Crystal-chemistry of Clinopyroxene From Sunda Volcanic Arc

The crystal-chemistry of clinopyroxene from a suite of Sunda are volcanic rocks was investigated by X-ray structure refinement and microprobe analysis. Relationships with clinopyroxene from volcanic rocks of the intraplate environment were highlighted. Sunda clinopyroxenes were distinguished into two groups. The first consists of clinopyroxene from SiO2-saturated rock types and crystal rim from SiO2-undersaturated rock types, the second of crystal core from SiO2-undersaturated rock types. The crystal chemical behaviour of the two groups differs significantly, mainly in the geometrical relationships between M2 and T sites. While in the first group <M2-O3> and <T-O3> bond lengths shorten and lengthen 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 lengths are forced to lengthen in spite of low Al-IV content, in order to mantain the local charge balance around the O3 oxygens. It is noteworthy that Sunda clinopyroxene shows strong analogies with that from basaltic rocks from the Ethiopian plateau and K-rich lavas from the Roman Province respectively, reflecting similarities between the composition and mineralogical assemblages of the host rocks, although they all came from different tectonic environments. A relationship between intracrystalline Mg-Fe2+ disorder and the explosive character of the volcanism is demonstrated.