Ca-rich pyroxene from basic and silicic volcanic rocks from the Cameroon Volcanic Line (West-Africa): crystal chemistry and petrological relationships

C2/c pyroxene (Cpx) phenocrysts from basic rocks (basanite-trachyphonolite suite and alkali basalts) and differentiated CIPW Ne- and Q-normative trachytes and rhyolites occurring in the continental sector of the Cameroon Volcanic Line were investigated by single-crystal X-ray diffraction combined with electron probe microanalysis. This technique provides accurate data on site occupancy and geometric variations. The investigated Cpx is the only liquidus Ca-rich crystallizing phase from both basic and differentiated volcanics. Cpx from basic alkaline rocks are characterized by large VT owing to their low Si(T), which requires high R3+ (AlVI + Fe3+ + Cr3+ + Ti4+) in M1 site, due to Si-AlIV substitution. High Ca content allows the appropriate netcharge for the overbonded O3 oxygens. Cpx from differentiated silicic volcanics have large VM1 related to high Fe2+ (M1), since Cpx crystallized from MgO-poor (< 0.5 wt%) melts at relatively low fO2 (QFM buffer). The high Fe2+ (M1) causes lengthening of 〈M1-O〉, which requires shortening of T-O1 and T-O2 (i.e. high Si), irrespective of the magmatic affinity of the Cpx in SiO2-undersaturated or oversaturated conditions. Therefore, Cpx from silicic rocks have similar (Ca + Na) contents to those formed from basic rocks, but their 〈M2-O3〉 is relatively longer, owing to the shortening of 〈T-O3〉. It follows that Cpx crystallizing from silicic melts show a relatively large VM2 increase, which is not related to an increase in (Ca + Na).