X-Ray-Emission Microanalysis of Rock-Forming Minerals. V. Orthopyroxenes

Forty-eight chemically analyzed orthopyroxenes have been analyzed by microprobe X-ray-emission techniques for Na, Mg, Al, Si, K, Ca, Ti, V, Cr, Mn, Fe, and Ni. As far as possible intergrown phases such as augite and ilmenite were avoided, and a systematic bias with respect to the bulk chemical analyses is to be expected. For each specimen the lowest Ca reading, which is assumed to give the best estimate for orthopyroxene uncontaminated by augite, increases on average for Fe-rich specimens. Microprobe analyses for Al tend to be lower than chemical results; for Fe-rich samples the Al content tends to be lower. Microprobe readings for Mn and Cr tend to be higher than chemical values, suggesting a bias in one or both of the techniques. Mn increases strongly while Ni and Cr decrease on the average as the Fe content increases. The highest Ti reading of 0.12 wt. per cent is much lower than the highest chemical value of 0.8 wt. per cent: this discrepancy results at least principally from ilmenite lamellae in the chemically analyzed material. Microprobe analyses of Ti show no correlation with the Mg/Fe ratio. Highest readings of Na and K for the few specimens analyzed are 0.04 per cent and 0.00 per cent. Oxide totals (assuming all iron is divalent) range from 97.3 to 101.3 per cent, with thirty-four between 99 and 101 per cent.