The Fountain Hill skarn, which was produced by thermal metamorphism of impure limestone in the Omey Granite aureole, consists of wollastonite, calcite, grossular-andradite garnet, diopside, B-free vesuvianite and small amounts of albite, K-feldspar and quartz. Zoning in vesuvianite is, overall, independent of birefringence but oscillatory concentric zoning in the mineral is controlled mainly by Ti. In addition to extensive within-site substitutions (e.g. F for OH in the OH-sites, Fe2+ for Mg, Fe3+ for Al, Ti for Al etc. in the Y-sites), there are significant cross-site combined substitutions involving Y-X and Y-Z but not X-Z sites so that ideal solution models for this mineral are not applicable. The thermodynamic mole fraction of Hoisch (1985) is modified to account for the excess of the ΣY cations in the Y sites and can be applied to both B-bearing and B-free vesuvianites. Using the thermodynamic dataset of Holland and Powell (1990) and taking into account the existence of andalusite, sillimanite and corundum in associated pelites, leads to the conclusion that the metamorphic conditions were about 640±20°C and 3.3±0.3 kb at 0.15±0.05 XCO2.

Sectored vesuvianite showing optically triclinic properties was studied by X-ray and P-FTIR analyses, and the origins of the internal optical texture are discussed. A monoclinic refinement (space group P2/n) suggests that site occupancies are slightly different among the Al(2) series, though the OH– dipole is randomly oriented in all sectors. A relationship between the surface and internal texture suggests that these sectoral structures were produced during crystal growth, not by phase transitions.

The new minerals manganvesuviante and tweddillite, both formed by hydrothermal alteration of primary manganese ores, are described from the Kalahari manganese fields (Republic of South Africa). In addition, single-crystal X-ray structure refinements of both new minerals are presented.

Manganvesuvianite is a tetragonal vesuvianite mineral with the simplified formula Ca19Mn3+(Al,Mn3+,Fe3+)10(Mg,Mn2+)2Si18O69(OH)9, characterized by Mn3+ occupying the five-coordinated position (square pyramid). The crystals simple prismatic forms: {100}, {110} terminated by {101} and exhibit deep maroon red colour. With polarized light the crystals are strongly pleochroic, yellowish parallel to E and dark red to lilac parallel to O.

Tweddillite is an epidote-group mineral (space group P21/m, a = 8.932(5), b = 5.698(4), c = 10.310(5) Å, β = 114.56(4), V = 477.3(8) Å3) with the simplified formula CaSr(Mn3+,Fe3+)2Al[Si3O12](OH), closely related to strontiopiemontite. The difference between strontiopiemontite and tweddillite is the concentration of octahedral Mn3+. Strontiopiemontite has Mn3+ mainly on the M3 site whereas tweddillite has Mn3+ with minor Fe3+ on M3 and M1. Tweddillite forms aggregates of very thin dark red {001} blades characterized by striking pleochroism. The crystals appear dark red parallel to b and orange-yellow parallel to a. Perpendicular to (001) the blades appear magenta to red.