M1 site splitting due to next nearest neighbor effects and ferric iron in tetrahedral site in clinopyroxene megacrysts

Abstract

It is well known that in pyroxene structure, there are two metal sites, M1 and M2. Generally speaking, ferrous iron in each of these sites would normally be expected to give rise to a doublet. However, anomalies have been found in the relative areas of the peaks in the room temperature spectra of some clinopyroxene (CPX) when the above assignment is followed. According to the calculation of Next Nearest Neighbor configurations of divalent cations in M1, we found that the four configurations of M1 can be divided into two groups. One group is 3Ca configuration that increases with the content of Ca (p.f.u.); the other group is made up of three No-3Ca configurations that decrease with the content of Ca. The two groups contribute to the spectrum structure of M1, so in this study we fit two doublets for ferrous iron in M1. Though there were several reports on Fe3+ in tetrahedral site previously, it was not sure that Fe3+ occupies the T site is an universal fact in CPX, despite of the content of Al. We found that the Fe3+ in the T site fitted by Mossbauer spectroscopy is negatively correlated to the Si content in the T site and positively correlated to the Fe3+ in the T site estimated on the supposition that Fe3+ and Al occupy the T site randomly. If it is true, it is important in the modeling of ion exchange geobarometries and geothermometries.