Rietveld refinement of a natural cobaltian mansfieldite from synchrotron data

A structural refinement of a natural sample of a Co-bearing mansfieldite, AlAsO4·2H2O [aluminium orthoarsenate(V) dihydrate], has been performed based on synchrotron powder diffraction data, with 5% of the octahedral Al sites replaced by Co. Mansfieldite is the aluminium analogue and an isotype of the mineral scorodite (FeAsO4·2H2O), with which it forms a solid solution. The framework structure is based on AsO4 tetrahedra sharing their vertices with AlO4(H2O)2 octahedra. Three of the four H atoms belonging to the two water molecules in cis positions take part in O—H⋯O hydrogen bonding.

A structural refinement of a natural sample of a Co-bearing mansfieldite, AlAsO 4 Á2H 2 O [aluminium orthoarsenate(V) dihydrate], has been performed based on synchrotron powder diffraction data, with 5% of the octahedral Al sites replaced by Co. Mansfieldite is the aluminium analogue and an isotype of the mineral scorodite (FeAsO 4 Á2H 2 O), with which it forms a solid solution. The framework structure is based on AsO 4 tetrahedra sharing their vertices with AlO 4 (H 2 O) 2 octahedra. Three of the four H atoms belonging to the two water molecules in cis positions take part in O-HÁ Á ÁO hydrogen bonding.
It often develops encrustations, crust-like or rounded aggregates on the matrix, and individual crystals are rarely observed.
While the structural data of synthetic mansfieldite were reported recently by Harrison (2000), no data regarding natural samples have been provided in literature up to now, probably because of the rare occurrence of crystalline material suitable for structural investigations. Rietveld refinement of a natural sample of a Co-bearing mansfieldite has now been carried out using synchrotron powder diffraction data (Fig. 1). to O3, respectively (Table 2). Such a structure framework displays channels along b. With respect to the synthetic mansfieldite (Harrison, 2000), the natural sample shows a slightly smaller unit cell volume, which is the effect of slightly smaller octahedral (9.110 versus 9.187 Å 3 ) and tetrahedral (2.428 versus 2.450 Å 3 ) volumes. Also the distortion index of both the polyhedra, 0.019 and 0.008 for the octahedral and tetrahedral site, respectively, is larger than that calculated for the synthetic material, viz. 0.014 and 0.002. A slight distortion of the structure is probably due to the small amount of incorporated Co and other elements present in the structure of the natural sample. Bond valence calculations show slightly overbonded values of 3.036 and 5.079 valence units for the cation in the octahedral site and in the tetrahedral site, respectively. The isotropic thermal parameters for O5W and O6W, 0.0182 (12) and 0.0164 (12) Å 2 , respectively, are slightly larger than those of the other oxygen atoms and reveal a certain degree of disorder of the water molecules along the channels, since they are not taking part in the metal-oxygen-metal chains of the structure.

Experimental
The specimen used in this study is from the locality of Mt. O. The excess Al resulting from the calculation has been arbitrarily assigned to the tetrahedral site. X-ray data collections of some single crystals, with a CCD equipped diffractometer, revealed that all the samples were actually polycrystalline aggregates and showed irregular and broadened spots typical of materials with high mosaicity. Refinements from single-crystal X-ray diffraction data yielded, in the best case, a not satisfactorily R F index of 6.54%. Fragments of pure mansfieldite were then ground and used for synchrotron X-ray data collection.

Refinement
Structural data were refined employing the Rietveld method and starting from the atomic coordinates provided by Harrison (2000), except for the H atom parameters that were not refined but included in the model. The site occupancies were assigned according to the composition of the idealised chemical formula (Al 0.95 Co 3+ 0.05 )AsO 4 .2H 2 O, with 5% Co at the octahedral Al sites. Fig. 1. The observed, calculated, background and difference X-ray diffraction profile for natural mansfieldite. Bragg reflection positions are shown at the bottom.