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

Phases of the cuspidine and goetzenite groups, in addition to khibinskite, are found in melitolite from the Pian di Celle volcano both as groundmass and as trapped/daughter phases of melt inclusions hosted by melilite and olivine. The crystallization sequence is believed to have been Zr,Ti-rich cuspidine ->> Zr-rich cuspidine ->> goetzenite, khibinskite. The Zr-rich cuspidine shows considerable variations in CaO (58.2-40.4 wt.%), ZrO (sub 2) (0.2-11.2 wt.%), Na (sub 2) O (0.5-4.5 wt.%) and F (10.1-8.1 wt.%), suggesting partial solid solution between monoclinic cuspidine, Ca (sub 4) [Si (sub 2) O (sub 7) ]F (sub 2) , and pseudo-monoclinic hiortdahlite-2, NaCa (sub 2) Zr[Si (sub 2) O (sub 7) ]OF, due to possible combined substitution 2Ca (super 2+) + F = Na (super +) + Zr (super 4+) + O (super 2-) . XRD and Raman spectroscopy support this hypothesis and show that the Pian di Celle cuspidine retains monoclinic symmetry with a maximum content of Zr+Ti (up to 0.5 pfu). Goetzenite, (Na,Ca) (sub 3.5) Ti (sub 0.5) [Si (sub 2) O (sub 7) ](F,O) (sub 2) , generally varies in CaO, REE (sub 2) O (sub 3) , TiO (sub 2) , ZrO (sub 2) and Na (sub 2) O, which may indicate the two substitutions 2Ca (super 2+) = REE (super 3+) + Na (super +) and Ti (super 4+) = Zr (super 4+) , with REE and Zr contents up to 0.14 pfu. The Zr-Ti cuspidine composition ranges from Na (sub 0.7) Ca (sub 2.8) (Tl,Zr) (sub 0.5) [Si (sub 2) O (sub 7) ]F (sub 1.7) O (sub 0.3) to Na (sub 0.8) Ca (sub 2.4) (Zr,Ti) (sub 0.8) [Si (sub 2) O (sub 7) ]F (sub 1.2) O (sub 0.8) ; the extreme Zr, Ti-rich variety approaches hiortdahlite-1 in composition. The khibinskite differs from the ideal composition in having high Na (up to 0.25 pfu) and Fe (up to 0.12 pfu).