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Holtstamite, Ca₃(Al,Mn³⁺)₂(SiO₄)_3-x(H₄O₄)_x, a new tetragonal hydrogarnet from Wessels Mine, South Africa

¹ Department of Mineralogy, Swedish Museum of Natural History, Box 50007, SE-104 05 Stockholm, Sweden
² Department of Inorganic Chemistry, Stockholm University, SE-106 91 Stockholm, Sweden
³ Department of Earth Sciences, Uppsala University, Villavägen 16, SE-752 36 Uppsala, Sweden

Corresponding author, e-mail: ulf.halenius{at}nrm.se

A new tetragonal hydrogarnet from Wessels Mine, South Africa is described. The mineral occurs in a calc-silicate skarn together with Mn-rich vesuvianite, calcite and henritermierite. It is the Al-dominant analogue of henritermierite, Ca₃Mn₂(SiO₄)₂(OH)₄, space group I4₁/acd. The new mineral, holtstamite, is optically uniaxial positive and shows lower refractive indices, = 1.718 (2) and = 1.746 (2), than henritermierite. The cell parameters of holtstamite are = 12.337 (3) and c = 11.930 (5) Å.

Extensive holtstamite-henritermierite solid solution is evident from compositionally zoned grains, which frequently show cores of henritermierite composition and rims of holtstamite. The octahedral occupancy in holtstamite is characterised by Al > Mn³⁺ and the most Al-rich tetragonal hydrogarnets in the present sample contain 69 mol. % of a hypothetical tetragonal end-member Ca₃Al²(SiO₄)₂(OH)₄.

A crystal structure refinement (R1 = 3.57 %) of a zoned holtstamite single crystal with the mean composition Ca₃(Al_0.96 Mn³⁺_0.68Fe³⁺_0.37)(SiO₄)_2.00(H₄O₄)_0.99 shows distinctly shorter octahedral bond lengths (mean M1-O = 1.976 Å) than for henritermierite (mean M1-O = 2.020 Å) and a considerably smaller octahedral axial ratio (1.085) as compared to henritermierite (1.144).

Crystal field splitting parameters, 10Dq, for Mn³⁺ in crystals on the join henritermierite-holstamite indicate strong structural relaxation, which is the most likely cause for the observed complex FTIR-spectrum of holtstamite in the OH-stretching region.

It is suggested that stabilisation of the tetragonal Al-dominant calcic hydrogarnet may require a fraction of Mn³⁺ at the octahedral M1-site. Micro-chemical analyses indicate that Si and OH are inversely correlated in holtstamite, with higher degree of H₄O₄-substitution for SiO₄ in Al-rich compositions. Due to variable OH-content as well as a necessary stabilisation by the presence of trivalent manganese, the holtstamite formula is defined as Ca₃(Al, Mn³⁺)₂(SiO₄)_3-x(H₄O₄)_x (with Al > Mn³⁺) rather than Ca₃Al₂(SiO₄)₂(OH)₄. In the present crystals 0.9 < x < 1.2, but the limits of H₄O₄-SiO₄ substitution as well as maximum Al-occupancy at the octahedral site need to be explored.

Key-words: new mineral, crystal structure, hydrogarnet, holtstamite, South Africa.