Crystal Chemistry and Site Configuration of the Clinopyroxene from Leucite-bearing Rocks and Related Genetic Significance: the Sabatini Lavas, Roman Region, Italy

A clinopyroxene suite from leucite–bearing lavas locatedin the Sabatini district of the Roman Volcanic Region has beeninvestigated by single–crystal X–ray diffractionmethods combined with electron probe microanalysis. The main aim of the study was to obtain crystallographic datanecessary to evaluate intracrystaUine order–disorder relationshipsin natural clinopyroxenes and related equilibria–disequilibriaphenomena in different host–rock types. Generally, the polyhedral site configurations of the clinopyroxenesinvestigated are interdependent and follow specific variationtrends related to specific host–rock types. The majorvariations are found in the tetrahedron and the octahedron;while the polyhedron M2 is essentially unchanged and occupiedby(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) clinopyroxenesthan in the corresponding higher–temperature (core) fromthe same crystals. This is' attributed to configuration relationshipsunfavourable to the Mg–Fe²⁺ ordering and largely dependingon the occupancy of M2. Polygenetic, early–crystallized clinopyroxenes, occasionallywith resorbed margins, indicate crystal-liquid disequilibriarelated to crystal and–or magma mixing processes.