Novel extraction protocol for evaluating abundances and structural features of amorphous SiO2

Abstract

Spectral data from satellite and rover missions on Mars identified significant abundances of amorphous phases in most samples analyzed, and SiO2 is the principal amorphous constituent in the Gale crater. Identifying and quantifying these short-range ordered, highly reactive phases is challenging but necessary to gain insight into the evolution of these materials. Terrestrial analogs are frequently employed to allow detailed analyses that cannot be performed on Martian samples. Historically, chemical extraction techniques have been extensively used to characterize amorphous materials in terrestrial soils, but most automated systems are complex, expensive, and limited to analyzing a single sample at one time. This study aims to develop a cost-effective apparatus that will allow latitude in choosing an extractant, process several samples simultaneously, enable rapid sampling over time without interruption and provide the resolution for quantitative differentiation of rapidly dissolving SiO2(a) phases in natural samples. Dissolution rates as a function of time were used as input for kinetic models to estimate the abundances of amorphous phases. When 2 M Na2CO3 is used as the extractant, dissolution rates differ significantly between secondary phases such as opal and primary glass phases. A stronger base, NaOH, is necessary for the complete dissolution of basaltic glass. Palagonitic tuffs from Iceland (proposed analogs of Martian soils) with >90% (w/w) amorphous composition were analyzed with 2 M Na2CO3 in the proposed apparatus, and both primary glass and secondary SiO2 appear to be present. Using the kinetic model of the dissolution, the palagonitic tuff has a composition of approximately 25% (w/w) of a rapidly reacting amorphous phase and 13% (w/w) of the slower reacting glass-like phase. The proposed high-efficiency analytical method can be applied to screen multiple terrestrial analogs and archive dissolution kinetics of many standard amorphous minerals. Although this paper focuses on extracting SiO2(a), the same setup can be applied to study time-based dissolution reactions using other extractants such as ammonium oxalate oxalic acid.