Rankamaite, a new tantalum mineral from Kivu, Congo

Rankamaite is a new alkali tantalate found in heavy mineral concentrates from alluvial deposits, Mumba area, Kivu, eastern Congo, as water-worn pebbles up to tens of grams in weight. The mineral constitutes the matrix in which corroded grains of simpsonite, minute crystals of cassiterite and some manganotantalite and muscovite are seen. It was probably formed by alteration of simpsonite; the source rock is probably a l i thium pegmatite. The chemical analysis gave: T a 2 0 5 69.47, N b 2 0 5 17.40, A1 2 0 3 3.40, L i a O O.u , N a 2 0 2.31, K 2 0 1.80, PbO 2.63, H 2 0 + 1.55, H 2 0 — 0.35, S i 0 2 0.96, sum 99.98 % . After correcting for muscovite, this gives for the unit cell content (Na3 .4 5 K ^ j Pb0 .5 5 Li0 .3 1 ) (Tai3.75 Nb6 .06 Al 2 . 3 5 ) 0 5 6 . 5 2 ( O H ) , . , or (Na, K, Pb, Li)5 .8 6 (Ta, Nb, Al ) 2 2 . 1 6 (O, OH)60 . The X-ray study shows that the powder pattern can be indexed on the basis of an orthorhombic unit cell, with a„ = 17.19, b0 = 17.70, c„ = 3.933 A. If only reflections, with h -f k = even, are considered, the possible space groups are C222, Cmml, Cm2m, and Cmmm. The powder data are comparable to those of tetragonal S r T a 4 O n and K 2 N b 8 0 2 l and also to those of orthorhombic PbNbaO„; all these synthetic compounds are structurally related to the tungsten bronzes. The strongest X-ray lines are 4 .112 (34), 3.935 (41), 3.467 (52), 3.375 (60), 3.011 (80), 2.970 (100), 1.736 (32). The colour is white to creamy-white. H is difficult to determine, but is approximately 3—4. G is about 5.5 (meas.), 5.84 (calc.). In thin section it is felt-like, somewhat resembling fibrous sillimanite. Elongation is positive, Z || c, X ± c, «s above 2.1. The name is in honour of Professor Kalervo Rankama of the University of Helsinki, who has made a major contribution to the geochemistry of tantalum and niobium. The name rankamaite was approved in advance of publication by the Commission on New Minerals and Mineral Names of the I .M.A.