Evaluation of the Rietveld method for determining content and chemical composition of inorganic X-ray amorphous materials in soils

Inorganic X-ray amorphous materials (iXAMs) such as vitreous phases, minerals having an insufficient number of repeating structural units to diffract X-rays, and inorganic solids with exclusively structural short-range order are ubiquitous in soils and are relevant for numerous environmental processes but are notoriously difficult to identify and quantify. To test for the quantification and chemical composition of iXAMs in soil, we prepared four mineral mixtures containing quartz, calcite, feldspars, and clay minerals in different proportions typical of soils and amended them with 10-70 wt% iXAMs in the form of a 1:1 weight mixture of ferrihydrite and opal-A. We quantified these iXAMs in mineral mixtures by analyzing powder X-ray diffraction (PXRD) data using the Rietveld method and compared the results for different sample preparation techniques (conventional and spray drying) based on the internal standard method in Rietveld analysis. The mineral mixtures were also analyzed for their chemical composition by X-ray fluorescence (XRF) spectrometry, and mass-balance calculations combining Rietveld and XRF data were carried out to estimate the chemical composition of iXAMs in mineral mixtures. Both sample preparation methods showed no significant difference in determined iXAM contents and yielded accurate results for iXAM contents within +/- 3 wt% at the 95% confidence level (2 sigma). The relative accuracy deteriorated with decreasing iXAM content but remained below 10% for iXAM contents >10 wt% (mean = 3%). The precision of iXAM content quantification in mineral mixtures prepared by spray drying was slightly better, though statistically equivalent to the conventionally prepared mixtures (2 sigma = 1.49 and 1.61 wt%). The average precision of both sample preparation methods was +/- 2 wt% at the 95% confidence level. Levels of detection and quantification of iXAMs in spray-dried mineral mixtures containing 1-10 wt% iXAMs were estimated at 0.8 and 4.0 wt%, respectively. The chemical composition of iXAMs in terms of major oxides was accurately assessed by mass-balance calculations with average relative errors for nominal SiO2 and Fe2O3 contents of 9.4 and 4.3%, respectively (range = 0.02-54.7%). Even though adsorbed H2O and structural H2O/OH- as quantified by the loss on ignition comprised an important portion of the iXAMs (15.3 wt%), their LOI in mineral mixtures as derived from mass-balance calculations could only be quantified with an average relative error of 67.2% (range = 1.30-371%). We conclude that iXAMs in soil and related geomaterials present at levels >4 wt% can be quantified by Rietveld analysis of PXRD data with an accuracy of +/- 3 wt% at best. Combined results of Rietveld and XRF analyses can yield accurate results for the chemical composition of iXAMs within a relative error of 10% for major oxides, provided iXAM contents exceed 10 wt%, and the content and chemical composition of all crystalline mineral phases are accurately assessed. The results presented in this study lay the foundation to explore iXAM contents and chemical compositions in soils and to examine their impact on soil physicochemical properties and biogeochemical element cycles.