Crystal chemistry and site configuration of the clinopyroxene from leucitite-bearing rocks and related significance: the Sabatini lavas, Roman region Italy

A clinopyroxene suite from leucite–bearing lavas located in the Sabatini district of the Roman Volcanic Region has been investigated by single–crystal X–ray diffraction methods combined with electron probe microanalysis. The main aim of the study was to obtain crystallographic data necessary to evaluate intracrystaUine order–disorder relationships in natural clinopyroxenes and related equilibria–disequilibria phenomena in different host–rock types. Generally, the polyhedral site configurations of the clinopyroxenes investigated are interdependent and follow specific variation trends related to specific host–rock types. The major variations are found in the tetrahedron and the octahedron; while the polyhedron M2 is essentially unchanged and occupied by(Ca+Na). The degree of Mg–Fe2+; ordering in M1–M2, respectively, depends on the configuration of these sites, particularly M2, and is generally lower in late–crystallized (rim) clinopyroxenes than in the corresponding higher–temperature (core) from the same crystals. This is' attributed to configuration relationships unfavourable to the Mg–Fe2+ ordering and largely depending on the occupancy of M2. Polygenetic, early–crystallized clinopyroxenes, occasionally with resorbed margins, indicate crystal-liquid disequilibria related to crystal and–or magma mixing processes.