Abstract

Hydrous 2:1 and 1:1 layered phyllosilicates offer multiple structure sites that can uptake/release nutrients under different biogeochemical conditions. We examined the clay fraction (<2 ƒÊm) of Ultisols from hardwood, pine, and cultivated sites in the Calhoun Critical Zone Observatory, located on Piedmont of South Carolina, United States. Shallow (< 1m) and deep (>5m) samples were examined with X]ray diffraction (XRD) as oriented mounts in Mgand K]saturated (Mg]sat and K]sat) states in both ethylene glycol (EG) and heated (110‹C) conditions. Calculated XRD patterns were forward modeled using NEWMOD2. Model results can attribute the variable low angle XRD responses to a combination of differences in a) preferred orientation (i.e., ..* as defined by Reynolds, 1986) and b) the presence of mixed layer clay structures (layer types and abundances) (Reynolds, 1980). In general, K]sat/EG has the effect of increasing low]angle scattering (<5‹ 2..) relative to Mg]sat/EG, while 110‹C has the effect of increasing scattering of both K] and Mg]sat relative to EG. The increase in lowangle scattering can be ascribed to commensurate increases ..*. Scattering differences in the range of 5 to 15 ‹2.. can be ascribed to changes in 1) layer dimensions, 2) relative number of layer types, and 3) distribution of coherent scattering domains. Layer types used in NEWMOD2 included: kaolinite (K), illite]like (I), vermiculite (V), and smectite (S). Interlayer cation and octahedral iron abundances for 2:1 structures were adjusted based on independent electron microprobe observations of large grains.

Mixed layers of varied proportions where exhibited in all samples. For example, the deep pine site progressively had in the Mg]sat/EG (IS73, IV74, IV64, KI90, KI50); K]sat/EG (IV85, IV72, KI90, KI34); Mg]sat/110‹ (IS68, IV79, IV64, KI96, KI47); K]sat/110‹ (IV85, IV72, KI34, KI90). XRD patterns for shallow samples exhibited less variability across all treatments because they contain lower abundances of vermiculite layers (smectite layers are not detected). Subtle changes in the intensity and peak position of the kaolinite (001) peak exhibited in the K]sat/110‹ pattern suggests that there are multiple combinations of mixed layered phases, possibly including three component mixed layered structures but are not able to be modeled using NEWMOD2 two layer only algorithms.

The demonstrable presence of mixed layer structures and their variability in response to cation saturations, hydration states, and location within the weathering profile suggests that these minerals can play an important role in the cycling of base cations in the critical zone. The added observation that there was a change in K]bearing layer types progressing from cultivated to pine forested to hardwood forested locations hints that these mixed layer phases play an important role in the uplift and cycling of K in vegetative ecosystems.

Citation

Schroeder, Paul A., Daniel D. Richter, and Jason C. Austin (2019): Quantification of mixed‐layer clays in multiple saturation states using NEWMOD2: Implications for the potassium uplift hypothesis. EuroClay 2019, Paris, France, 1-5 July 2019.

This Paper/Book acknowledges NSF CZO grant support.