Complex colour and chemical zoning of sodalite-group phases in a hau« ynophyre lava from Mt.Vulture, Italy A. DI MURO 1, *, E. BONACCORSI 2 AND C. PRINCIPE 3 1 Laboratoire de Physique et Chimie des Syste `mes Volcaniques, IPGP-Paris VI, Case Courrier 109, Place Jussieu 4, Paris Cedex 05, France 2 Dipartimento di Scienze della Terra, Universita ` di Pisa, Via S. Maria 53, 56126 Pisa, Italy 3 Istituto di Geoscienze e Georisorse-CNR, Via G. Moruzzi 1, 56124 Pisa, Italy ABSTRACT The hau ¨ynophyre 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 hau ¨ynes, 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 SO 4 in black hau ¨ynes and in white noseans, while it is partly reduced to form S 3 À groups in blue lazurites, which also contain H 2 O molecules. Structural and chemical data strongly question the validity of the Hogarth and Griffin (1976) method widely used to resolve the ratio S 6+ /S 2À 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 hau ¨ynes have highly variable S/Cl and slightly lower SiO 2 /Al 2 O 3 ratios, larger Fe TOT contents and more compatible trace elements than lazurites. Thin opaque nosean- sodalite 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 SiO 2 and a H 2 O of the silicate melts. KEYWORDS: hau ¨yne, nosean, lazurite, Raman spectrometry, Mt. Vulture, Italy. Introduction THE Vulture volcanic complex, active between ~0.74 and 0.13 Ma (Brocchini et al., 1994), is the most voluminous eruptive centre of the Italian Ultra-alkaline Intermontane Province (IUP) (Lavecchia and Boncio, 2000). The volcanic edifice is formed by a main feldspar-bearing series of pyroclastic rocks and subordinate lava flows, ranging in composition from basanite and foidite to phonolites (synthems 4,6,7; Fig. 1) and a subordinate coeval series of melilite-bearing lavas and pyroclastic products (synthem 5; Fig. 1) (De Fino et al., 1986; Caggianelli et al., 1990; Melluso et al., 1996). Carbonatite-melilitite magmas fed the final phase of volcanism producing maar-type craters (Synthem 3; Fig. 1) (Stoppa and Principe, 1997). The large Na and S contents in the Vulture magmas (Marini et al., 1994) result in the widespread presence of sodalite-group phases among the feldspathoids (De Fino et al., 1982; Stoppa et al., 2002), whereas leucite dominates the magmas of the Roman Comagmatic Region that lies west of the IUP. However, no systematic mineralogical and chemical studies have yet been carried out on the Vulture sodalite-group minerals. The famous hau ¨ynophyre (De Lorenzo, 1899) represents a unique opportunity to observe and study almost the whole range of sodalite-group phases occur- ring at Vulture volcano. The hau ¨ynophyre is a * E-mail: dimuro@ipgp.jussieu.fr DOI: 10.1180/0026461046840206 Mineralogical Magazine, August 2004, Vol. 68(4), pp. 591–614 # 2004 The Mineralogical Society