Correlation of some igneous rocks of New Mexico by the fusion method

Abstract

Thirty‐six samples of igneous rocks and ten of metamorphic rocks, for which complete chemical analyses have been obtained, were powdered and fused into artificial glasses in the carbon arc of a spectrograph. Curves showing the relation between the indices of refraction of such glasses and the composition of the rock samples may be used to estimate the approximate composition and may aid in the correlation of igneous rocks. By this method, the SiO2 content of an unknown rock sample may be estimated to within three per cent of accuracy, whereas MgO, CaO, and the combined Fe2O3 + FeO may be estimated to within two per cent of accuracy. The method is not so satisfactorily applied to rhyolite and quartz latite, because these rocks do not readily form homogeneous glasses in the carbon arc.Aside from the effect of changing proportions of SiO2, the indices of refraction of the artificial glasses are determined, in decreasing order, by the oxides Fe3O2, TiO2, FeO, MgO, CaO, Na20, and K20.From comparative studies of variation diagrams, it can be shown why the method is particularly effective for volcanic rocks of intermediate composition, but may not be applied satisfactorily to altered, metamorphic, or other igneous rocks.