X-Ray-Emission Microanalysis of Rock-Forming Minerals III. Alkali Feldspars

Microprobe X-ray-emission analyses have been made for sodium, magnesium, aluminum, silicon, phosphorus, potassium, calcium, titanium, manganese, iron, rubidium, strontium, cesium, and barium in sixty chemically analyzed alkali feldspars. The highest concentrations (weight per cent) of the minor elements are: magnesium, <0.02; phosphorus, 0.10; titanium, 0.07; manganese, <0.01; iron, 0.80; rubidium, <0.04; strontium, 0.80; cesium, <0.02; barium, 1.80. Agreement with chemical data for the minor elements is poor; for example, most microprobe analyses of iron are lower, and most analyses of barium are higher, than values previously reported. Agreement for titanium and phosphorus could not be expected because of possible inclusions of ilmenite and apatite. Older gravimetric analyses of sodium and potassium show considerable scatter with the microprobe data, but there is excellent agreement between the latter and recent flame-photometer analyses. Empirical calibration curves were derived by comparison with flame-photometer data, from theoretical X-ray-emission formulas, and from the expectation that feldspars will be nearly stoichio-metric. The calibration curve for sodium differs from the one calculated using the formulas of Part I. The calibration curves for silicon and aluminum were adjusted empirically so that the average atomic proportions agreed with those calculated from the alkali and alkaline-earth metals. The resulting correction factors agree with those calculated from the formulas of Part I. Microprobe values of silicon are higher than the chemical data, while those of aluminum are lower, in conformity with the bias previously suggested in co-operative studies of the test specimens Gl and Wl. Only seven of the analyses give oxide totals outside 99-101 per cent. Totals of the feldspar molecules calculated from the alkali and alkaline-earth metals range more widely: forty-seven lie between 99 and 101 per cent with others as low as 97.1 and as high as 102.1 per cent. The scatter increases with sodium content, reflecting the lower accuracy for this element.