Abstract

Representative modal analyses of rocks with an average grain size greater than 2 or 3 mm., and of rocks which contain large, irregularly distributed crystals, cannot be made from standard 1−½ square thin sections. Rapid, moderately precise modes of many such rocks can be obtained by sawing and grinding or polishing a smooth surface on a hand specimen, etching and staining the surface to aid in mineral identification, applying dot pattern Zip-A-Tone to the surface, and making a point count with the aid of a magnifier. The method has been developed for obtaining modes of medium- and coarse-grained granitic rocks from the Sierra Nevada and subsilicic rocks from the Stillwater complex. Detailed techniques are presented for etching, staining, and counting rocks ranging in composition from granite to dunite, but the general method should be applicable to a wider variety of rock types.

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