Galloplumbogummite from Tsumeb, Namibia, a new member of the alunite group with tetravalent charge balance
Schlüter, Jochen; Malcherek, Thomas; Mihailova, Boriana
published: Jun 1, 2014
ArtNo. ESP154019103005, Price: 29.00 €
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 R3m. Single crystal studies gave a = 7.083(5) Å, c = 16.742(3) Å, V = 727.4(7) Å3 and Z = 3. The strongest reflections in the X-ray powder diffraction data are as follows d obs. (Å) (I, hkl): 5.730 (100, 101), 2.983 (78, 213), 3.528 (24, 210) and 2.225 (19, 322). 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 14 O atoms is (Pb1.04Ca0.05)Σ1.09(Ga1.41Al1.35Ge0.38Fe0.02)Σ3.16(P1.91S0.14)Σ2.05O8.44(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, Al)3–x GexH1–x (PO4)2(OH)6 for 0 ≤ × ≤ 1 or Pb(Ga,Al)2Ge(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/cm3, the calculated mean refractive index is 1.82.