DORALLCHARITE, TL0.8K0.2FE3(SO4)2(OH)6, A NEW MEMBER OF THE JAROSITE-ALUNITE FAMILY

Dorallcharite, ideally Tl+Fe3+3 (SO4)2(OH)6, is a Tl-Fe member of the alunite-jarosite family. Electron microprobe measurements and a crysta l-structure determination show that K substitutes 8.3 to 29.5% (mean 1 8.6%) of the Tl atoms in the investigated sample. A complete solid-sol ution series is assumed to exist between dorallcharite and jarosite. T he crystal structure was refined from powder X-ray diffraction data us ing the Rietveld method (Rw = 5.86%). The (hexagonal) unit cell parame ters are: a = 7.3301(3) angstrom, c = 17.6631(7) angstrom, space group R3mBAR, Z = 3, rho(calc) = 3.85 g/cm3 (at 15-degrees-C). Structural d ata indicate a possible displacement of the Tl atom along the c axis d irection (statistically distributed through the structure) owing to th e lone-electron-pair effect. The strongest lines in the X-ray powder d iffraction pattern are [d(angstrom), 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 rho imbohedral crystals under 10 mum in diameter with a distinct {0001} cl eavage. It is uniaxial negative with n(omega) = 1.822(3), n(epsilon) = 1.768(2), with normal interference colours and no pleochroism. Dorall charite is found in Allchar, Republic of Macedonia, a deposit known fo r Tl sulphides and sulphosalts, where it has formed in the oxidation z one of the ore body. It is often associated with an amorphous Fe-Mn su lphate-arsenate free of Tl. The observed paragenesis suggests that, in the oxidation zone of the sulphides, Tl concentrates preferentially i n a material of jarositic composition.