Jorgkellerite, Na3Mn33+(PO4)(2)(CO3)O-2 center dot 5H(2)O, a new layered phosphate-carbonate mineral from the Oldoinyo Lengai volcano, Gregory rift, northern Tanzania

Jorgkellerite, ideally Na3Mn3+ (3)(PO4)(2)(CO3)O-2 center dot 5H(2)O, is a new layered phosphate-carbonate from the Oldoinyo Lengai volcano in the Gregory Rift (northern Tanzania). The mineral occurs as spherulites, up to 200 mu m in diameter, consisting of plates up to 10 mu m in thickness in shortite-calcite and calcite carbonatites. Jorgkellerite is brown with a vitreous lustre and has a perfect micaceous cleavage on {001}, Mohs hardness is 3. The calculated density is 2.56 g/cm(3). Jorgkellerite is uniaxial (-), omega = 1.700(2), epsilon = 1.625(2) (Na light, 589 nm) with distinct pleochroism: O = dark brown, E = light brown. The empirical formula of the mineral (average of 10 electron microprobe analyses) is (Na2.46K0.28Ca0.08Sr0.04Ba0.02)(Sigma 2.88)(Mn3+ Fe-2.39(3+) (0.56))(Sigma 2.95)((PO4)(1.95)(SiO4)(0.05)))(Sigma 2.00)(CO3)(O-1.84(OH)(0.16))(Sigma 2.00)center dot 5H(2)O. The oxidation state of Mn has been determined by XANES. Jorgkellerite is trigonal, space group P-3, a = 11.201(2) , c = 10.969(2) angstrom, V = 1191.9(7) angstrom(3) and Z = 3. The five strongest powder-diffraction lines [d in angstrom, (I/I-o), (hkl)] are: 10.970 (100) (001), 5.597 (15) (002), 4.993 (8) (111), 2.796 (14) (220) and 2.724 (20) (004). The crystal structure is built up of the layers composed of disordered edge-sharing [MnO6] octahedra. Each fourth Mn site in octahedral layer is vacant that results in appearance of ordered system of hexagonal "holes" occupied by (CO3) groups. The overall composition of the layer can be expressed as [Mn3O8(CO3)]. These manganese-carbonate layers are linked in the third dimension by (PO4) tetrahedra and Na-polyhedra. The origin of jorgkellerite is related to low-temperature oxidative alteration of gregoryite-nyerereite carbonatites.