• journal_title：Geological Society of America Bulletin
• Contributor：B. N. CHURCH ; W. M. JOHNSON
• Publisher：Geological Society of America
• Date：1980-
• Format：text/html
• Language：en
• Identifier：10.1130/0016-7606(1980)91<619:COTRIO>2.0.CO;2
• journal_abbrev：Geological Society of America Bulletin
• issn：0016-7606
• volume：91
• issue：10
• firstpage：619

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.