X-Ray-Emission Microanalysis of Rock-Forming Minerals IV. Plagioclase Feldspars

Forty-two chemically analyzed and eleven synthetic plagioclase feldspars have been analyzed by micro-probe X-ray-emission methods for sodium, magnesium, aluminum, silicon, phosphorus, potassium, calcium, titanium, manganese, iron, strontium, cesium, and barium. The highest concentrations (weight per cent) of the minor elements are: phosphorus, 0.11; potassium, 0.55; titanium, 0.04; iron, 0.53; strontium, 0.13. Magnesium, manganese, cesium, and barium were not detected at the 0.02 wt. per cent level, although barium was found in intergrown potassium feldspar. With a few exceptions, phosphorus, potassium, titanium, and iron were found at lower levels than those given by bulk chemical analyses, as might be expected from the occurrence of inclusions in many specimens. Strontium analyses for one suite of specimens showed a systematic bias, probably attributable to a calibration error in the optical spectrographic technique. Major elements were referred to synthetic plagioclase glasses and to devitrified synthetic albite as standards. For sodic plagioclases (especially glasses) it was necessary to use either low counting rates or wide electron beams to reduce alkali loss. Microprobe and chemical analyses for calcium show a good, almost unbiased, correlation, but for sodium they have a poor correlation, microprobe results tending to be higher than chemical. Microprobe analyses of aluminum and silicon tend to be lower and higher, respectively, than chemical analyses. Only three of forty analyses yield oxide totals outside the 99.0-101.0 per cent range, but totals of feldspar molecules calculated from sodium, potassium, calcium, and strontium have a wider range, 97.5-102.5 per cent, with thirty-three between 99 and 101 per cent. The average iron content increases with calcium as expected from the increasing Al³⁺ sites and the generally higher temperatures of formation of calcic plagioclases. Strontium and potassium reach their highest concentrations in the range An₂₀-An₄₀.