Ab initio X-ray powder structure determination of parascorodite, Fe(H2O)2AsO4

Cuspidine and götzenite were found as accessories in the pegmatitoid facies of venanzite from the Pleistocene lava flow of Pian di Celle. The modal compositions of venanzite and its pegmatitoid facies have been defined through combined XRF and Rietveld quantitative analyses. Cuspidine and götzenite were unambiguously identified by combining microprobe analyses and structural studies. The structure analyses werecarried on with single-crystal XRD data and the refinements converged to R1 = 0.048 and 0.053 for cuspidine and götzenite respectively. Cuspidine from Pian di Celle, ideally (Ca,Zr) Ca2 (Ca,Na) Si 2O7 F2, Z = 4, space group P2 1/a, a = 10.919(2), b = 10.485(1), c = 7.485(1) Å , β = 109.55(1)°, represents a Zr,Na-rich variety of cuspidine. Götzenite from Pian di Celle, ideally (Ca,Zr) (Ca,Na) Ca (Na,Ca) 0.5 Ti0.5 Si2O7 (F,OH,O)2, Z = 2, space group P1, a = 9.636(3), b = 7.341(2), c = 5.737(1) Å, α = 89.94(2), β = 100.74(2), γ = 101.06(3)°, represents a Zr-rich variety of götzenite. Zirconium and sodium enter in the structures substitutingpart of calcium, according to the main substitution indicated by the chemical data: 2Ca2+ + F- ↔ Na+ + (Zr+Ti)4+ + O2-.