Structure solution from powder data of the phosphate hydrate tinticite

The crystal structure of the mineral tinticite has been solved by direct me thods from integrated intensities of X-ray powder diffraction data and subs equently refined with the Rietveld technique. The sample used for the struc ture solution comes from the Gava-Bruguers area (20 lan SW of Barcelona), w hich contains a large variety of phosphates, some of which were exploited i n gallery mines during the ancient neolithic. Tinticite crystallizes in the triclinic space group P (1) over bar with unit cell parameters a = 7.965(2 ) Angstrom, b = 9.999(2) Angstrom, c = 7.644(2) Angstrom, alpha = 103.94(2) degrees, beta = 115.91(2)degrees, gamma = 67.86(2)degrees and cell content Fe-5.34(3+)(PO4)(3.62)(VO4)(0.38)(OH)(4). 6.7 H2O; rho(exp) = 2.94 g/cm(3); rho(calc) = 2.88 g/cm(3). The Rietveld refinement of the data set converge d to R-wp = 13.1 % and chi(2) = 3.3. Due to the complexity of the disorder in this structure, the refined structure model could only account for part of it. The octahedrally coordinated Fe3+ ions form dreier single chains of general formula(infinity) (1)[Fe3O14] at y = 0 and trimers of type cis-[Fe3 O14] placed at y = 1/2. While the dreier single chains are linked to each o ther by fully occupied PO4 groups yielding in this way predominantly ordere d layers, the trimers are partially disordered and connected to each other and to the ordered layers both by PO4 groups and through H-bonds. The highe r stability of the ordered layers is consistent with the observed platy nat ure of the microcrystals of tinticite.