A method is presented for determining the grain-shape—optic-axis relationship of clastic quartz grains and for constructing three-dimensional grain-shape fabric diagrams from optic-axis fabric diagrams.

Quartz grains from a channel sandstone and a shale were loosened and mounted so that the angle between the optic axis and physical dimension (maximum projection area and longest dimension) of each grain could be measured on the universal stage. Frequency-distribution curves of these angles indicate that clastic quartz grains tend to be enlarged parallel to the unit rhombohedron and, to a lesser extent, to the prism. Because grain shape is of primary importance in interpreting fabric diagrams of sediments, optic-axis fabrics were measured from oriented thin sections adjacent to the source of loosened grains. These diagrams are converted into grain-shape diagrams by applying to the optic-axis maxima the relationship between the optic axis and grain shape.

Frequency-distribution curves of the spatial relationship between the optic axis and grain shape are presented for the maximum projection area and the longest dimension. Possible application of the method is suggested for sedimentary studies, stratigraphic correlation, and foundation investigation.

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