Abstract

Carlfrancisite, Mn32+(Mn2+,Mg,Fe3+,Al)42(As3+O3)2(As5+O4)4[(Si,As5+)O4]6[(As5+,Si)O4]2(OH)42, is a new mineral from the Kombat mine, Otavi Valley, Namibia, and occurs as curved platy aggregates ~2 cm across on a matrix of Mn arsenates and oxides. It is yellowy orange to pale yellow with a very pale-yellow streak, translucent with a vitreous to opalescent luster, and does not fluoresce under ultraviolet light. Cleavage is micaceous on {001}, and no parting or twinning was observed. Mohs hardness is 3, and carlfrancisite is brittle with a hackly fracture. The calculated density is 3.620 g/cm3. Optical properties were measured with a Bloss spindle stage for the wavelength 590 nm using a gel filter. The indices of refraction are ɛ = 1.756, ω = 1.758, and it is non-pleochroic. Carlfrancisite is trigonal, space group Rc, a = 8.2238(2), c = 205.113(6) Å, V = 12013.5(4) Å3, Z = 6, c:a = 1:24.941. The seven strongest lines in the X-ray powder-diffraction pattern are as follows: d (Å), I, (hkl): 2.826, 100, (2̄ 2 44); 2.371, 88, (2̄ 3 40, 1̄ 3 41); 1.552, 84, (1̄ 5 0); 2.676, 63, (2̄ 3 7); 3.243, 54, (0 1 56, 1̄ 2 39); 4.107, 48, (1̄ 2 0); 2.918, 47, (0 2 40). Chemical analysis by electron microprobe and crystal-structure refinement gave As2O5 13.07, As2O3 3.18, P2O5 0.50, V2O5 0.74, SiO2 8.96, Al2O3 0.78, FeO 0.22, MnO 53.25, MgO 9.37, H2O(calc) 8.42, sum 98.49 wt%. The H2O content and the valence states of As were determined by crystal-structure analysis. The empirical formula is Mn2+33.55Mg10.39Fe2+0.14Al0.68 As3+1.44(Si6.67P0.32V5+0.37As5+5.08)O54 (OH)42 on the basis of 96 anions with (OH) = 42 apfu. The structure of carlfrancisite is closely related to that of mcgovernite and turtmannite.

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