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
European Journal of Mineralogy Volume 16 Number 6 (2004), p. 957 - 969
published: Dec 28, 2004
Abstract
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−.
Keywords
cuspidine • gotzenite • microprobe chemical data • single-crystal x-ray diffractometry • crystal structure