Lukrahnite, CaCuFe3+(AsO4)(2)[(H2O)(OH)], the calcium analogue of gartrellite

Lukrahnite is a new member of the tsumcorite group and represents the calci um analogue of gartrellite. It occurs on samples from the Tsumeb mine, Nami bia; associated minerals are beudantite, cuprian adamite, conichalcite, wul fenite, chalcocite, and quartz. Lukrahnite forms spherical aggregates up to 0.5 mm; no individual crystals could be observed. The colour is yellow and the streak is light yellow; the lustre is subadamantine. Mobs' hardness is 5, VHN25 630 kg/mm(2) D(calc.) 4.18 g/cm(3), no distinct cleavage was obse rved. Lukrahnite is biaxial positive, 2V 30(5)degrees (from spindle-stage m easurements at 590 nm), n(x) 1.83(1), n(y) 1.834(calc.), n(z) 1.89(2); pleo chroism is moderate with X yellow, Y, Z pale yellow. Electron-microprobe in vestigations gave CaO 11.42, NiO 0.05, CoO 0.15, CuO 10.00, ZnO 8.19, PbO 0 .69, Al2O3 0.37, Fe2O3 13.75, P2O5 0.16, As2O5 47.72, SO3 0.09, H2O (calc.) 5.98, total 98.57 wt. %. The empirical formula based on 10 oxygen atoms is (Ca0.98Pb0.02)(Sigma1.00)(Cu0.60Fe0.83Zn0.48Co0.01Al0.04)(Sigma1.96)[(AsO4 )(1.99)(PO4)(0.01) (SO4)(0.01)](Sigma2.01) [(H2O)(1.22)(OH)(0.74)]Sigma1.96 . The simplified formula is Ca(Cu,Zn)(Fe,Zn) (AsO4)(2)(H2O,OH)(2), the idea l formula is CaCuFe3+ (AsO4)(2)[(H2O)(OH)]. The 5 strongest lines in the X- ray powder diffraction pattern are [d in Angstrom (I-obs) (hkl)]: 3.416 (10 0) (112), 3.186 (40) (012, 102), 2.927 (64) (111), 2.832 (26) (111, 111), 2 .700 (30) (211), 2.533 (30) (201, 212). Lukrahnite is triclinic, space grou p P! (by analogy with gartrellite), a 5.457(3), b 5.539(4), c 7.399(6) Angs trom, alpha 68.43(5), beta 68.90(4), gamma 69.44(5)degrees, V 187.8(1) Angs trom (3), Z= 1.