Electron-Excited Luminescence as a Petrologic Tool

Abstract

Preliminary studies of cathodo-luminescence of rock-forming minerals have shown that several minerals can be rapidly identified, and that structural and composition variations with potential genetic implications can be revealed. Detrital and secondary quartz can be distinguished by color differences. Radiation halos in quartz, caused by zircon inclusions, have been detected. Apatites show a variety of colors, which may be of diagnostic value. Carbonates show red luminescence, except for iron-bearing specimens which yield reduced or no luminescence. Zoning in dolomites is readily detected from variation in intensity of luminescence. Mottled texture probably results from inheritance of manganese and iron from the carbonate replaced by the dolomite. Residual skeletal structure in fossils can be easily delineated by cathodo-luminescence, and it may be possible to ascribe a featureless luminescence to recrystallization. A faint blue luminescence, which rapidly disappeared, has been seen in ovoid structures of a Precambrian chert, and has been ascribed to organic remains. Luminescence may help to establish the validity of doubtful fossil remains. Separate phases in feldspar perthites can be easily detected by color and intensity variations of the luminescence. Analysis of potassium X-rays from lamella in orthoclase-microcline-albite perthites showing mottled luminescence has led to distinction between orthoclase and microcline, and to the possibility that the latter nucleates at the boundary between orthoclase and albite. Occurrence of coarse red and blue banding crossed by fine K- and Na-rich lamellae distinguished by different intensities of luminescence has led to the suggestion of an earlier stage of exsolution now revealed only by the luminescence. Variation of intensity of luminescence in albite lamellae of microcline perthites may result either from formation over a temperature range or from limited diffusion. Cathodo-luminescence has proved useful in characterizing fine-scale impurities, such as apatite in a feldspar host, and in checking homogeneity of natural and synthetic minerals. Fifteen illustrations in color are given.