Estimating modal abundances from the spectra of natural and laboratory pyroxene mixtures using the modified Gaussian model

Abstract

Spectra of samples containing multiple pyroxene components are explored as a function of modal abundance using the modified Gaussian model (MGM). The MGM, unlike other approaches, allows spectra to be analyzed directly, without the use of actual or assumed end‐member spectra and therefore holds great potential for remote applications. Quantitative understanding of the spectral characteristics of lithologies which include mixtures of two or more pyroxenes is fundamental to analyzing remotely acquired spectra of terrestrial and extra‐terrestrial targets. A series of mass fraction mixtures created from several different particle size fractions were analyzed with the MGM to quantify the properties of pyroxene mixtures as a function of both modal abundance and grain size. Results of this MGM analysis indicate that band centers, band widths, and relative band strengths of absorptions from individual pyroxenes in mixture spectra are largely independent of particle size. In addition, systematic changes in relative band strength as a function of modal abundance are observed, which yield particle size independent relationships that can be used to estimate modal abundances from the spectra of unknown samples. Spectra of natural samples exhibiting both zoned and exsolved pyroxenes are evaluated as examples of spectra likely to be measured from actual lithologies. Spectral properties of both pyroxene components are resolved in exsolved samples using the MGM, and modal abundances are accurately estimated to within 5–10% without predetermined knowledge of the end‐member spectra. In contrast, the spectra of samples exhibiting zoned compositions are consistent with one dominant pyroxene component. This single pyroxene component has anomalously wide absorption bands and appears to represent an average composition.