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, ω = 1.718 (2) and ϵ = 1.746 (2), than henritermierite. The cell parameters of holtstamite are α = 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(Al0.96 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.