Chevkinite-group minerals from salic volcanic rocks of the East African rift

Electron microprobe analyses are presented of chevkinite-group minerals occurring as micropheno-crysts in peralkaline rhyolites of the Greater Olkaria Volcanic Complex (Kenya) and as a groundmass phase in a peralkaline quartz trachyte lava from the Tarosero volcano (Tanzania), both in the East African Rift Valley. Their compositions conform closely to the formula: (REE, Ca, Th)4 Fe 2+ (Fe2+, Al, Ti, Zr, Nb)2 Ti2 (Si4O22). Compared to published analyses of chevkinite-group minerals, the Olkaria phases are relatively enriched in Nb and the LREE; the Tarosero phase is more calcic and relatively Zr-and Nb-rich. The main substitution in the A site at Olkaria is Ca> Ce. The overall charge-balancing substitution seems to be (McDowell, 1979): Ca2+A + T