CRYSTAL-CHEMISTRY OF CLINOPYROXENE FROM SUNDA VOLCANIC ARC

The crystal-chemistry of clinopyroxene from a suite of Sunda are volca nic rocks was investigated by X-ray structure refinement and microprob e analysis. Relationships with clinopyroxene from volcanic rocks of th e intraplate environment were highlighted. Sunda clinopyroxenes were d istinguished into two groups. The first consists of clinopyroxene from SiO2-saturated rock types and crystal rim from SiO2-undersaturated ro ck types, the second of crystal core from SiO2-undersaturated rock typ es. The crystal chemical behaviour of the two groups differs significa ntly, mainly in the geometrical relationships between M2 and T sites. While in the first group <M2-O3> and <T-O3> bond lengths shorten and l engthen respectively due to Ca and Al-IV increase, in the second group in which M2 site is almost fully coordinated by Ca, <T-O3> bond lengt hs are forced to lengthen in spite of low Al-IV content, in order to m antain the local charge balance around the O3 oxygens. It is noteworth y that Sunda clinopyroxene shows strong analogies with that from basal tic rocks from the Ethiopian plateau and K-rich lavas from the Roman P rovince respectively, reflecting similarities between the composition and mineralogical assemblages of the host rocks, although they all cam e from different tectonic environments. A relationship between intracr ystalline Mg-Fe2+ disorder and the explosive character of the volcanis m is demonstrated.