Abstract

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 Gavá-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 P1̄ with unit cell parameters a = 7.965(2) Å, b = 9.999(2) Å, c = 7.644(2) Å, α = 103.94(2)°, β = 115.91(2)°, ω = 67.86(2)° and cell content Fe3+5.34(PO4)3.62(VO4)0.38(OH)4·6.7 H2O; ρexp = 2.94 g/cm3; ρcale = 2.88 g/cm3. The Rietveld refinement of the data set converged to Rwp = 13.1 % and χ2 = 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 1[Fe3O14] at y = 0 and trimers of type cis-[Fe3O14 placed at y = 1/2. While the dreier single chains are linked to each other by fully occupied PO4 groups yielding in this way predominantly ordered layers, the trimers arc partially disordered and connected to each other and to the ordered layers both by PO4 groups and through H-bonds. The higher stability of the ordered layers is consistent with the observed platy nature of the microcrystals of tinticite.

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