The new mineral erikapohlite, Cu_3(Zn,Cu,Mg)_4Ca_2(AsO_4)_6 ? 2H_2O, the Ca-dominant analogue of keyite, from Tsumeb, Namibia

摘要

The new mineral erikapohlite (IMA-CNMNC No. 2010-090) has the chemical formula Cu_3(Zn,Cu,Mg)_4Ca_2(AsO_4)_6 ? 2H_2O. It is the Ca-dominant analogue of keyite, Cu_3(Zn,Cu,Mg)_4Cd_2(AsO_4)_6 ? 2H_2O. Erikapohlite is metrically monoclinic with space group C2/c. X-ray powder diffraction studies gave a= 12.6564(6) A, b= 12.7282(8) A, c = 6.9148(3) A and (3= 113.939(4)°, V= 1018.11(6) A~3, Z_=2. The strongest reflections in the X-ray powder diffraction data are as follows d_(obs). (A) (I, hkl): 2.8919 (100, 400), 3.3043 (49, 112), 2.7880 (40, 240), 3.1600 (32, 002) and 2.7638 (14, 402). Chemical analyses gave (wt.%): CuO 22.42, ZnO 14.45, CaO 8.21, MgO 2.21, FeO 0.03, As_2O_5 51.02, H_2O_(by difference) 1.66, sum 100.00 wt.%. The resulting empirical formula based on 26 anions is Cu_3(Zn_(2.48)Cu_(0.93)Mg_(0.77)Fe_(0.01))_(Σ4.19)Ca_(2.04)As_(6.20)O_(24.71) ? 1.29H_2O. Small zones in erikapohlite are Mg- or Cu-dominant, thus representing new mineral species, which still have to be characterized. The type specimen was found on level 44, Tsumeb ore deposit, Tsumeb, Namibia. It was bought by one of the authors (GG) together with other minerals at the Tsumeb mine in 1994. Erikapohlite occurs in up to 0.7 mm wide seams of very thin lamellar microcrystals in masses of granular quartz. As a breakdown product of tennantite it is closely associated with lammerite and conichalcite and an unknown, transparent, amorphous, brownish Zn-Fe-Cu-(Mg-Ca) arsenate. Erikapohlite is transparent with a deep blue colour. The calculated density is 4.55 g/cm3, the calculated mean refractive index is 1.78.