Tetrahedral vacancies and cation ordering in low-temperature Mn-bearing vesuvianites: Indication of a hydrogarnet-like substitution

Strongly zoned Mn-rich vesuvianites with MnO concentrations up to 14.3 wt% from the N’chwaning II mine of the Kalahari manganese field (South Africa) crystallized at hydrothermal conditions below 450 °C. These vesuvianites are by far the most Mn-rich samples hitherto described and have either space group P4nc or P4/n due to partial long-range ordering. Most crystals are assembled of P4nc and P4/n domains yielding P4 average symmetry. The crystal structure of one Mn-rich crystal of average composition Ca₁₉Mn_3.5Al_9.5Si_17.4(O,OH)₇₈ was refined from single-crystal X-ray data (R1 = 3.85%) in space group P4/n (a = 15.571(2), c = 11.789(2) Å). Mn²⁺ and Mn³⁺ are concentrated on the fivefold-coordinated square pyramidal Y′ site. Additional Mn³⁺ was located on the octahedral sites Y2a (35%), Y1a (22%), Y2b (13%) and Y1b (8%). Electron microprobe analyses and crystal-structure refinements indicated tetrahedral vacancies in the orthosilicate tetrahedra (Z1 and Z2) but not in the disilicate units (Z3). Z1 tetrahedra with up to 17% vacancies have strongly increased Z1-O distances of 1.67 Å. Structural and chemical evidence combined with the similarity of the structures of vesuvianite and garnet suggest a partial hydrogarnet-like substitution of SiO₄ tetrahedra by H₄O₄.