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A new method is described for determining the direction and relative degree of preferential quartz grain optic axis orientation in rocks principally composed of quartz. It is intended primarily as a tool in the petrofabric analysis of sandstones, but could also be used in a study of metamorphic rocks. The study of quartz grain orientation in sandstones has obvious applications in the search for and development of stratigraphically controlled deposits of economic minerals. Older methods involve observation of many individual grains, and in order to sample the unit properly many tedious and time-consuming measurements must be made. A few simple observations using this new method will result in a determination of the integrated effect of hundreds, and in many cases thousands of grains in contrast to the measurement of each of these individual grains.
The technique consists of measuring by means of an attached photometer the variation in the intensity of monochromatic light passed through a standard thin section of sandstone on the stage of a petrographic microscope with gypsum plate inserted and nicols crossed during a 360° rotation of the stage. Minimum intensity of light occurs when the trend of the optic axes of the quartz grains lies parallel with the slow direction of vibration of light in the gypsum plate. Experimental data obtained by this method have been interpreted by the writer to indicate that the trend of the long axes of the quartz grains also lies parallel with this direction for some samples.
A petrofabric study, using this method, was made of a sandstone from the Bell Canyon formation, cropping out in the vicinity of the El Capitan Reef Escarpment, near Nickel in Culberson County, Texas. The direction of c-axis lineation of quartz grains, generally northwest-southeast, was found to correspond with the direction of alignment of oriented fusulinids and to be approximately at right angles to the trend of ripple marks also found in this formation. From this evidence, it was concluded that c-axis orientation determined by this method could have been safely used as evidence for deducing current directions if the other sedimentary features had not been visible.
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