M(1) and M(3) sites, and local bond-valence requirements are satisfied by the presence of Mn3+, which assumes a strongly distorted coordination due to its degenerate eg electronic state. Therefore, the inverse patterns observed for both cation-ordering and deformation of the octahedra are incompatible with solid-solution between these two species that coexist at the Hoskins mine.