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) Si2O7 F2, 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 P1̄, 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−.

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