Original paper

Chemical and structural study of the Zr, Ti-disilicates in the venanzite from Pian di Celle, Umbria, Italy

Bellezza, Marco; Merlino, Stefano; Perchiazzi, Natale


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 were carried 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)Si2O7F2, Z = 4, space group P21/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 substituting part of calcium, according to the main substitution indicated by the chemical data: 2Ca2+ + F ↔ Na+ + (Zr+Ti)4+ + O2−.


cuspidinegotzenitemicroprobe chemical datasingle-crystal x-ray diffractometrycrystal structure