Editor: R.C. Emmons
Our study has been confined almost entirely to the feldspars of routine rocks and has been directed away from the feldspars of pegmatites. Actual separations were made magnetically and by heavy liquids, largely by Gates and Clabaugh (Chapter 1), and the fractions were chemically analyzed (Chapter 2). To the extent that our analyzed feldspars are representative of all such rocks, we conclude that the potassic content of plagioclase is much lower than we have been led to believe; also, that perthitic feldspars are probably composed of members which are quite normal in composition as defined by the analyses of the feldspars given. We believe that soda-rich "orthoclase" will prove to be as difficult to find as is potash-rich plagioclase. The optical study of our samples and the correlation with the analyses were done by Crump and Ketner (Chapter 3). They have learned that the refractive indices of plagioclase give the most reliable information on the chemical composition. The potash feldspars have not yet been studied in the same way. The Fedorow migration curves are less reliable in our present state of understanding. We hope that the irregularities they display may ultimately prove to be their greatest asset in that, when understood, they will yield even more detailed information. That will be our next endeavor. Extinction-angle curves, properly selected and used, are apparently more reliable than are the Fedorow migration curves but are less reliable than the refractive indices. We have made no progress toward a recognition of the potash content of plagioclase from optical data…
Published: 1/15/1953
Pages: 142
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