SANJUANITE: AB INITIO CRYSTAL-STRUCTURE SOLUTION FROM LABORATORY POWDER-DIFFRACTION DATA, COMPLEMENTED BY FTIR SPECTROSCOPY AND DT-TG ANALYSES

Sanjuanite, Al-2(PO4)(SO4)(OH)center dot 9H(2)O, is a microcrystalline fibrous mineral found in Pennsylvanian slates belonging to the Los Jejenes Formation at the Pocito Department, San Juan Province, Argentina. Sanjuanite is monoclinic, space group P2(1)/n, with unit-cell dimensions a 13.9163(5), b 17.2422(5), c 6.1125(3) angstrom, beta 98.255(4)degrees, V 1450.7(5) angstrom(3) and Z = 4. Its crystal structure has been solved and refined from laboratory powder-diffraction data (Bragg-Brentano geometry, CuK alpha(1,2) radiation, scintillation detector). Integrated intensities extending to a resolution of 1.15 angstrom were introduced in XLENS and processed with the S-FFT algorithm. Except for one O atom, the other atoms appeared in the Fourier map of the best direct-methods solution. Subsequent restrained Rietveld refinement converged to a chi(2) value of 2.47. The structure of sanjuanite is composed of infinite aluminophosphate chains running parallel to c and located at (x, y) = (0,0) and (1/2, 1/2). Isolated (SO4)(2-) groups and H2O molecules connect the groups of chains. Hydrogen bonding plays a key role in the stabilization of the structure. The group formed by pairs of double chains of alternating (PO4) and (AlO6) present in sanjuanite bears no close resemblance to any known mineral structure. The FTIR spectrum and DTA-TGA curves of sanjuanite are provided. Chemically related species include kribergite and hotsonite, whose structures remain unknown. The formation of sanjuanite is probably related to the oxidation of sulfides (mostly pyrite and marcasite) under arid conditions, which produces sulfuric acid that releases Al from silicates in pelitic rocks. The source of P remains unknown.