Manganvesuvianite and tweddillite, two new Mn³⁺-silicate minerals from the Kalahari manganese fields, South Africa

Abstract 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 Ca 19 Mn 3+ (Al,Mn 3+ ,Fe 3+ ) 10 (Mg,Mn 2+ ) 2 Si 18 O 69 (OH) 9 , characterized by Mn 3+ 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 P 2 1 / m , a = 8.932(5), b = 5.698(4), c = 10.310(5) Å, β = 114.56(4), V = 477.3(8) Å 3 ) with the simplified formula CaSr(Mn 3+ ,Fe 3+ ) 2 Al[Si 3 O 12 ](OH), closely related to strontiopiemontite. The difference between strontiopiemontite and tweddillite is the concentration of octahedral Mn 3+ . Strontiopiemontite has Mn 3+ mainly on the M 3 site whereas tweddillite has Mn 3+ with minor Fe 3+ on M 3 and M 1. 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.