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

FERNANDO COLOMBO; JORDI RIUS; ELISA V. PANUNZIO-MINER; JOSÉ C. PEDREGOSA; GERARDO E. CAMÍ; RAÚL E. CARBONIO

CANADIAN MINERALOGIST

MINERALOGICAL ASSOC CANADA

Año: 2011 vol. 49 p. 441 - 453

0008-4476

Sanjuanite, Al2(PO4)(SO4)OH•9H2O, is a microcrystalline fibrous mineral found in Pennsylvanian slates belonging to theLos Jejenes Formation at the Pocito Department, San Juan Province, Argentina. Sanjuanite is monoclinic, space group P21/n,with unit-cell dimensions a 13.9163(5), b 17.2422(5), c 6.1125(3) Å, b 98.255(4)%, V 1450.7(5) Å3and Z = 4. Its crystalstructure has been solved and refined from laboratory powder-diffraction data (Bragg–Brentano geometry, CuKa1,2 radiation,scintillation detector). Integrated intensities extending to a resolution of 1.15 Å were introduced in XLENS and processed withthe 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 c2 value of 2.47. The structure of sanjuanite is composed of infinitealuminophosphate chains running parallel to c and located at (x, y) = (0,0) and (½,½). Isolated (SO4)2– groups and H2O moleculesconnect the groups of chains. Hydrogen bonding plays a key role in the stabilization of the structure. The group formed bypairs of double chains of alternating (PO4) and (AlO6) present in sanjuanite bears no close resemblance to any known mineralstructure. The FTIR spectrum and DTA–TGA curves of sanjuanite are provided. Chemically related species include kribergiteand 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.