HENNOMARTINITE AND KORNITE, 2 NEW MN3-MINE, KALAHARI, SOUTH-AFRICA( RICH SILICATES FROM THE WESSELS)

Hennomartinite, SrMn23+[Si2O7](OH)2 . H2O, is an orthorhombic (Cmcm, a = 6.245[1], b = 9.031[2], c = 13.404[2] angstrom, V = 756.0[1] angstr om3, Z = 4) disilicate of the lawsonite structure type which occurs as anhedral poikiloblasts up to 1 mm, strongly intergrown with sugilite and serandite-pectolite. The mineral is translucent in thin section, p leochroic from yellowish brown to dark red-brown and has indices of re fraction greater than 1.82. The calculated density is 3.68 g/cm3. The chemical composition was determined by electron microprobe analyses an d IR-spectroscopy. The trivalent character of M-n is derived from stoi chiometry and bond lengths obtained from a crystal structure analysis. Kornite, (K,Na)(Na,Li)2(Mg,Mn3+,Fe3+,Li)5Si8O22(OH)2, is a monoclinic amphibole (P lattice, a = 9.94[1], b = 17.80[2], c = 5.302[4] angstro m, beta = 105.5[2], V = 904[4] angstrom3, Z = 2). The calculated densi ty is 3.15 g/cm3. The mineral forms bundles of extremely fine fibers ( 5-20 mu in diameter), often bent, maximum fibre length 0.2 mm. The min eral is dark red to brownish-lilac, biaxial (2 V(x) = 88-92-degrees) w ith indices of refraction alpha = 1.654(4), beta = 1.675 (calc), gamma = 1.696(4) determined under white light. The optic orientation is Z = b, Y: c = 60-65-degrees (in obtuse angle beta). The mineral is strong ly pleochroic: X = pink, Y = dark red, Z = orange-red. The chemical co mposition was determined by electron and ion microprobe analyses. Both new minerals occur together with idiomorphic sugilite (70%), serandit e-pectolite (25%) as hydrothermal reaction products of primary Mn-ore at the Wessels Mine, Kalahari Manganese Field, Republic of South Afric a.