Methods for Estimating Fe Redox in Mesoamerican Jadeite Jade

Abstract

Green jadeite has been selectively mined and used by Mesoamerican Indigenous cultures for centuries, with the green hues resulting from interactions among various chromophores, including Cr3+, Fe3+, and Fe2+. This study employed Fe K-edge X-ray absorption near-edge structure (XANES) measurements using a mm-scale X-ray beam to estimate the iron-redox ratio in jadeitite, with scanning electron imaging and X-ray microanalysis employed to determine mineral compositions and proportions within the synchrotron analysis zones. Results for the XANES measurements were processed using two distinct approaches based on different underlying assumptions: peak fitting and machine learning. Despite their differences, both peak fitting and machine learning approaches yielded statistically consistent results, suggesting that the analysis assumptions were valid for this system and that XANES data can provide reliable Fe redox estimates for jadeites. The Fe3+/FeTotal range for the jadeites measured by XANES was estimated to be between 75% and 91% after accounting for phase mixing (omphacite with jadeite) within samples. Low Fe concentrations in jadeites precluded the use of quantitative compositional analysis to estimate Fe redox using the charge balance method.