Galloplumbogummite from Tsumeb, Namibia, a new member of the alunite group with tetravalent charge balance

The new mineral galloplumbogummite (IMA 2010-88), Pb(Ga,Al)(3-x) GexH1-x(PO4)(2)(OH)(6) for 0 <= x <= 1, is the Ga-dominant analogue of plumbogummite and belongs to the plumbogummite subgroup of the alunite supergroup. Galloplumbogummite is trigonal with space group R (3) over barm. Single crystal studies gave a= 7.083(5) angstrom, c= 16.742(3) angstrom, V= 727.4(7) angstrom(3) and Z=3. The strongest reflections in the X-ray powder diffraction data are as follows d(obs). (angstrom) (I, hkl): 5.730 (100, 101), 2.983 (78, 2 (1) over bar3), 3.528 (24, 2 (1) over bar0) and 2.225 (19, 3 (2) over bar2). Chemical analyses gave (wt%): PbO 34.45, CaO 0.42, Al2O3 10.19, Ga2O3 19.64, GeO2 5.93, Fe2O3 0.20, P2O5 20.04, SO3 1.71, H2O (by difference) 7.42, sum 100.00 wt.%. The resulting empirical formula based on 14O atoms is (Pb1.04Ca0.05)(Sigma 1.09) (Ga1 41Al1.35Ge0.38Fe0.02)(Sigma 3.16)(P1.91S0.14)(Sigma 2.05)O-8.44(OH)(5.56). Galloplumbogummite has a relevant Ge4+ component on the trivalent cation site, which can act as a charge compensating cation in much the same way as the H+ atom in the (PO3OH)(-2) anion of other alunite group members. Thus its formula can be given as Pb(Ga,A1)(3-x)GexH1-x(PO4)(2)(OH)(6) for 0<=x<=1 or Pb(Ga,Al)(2)Ge(PO4)(2)(OH)(6) for the end member with x =1. The new mineral was discovered in a sample bequeathed to the Mineralogical Museum Hamburg by Prof Dr. Hermann Rose (1883-1976). The sample originates from the second oxidation zone of the Tsumeb ore deposit, Tsumeb, Namibia. The new mineral formed as a secondary phase derived from oxidation of primary Ge-Ga ores of the Tsumeb deposit and was found in two millimeter-sized vugs in massive germanite-renierite ore associated with chalcocite, Cd-rich sphalerite, galena and pyrite. Galloplumbogummite occurs as colourless but frosted white rhombohedral crystals up to 0.15 mm in size. Its calculated density is 4.62 g/cm(3), the calculated mean refractive index is 1.82.