Dorallcharite Tl0.8K0.2Fe3(SO4)2(OH)6, a new member of the jarosite-alunite family

Dorallcharite, ideally Tl + Fe 3 3+ (SO 4 ) 2 (OH) 6 , is a Tl-Fe member of the alunite-jarosite family. Electron microprobe measurements and a crystal-structure determination show that K substitutes 8.3 to 29.5% (mean 18.6%) of the Tl atoms in the investigated sample. A complete solid-solution series is assumed to exist between dorall-charite and jarosite. The crystal structure was refined from powder X-ray diffraction data using the Rietveld method (Rw = 5.86%). The (hexagonal) unit cell parameters are: a = 7.3301(3) A, c = 17.6631(7) A, space group R3m, Z = 3, ρ calc = 3.85 g/cm 3 (at 15°C). Structural data indicate a possible displacement of the Tl atom along the c axis direction (statistically distributed through the structure) owing to the lone-electron-pair effect. The strongest lines in the X-ray powder diffraction pattern are [d(A), I, hkl] 5.974, 87, 101; 3.666, 34, 110; 3.112, 100, 021 + 113 + 015; 2,988, 22, 202; 2,577, 21, 024; 1.991, 29, 303 + 033 + 125; and 1.833, 23, 220. The mineral has a golden yellow colour and occurs in earthy and fine-grained aggregates of rhombohedral crystals under 10 μm in diameter with a distinct {0001} cleavage. It is uniaxial negative with n ω = 1.822(3), n e = 1.768(2), with normal interference colours and no pleochroism. Dorallcharite is found in Allchar, Republic of Macedonia, a deposit known for Tl sulphides and sulphosalts, where it has formed in the oxidation zone of the ore body. It is often associated with an amorphous Fe-Mn sulphate-arsenate free of Tl. The observed paragenesis suggests that, in the oxidation zone of the sulphides, Tl concentrates preferentially in a material of jarositic composition