Concentrations of Ba, Sr, Rb, and Cs, X-ray monoclinic and triclinic ordering, and Ab content in 66 samples of K-feldspars (Kfs) from different pegmatites of the Appalachians and Rocky Mountains have been determined. Maximal contents of Ba and Sr and minimal Rb and Cs are found in the earliest generations of Kfs in muscovite pegmatites. The Kfs of raremetal pegmatites contain maximal Rb and Cs but minimal Ba and Sr. As a rule, the Kfs of raremetal-muscovite and non-specialized pegmatites have intermediate geochemical characteristics.
In all pegmatites Rb (and Cs) contents increase and Ba (and Sr) contents decrease from earliest to latest generations of Kfs. The Ba:Rb ratio is a sensitive indicator of pegmatite specialization (using early Kfs) or recrystallization (using comparison of Kfs generations). It is suggested that high pressure can enhance the isomorphism of Ba and Sr in Kfs and inhibit that of Rb and Cs.
Maximum microclines are predominant among Kfs of U. S. pegmatites. Orthoclase or high and intermediate microcline structures are characteristic of Kfs containing large quantities of Ab, Ba, and Rb, if the pegmatites have not been altered.