Analyzing Mg and Fe in Olivines, Pyroxenes, and Garnets: Systematic Discrepancies

Abstract

Quantitative analysis of Mg and Fe in olivines, pyroxenes, and garnets is important for geochemical studies. Natural olivines have compositions close to the forsterite (Mg2SiO4) and fayalite (Fe2SiO4) binary system, with relatively minor substitution of other components. Pyroxenes and garnets are problematic in that they exhibit more diverse solid solution, and may contain both Fe2+ and Fe3+, the relative concentrations of which cannot be directly measured with the microprobe. The forsterite-fayalite olivine binary is a convenient system for studying the accuracy of Mg and Fe analysis without other complicating factors.Bence and Albee utilized the a-factor approach for quantitative analysis of silicates, based on the empirical observation:where CA,AB is the weight fraction of the oxide A in a binary oxide system AB, and KA,AB is the measured x-ray intensity of A relative to that of the pure oxide A.