Krieselite, AI_2GeO_4(F,OH)_2: A new mineral from the Tsumeb mine, Namibia, representing the Ge analogue of topaz

摘要

The new mineral krieselite (IMA-CNMNC No. 2000-043a) has the ideal chemical formula Al_2GeO_4(F,OH)_2 and is the Ge analogue of topaz. Krieselite was found in the Tsumeb mine, Tsumeb, Namibia. It occurs as beige to white, hemispherical ag-gregates and crusts of fibrous crystals and is closely associated with quartz, wulfenite, anglesite, and graphite. The fibers are up to 50 mu m long and up to 5 mu m thick. Krieselite aggregates are translucent with a greasy luster. The streak is white. The name of the new mineral is in honor of F. W. KRIESEL, who was the chief chemist and head of the Tsumeb mine laboratory around 1920. Chemi-cal analyses by electron microprobe, neutron activation, and proton-induced X-ray fluorescence yielded the chemical formula (Al_(1.860) Ga_(0.102),As_(0.036)~(+3),Zn_(0.020),Mg_(0.016),Fe_(+0.012)~3,Na_(0.009),Sb_(+0.005)~3,Ti_(0.003)Cu_(0.001))_(Σ2.064)(Ge_(0.844),Al_(0.143),Si_(0.013))_(Σ1.000)O_4(F_(1.103)OH_(0.897)_(Σ2.000). Krieselite crystallizes in the space group Pbnm (62). Unit cell parameters refined from X-ray powder diffraction data are a = 4.809(2) A, b = 9.111 (3) A, c = 8.536(3) A, V = 374.0(3) A~3, Z = 4. Using Cu Kα radiation the five strongest reflections in the X-ray powder diffraction data are as follows: d_(obs)(A) (I, hkl): 3.016 (100, 112), 3.811 (78, 111), 3.315 (48, 012), 2.247 (38, 211) and 2.417 (27, 023/200). The calculated density and calculated mean refractive index are 4.07 g/cm~3and 1.74, respectively.