Zr-Ti disilicates from the Pian di Celle volcano, Umbria, Italy
Sharygin, Victor V.; Stoppa, Francesco; Kolesovc, Boris A.
European Journal of Mineralogy Volume 8 Number 5 (1996), p. 1199 - 1212
published: Oct 30, 1996
manuscript accepted: May 15, 1996
manuscript received: Aug 23, 1995
ArtNo. ESP147050805022, Price: 29.00 €
Abstract Phases of the cuspidine and gotzenite groups, in addition to khibinskite, are observed in melilitolite from the Pian di Celle volcano both as groundmass minerals and as trapped/daughter phases of melt inclusions hosted by melilite and olivine. The approximate crystallisation sequence, as inferred from their relationships in melilitolite, is thought to be as follows: Zr,Ti-rich cuspidine → Zr-rich cuspidine → gotzenite, khibinskite. Zr-rich cuspidine shows notable variations in CaO (58.2-40.4 wt.%), ZrO2 (0.2-11.2 wt.%), Na2O (0.5-4.5 wt.%) and F (10.1-8.1 wt.%). These variations suggest partial solid solution between monoclinic cuspidine, Ca4[Si2O7]F2, and pseudo-monoclinic hiortdahlite-2, NaCa2 Zr[Si2O7]OF, due to possible combined substitution 2Ca2+ + F' ↔ Na+ + Zr4+ + O2. The X-ray patterns of the Pian di Celle cuspidines, with a different abundance of ZrO2, fall between those of pure cuspidine and hiortdahlite. Raman spectroscopy supports the hypothesis of the above solid solution and shows that the Pian di Celle cuspidine retains monoclinic symmetry with a maximum content of Zr+Ti (up to 0.5 a.f.u.). Götzenite, (Na,Ca)3.5Ti0.5[Si2O7](F,O)2, generally varies in CaO, REE2O3, TiO2, ZrO2 and Na2O, which may indicate two main cation substitutions: 2Ca2+ ↔ REE3+ + Na+ and Ti4+ ↔ Zr4+, while REE and Zr contents do not exceed 0.14 a.f.u.. Both diffraction and spectroscopic data of Pian di Celle gotzenite show negligible differences from gotzenite from other localities. The Zr-Ti-cuspidine composition ranges from Na0.7Ca2.8(Ti,Zr)0.5[Si2O7]F1.7O0.3 to Na0.8Ca2.4(Zr,Ti)0.8[Si2O7]F1.2O0.8. Chemical, diffraction and Raman data suggest that the extreme Zr,Ti-rich composition approaches that of hiortdahlite (possibly, hiortdahlite-1). Khibinskite differs from the ideal composition K4Zr2[Si4O14] in having high Na (up to 0.25 a.f.u) and Fe (up to 0.12 a.f.u) contents.