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Quartz Grain Orientations--1 (The Photometric Method)
R. F. Sippel
Sandstone grain orientation measurements must be made in much greater numbers than heretofore, to establish the validity of diverse applications which have been suggested. Adequate depth of data can never be supplied by the tedious particulate methods which have come into general use. Among several rapid methods which have been suggested, the photometric method has much to recommend it. It has been little used for reasons stemming partly from inadequacies in the original presentation, and partly from criticisms which have been directed against it. The chief criticism is: since the method is a crystallographic one, that without prior knowledge of the relation between c axes and longest shape axes, the photometer measurements cannot be interpreted in terms of dimensional orientat ons. This criticism is not valid. Crystallographic methods are applicable except in very rare instances, and have unique advantages compared to the particulate methods.
This paper employs the Jones Calculus, a powerful method for polarized light calculations. With its help a quantitative theory of the photometer is presented. An optimum form of the photometer is derived and compared with the form originally suggested. Photometer action is simulated in the computer, and the two forms of instrument are compared in their ability to analyze assumed three-dimensional distributions of quartz grain c axes. Effect of porosity, thin section thickness, inclination of the preferred direction, and strength of preferred orientation are all evaluated for each instrument. The optimum photometer is superior in all respects but not greatly so.
Inadequacies of the particulate methods in addition to their tedium are discussed. It is also suggested that the Jones Calculus be used as the fundamental tool in teaching the theory of the petrographic microscope.
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