INFLUENCE OF MAGMA COMPOSITION AND OXYGEN FUGACITY ON THE CRYSTAL-STRUCTURE OF C2/C CLINOPYROXENES FROM A BASALT-PANTELLERITE SUITE

The crystallochemical variations of clinopyroxene in response to changes in f(O2) and melt composition have been determined for a basalt-pantellerite suite (Boseti Complex, Main Ethiopian Rift) by crystal structure refinement and microprobe analysis. The pyroxene evolutionary trend has both a "Ca-minimum" and late iron enrichment. During crystallization from basalts to trachytes, clinopyroxene geometry depends mainly on the relationships between T and M2 sites; for example, high SiO2 activity in the magma causes high Si occupancy in T site, which in turn requires low Ca occupancy in M2 site in order to fulfill the local charge balance requirements. In contrast, clinopyroxene crystallized from acid melts is characterized by high Fe2+ (M1) content and therefore by a very large M1 site. Longer <M1-O1> and M1-O2 bond lengths required shorter T-O1 and T-O2 bond lengths and high Si occupancy in T site. It is concluded that the "Ca-minimum" in the clinopyroxene structure is regarded as the lowest value at which the charge balance requirements are satisfied in a C2/c clinopyroxene structure.