Lithium in nigerite-group minerals

Nigerite-group minerals (polysomatic tin-bearing oxides) from the type locality Egbe District, Kabba Province (Nigeria) and from the abandoned tin mine of Tsomtsaub, between Omaruru and Uis in Namibia have been reinvestigated chemically and by single-crystal X-ray diffraction. At both localities nigerite occurs in quartz-sillimanite rocks associated with pegmatites. Both nigerites can be classified as 'zinconigerite-6N6S', space group R3m, a = 5.71, c = 55.4 A. In both structures the octahedral M2 site in the T (sub 1) -layer of the nolanite module is completely occupied by Sn with minor Ti. Charge balance for the tetravalent cations is achieved by partial Li substitution for Al in adjacent octahedral sites (O-layer) sandwiched between two T (sub 1) -layers. For the first time the presence of essential Li in nigerite-group minerals has been demonstrated by Secondary Ion Mass Spectrometry (SIMS) yielding 0.61(6) and 0.70(7) wt.% Li (sub 2) O for nigerite from Nigeria and Namibia, respectively. Associated gahnite in the Nigerian sample is free of Li (sub 2) O. Both investigated nigerite-group minerals have tetrahedral Zn:Fe > 1 in the spinel module and are therefore assigned to the zinconigerite subgroup. It is suggested that Sn-rich nigerite-group minerals (6N6S and 2N1S polysomes) from rare-element pegmatites or skarns possess essential Li for charge balance whereas low Sn, Ti-bearing nigerites, displaying an extended solid solution with hogbomite, do not require Li for charge balance.