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

Dipartimento di Scienze della Terra, Università di Pisa, Via S. Maria 53, I-56126 Pisa, Italy

^* E-mail: bellezza{at}dst.unipi.it

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 Ti_0.5 Si₂O₇ (F,OH,O)₂, Z = 2, space group P, a = 9.636(3), b =7.341(2), c =5.737(1) Å, = 89.94(2), β = 100.74(2), = 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⁺ + O^2–.

Key-words: cuspidine, götzenite, microprobe chemical data, single-crystal X-ray diffractometry, crystal structure.

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