CICTERRA   20351
CENTRO DE INVESTIGACIONES EN CIENCIAS DE LA TIERRA
Unidad Ejecutora - UE
artículos
Título:
SANJUANITE: CRYSTAL-STRUCTURE SOLUTION FROM LABORATORY POWDER-DIFFRACTION DATA, COMPLEMENTED BY FTIR SPECTROSCOPY AND DT-TG ANALYSES
Autor/es:
COLOMBO, FERNANDO; RIUS, JORDI; PANNUNZIO MINER, ELISA VICTORIA; PEDREGOSA, JOSÉ C.; CAMÍ, GERARDO E.; CARBONIO, RAÚL E.
Revista:
CANADIAN MINERALOGIST
Editorial:
MINERALOGICAL ASSOC CANADA
Referencias:
Año: 2011 vol. 49 p. 835 - 847
ISSN:
0008-4476
Resumen:
Sanjuanite, Al2(PO4)(SO4)OH·9H2O,
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 P21/n, with unit-cell dimensions a 13.9163(5), b 17.2422(5), c 6.1125(3)
Å, β 98.255(4)º, V 1450.7 Å3 and Z=
4. Its crystal structure has been solved and refined from laboratory powder diffraction
data (Bragg-Brentano geometry, CuKα1,2 radiation, scintillation detector). Integrated
intensities extending to a resolution of 1.15 Å 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 χ2 value of
2.47. The structure of sanjuanite is
composed of infinite alumino-phosphate 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 given. 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.