Structure solution from powder data of the phosphate hydrate tinticite

The crystal structure of the mineral tinticite has been solved by direct methods from integrated intensities of X-ray powder diffraction data and subsequently refined with the Rietveld technique. The sample used for the structure solution comes from the Gava-Bruguers area (20 km SW of Barcelona), which contains a large variety of phosphates, some of which were exploited in gallery mines during the ancient neolithic. Tinticite crystallizes in the triclinic space group P1 with unit cell parameters a = 7.965(2) A, b = 9.999(2) A, c = 7.644(2) A, alpha = 103.94(2) degrees , beta = 115.91(2) degrees , gamma = 67.86(2) degrees and cell content Fe (super 3+) (sub 5.34) (PO (sub 4) ) (sub 3.62) (VO (sub 4) ) (sub 0.38) (OH) (sub 4) .6.7 H (sub 2) O; p (sub exp) = 2.94 g/cm (super 3) ; = 2.88 g/cm (super 3) . The Rietveld refinement of the data set converged to R (sub wp) = 13.1 % and Chi = 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 Fe (super 3+) ions form dreier single chains of general formula (sub infinity) (super 1) [Fe (sub 3) O (sub 14) ] at y = 0 and trimers of type cis-[Fe (sub 3) O (sub 14) ] placed at y = 1/2. While the dreier single chains are linked to each other by fully occupied PO (sub 4) groups yielding in this way pre- dominantly ordered layers, the trimers are partially disordered and connected to each other and to the ordered layers both by PO (sub 4) groups and through H-bonds. The higher stability of the ordered layers is consistent with the observed platy nature of the microcrystals of tinticite.