Content and composition of inorganic X-ray amorphous materials in temperate climate soils determined by the Rietveld method

Abstract

Inorganic X-ray amorphous materials (iXAMs) are reactive soil components, including nanocrystalline minerals and mineraloids, often overlooked by mineralogical analyses. We evaluated their abundance and composition in temperate climate soils (Cambisol, Podzol, Chernozem, and Stagnosol) using Rietveld analysis of powder X-ray diffraction data and X-ray fluorescence spectrometry. Additionally, we explored iXAM extractability using wet-chemical methods, assessed selected clay fractions (< 0.2 µm) by transmission electron microscopy, and analyzed specific surface area and nanoporosity by N 2 gas adsorption. Inorganic XAMs made up 8–16 wt% in bulk soils and up to 54 wt% in clay fractions. These values align with those from tropical and (sub)arctic soils. Inorganic XAMs were enriched in horizons affected by illuviation processes and oxidation after waterlogging. Chemically, iXAMs contained Al, Fe, Si, and loss on ignition (LOI); Si decreased with decreasing particle size, whereas metals and LOI increased, indicating a more diverse iXAM composition in finer fractions. Wet-chemical extractions of iXAMs were incomplete. Co-extraction of Al and Si by Tiron and NaOH indicated that poorly ordered clay minerals constitute part of iXAMs. Amorphous silica explained one-third of iXAM-Si in bulk soils. Inorganic XAMs were negatively correlated to feldspar content, and positively correlated to phyllosilicate clay content, indicating a role as phyllosilicate precursors precipitated from weathering solutions. Together with phyllosilicate clays, iXAMs contributed significantly to soil specific surface area and nanoporosity, with poorly crystalline Fe (oxyhydr)oxides identified as key influencers. In sum, iXAMs are quantitatively important soil components, which should be considered in the assessment of physicochemical soil properties and functions.