The prediction of refractive index from silicate analyses increases the scope of Mathews' arc fusion method for the determination of fine-grained igneous rocks. This is possible by replacing the specific gravity factor in the classical Larsen (Gladstone) equation by a set of empirically derived coefficients for the major oxides. This yields the new equation where (n) is the refractive index, (n − 1) the refractivity, and (W.F.) the weight fraction of the oxides present. Calculations of (n − 1) values for test samples are precise to ± 0.007 which is about the same as the estimated error at source due to in-homogeneities in the prepared glass.

The refractive index of rock glass is a measure of basicity that provides a ready means of linking microscope observations with chemical composition to assist classification, petrological interpretation, and mapping of volcanic or other aphanitic igneous rocks.

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