Zr-Ti disilicates from the Pian di Celle volcano, Umbria, Italy

Phases of the cuspidine and götzenite groups, as well as khibinskite have been observed in melilitolite of the Pian di Celle volcano both as groundmass minerals and as trapped/daughter phases of melt inclusions hosted by melilite and olivine. Their approximate crystallisation sequence deduced from relationships of these minerals in melilitolite might be as follows: Zr,Ti-rich cuspidine Zr-rich cuspidine götzenite, 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 may suggest partial solid solution between monoclinic cuspidine, Ca4[Si2O7]F2, and pseudomonoclinic 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 possibility of the above solid solution and shows that the Pian di Celle cuspidine reserves 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 götzenite show negligible differences from götzenite elsewhere. 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. The extreme Zr,Ti-rich composition approaches that of hiortdahlite (possibly, hiortdahlite-1) on chemical, diffraction and Raman data. Khibinskite differs from the ideal composition K4Zr2[Si4O14] in high Na (up to 0.25 a.f.u) and Fe (up to 0.12 a.f.u) contents.