X-ray Intensity Measurements on Perthitic Materials: II. Data from Natural Alkali Feldspars

Abstract

Compositional data and () peak-intensity X-ray measurements on 79 alkali feldspar specimens from intrusive rocks of the western United States demonstrate a considerable variation in the () peak-intensity ratios , where o refers to the K-rich and a to the Na-rich phases) of the perthitic phases comprising the alkali feldspar, such variation being a function of bulk composition and thermal history. In agreement with theoretical expectations, sanidines and incompletely unmixed orthoclase perthites show a higher () peak-intensity ratio than orthoclase perthites with a greater unmixing and withsimilar bulk composition. Microcline perthite specimens having similar bulk composition have even lower () peak-intensity ratios, with the exception of a specimen from a mylonite. Under conditions of relatively placid crystallization, it appears that for any bulk composition the lower the temperature-structural state, the lower the () peak-intensity ratios. Such variations in () peak-intensity ratios are found even in related rocks of a consanguineous igneous sequence, suggesting that studies of this nature may provide a useful isograd-like zoning of many intrusive rocks. Alkali feldspars even within a small intrusive rock unit may show a considerable range in bulk composition.