Complex colour and chemical zoning of sodalite-group phases in a hauynophyre lava from Mt. Vulture, Italy

摘要

The haüynophyre emitted from a parasitic vent of the Vulture stratovolcano is a S- and Cl-rich, leucite-melilite-bearing lava flow containing an unusually large amount of sodalite-group minerals (>23 vol.%). Mineralogical and chemical study of phenocrysts has led to the identification of black haüynes, blue lazurites and of Cl-rich white or black noseans. X-ray diffraction (XRD) study confirms the occurrence of nosean having a low symmetry (P23). Raman spectra and XRD data show that S is fully oxidized to SO₄ in black haü ynes and in white noseans, while it is partly reduced to form S₃– groups in blue lazurites, which also contain H₂O molecules. Structural and chemical data strongly question the validity of the Hogarth and Griffin (1976) method widely used to resolve the ratio S6+/S2– in sodalite-group phases from EMPA data. Among euhedral phenocrysts, large lazurites are only faintly zoned. All other phases show variable core-rim chemical zoning and many phenocrysts are partially resorbed and/or colour-zoned. Black haüynes have highly variable S/Cl and slightly lower SiO₂/Al₂O₃ ratios, larger Fe_TOT contents and more compatible trace elements than lazurites. Thin opaque noseansodalite rims surrounding all crystals are interpreted as a result of rapid crystallization driven by exsolution of a S-scavenging fluid phase. We suggest that the extreme complexity of the mineralogical assemblage reflects variable a_SiO2 and a_H2O of the silicate melts.