Clinopyroxenes from Somma-Vesuvius: Implications of Crystal Chemistry and Site Configuration Parameters for Studies of Magma Genesis

A clinopyroxene suite from Somma-Vesuvius clinopyroxenite cjectaand mcgacrysts has been investigated by single crystal X-raydiffraction combined with electron probe microanalysis. Theaim of the study was to characterize the intracrystalline configurationof the clinopyroxenes and delineate their possible parentalliquid(s). Vesuvius megacrysts and nodule clinopyroxenes show the initial‘signature’ of the leucitite (sensu lato) magmawhile Somma clinopyroxenes are characteristic of leucite phonolitemagma. The crystal chemical variations demonstrate that thespecimens investigated had originated by shallow-level crystallizationof leucitite and its derivatives, under reducing conditions.T-site (Si + Al) deficiency, typical of clinopyroxene from lamproitesis reported in some specimens from the Roman Volcanic Regionfor the first time. Intracrystalline Mg-Fe²⁺ ordering in M1-M2 appears to be configuration-dependent,yielding the largest K_D variation in the clinopyroxene fromphonolite. This is contrary to expectation.