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Weathering of Almandine Garnet: Influence of Secondary Minerals on the Rate-Determining Step, and Implications for Regolith-Scale Al Mobilization

Published online by Cambridge University Press:  01 January 2024

Jason R. Price*
Affiliation:
Department of Earth Sciences, P.O. Box 1002, Millersville University, Millersville, PA 17551-0302, USA
Debra S. Bryan-Ricketts
Affiliation:
Department of Geological Sciences, 206 Natural Science Building, Michigan State University, East Lansing, Michigan 48824-1115, USA Environmental Protection Division, Los Alamos National Laboratory, Box 1663, Los Alamos, NM 87544, USA
Diane Anderson
Affiliation:
Department of Earth Sciences, P.O. Box 1002, Millersville University, Millersville, PA 17551-0302, USA Celerity, 4720 Gettysburg Rd., Suite 204, Mechanicsburg, PA 17055, USA
Michael A. Velbel
Affiliation:
Department of Geological Sciences, 206 Natural Science Building, Michigan State University, East Lansing, Michigan 48824-1115, USA
*
*E-mail address of corresponding author: Jason.Price@millersville.edu

Abstract

Secondary surface layers form by replacement of almandine garnet during chemical weathering. This study tested the hypothesis that the kinetic role of almandine’s weathering products, and the consequent relationships of primary-mineral surface texture and specific assemblages of secondary minerals, both vary with the solid-solution-controlled variations in Fe and Al contents of the specific almandine experiencing weathering.

Surface layers are protective (PSL) when the volume of the products formed by replacement is greater than or equal to the volume of the reactants replaced. Under such circumstances, reaction kinetics at the interface between the garnet and the replacing mineral are transport controlled and either transport of solvents or other reactants to, or products from, the dissolving mineral is rate limiting. Beneath PSLs, almandine garnet surfaces are smooth, rounded, and featureless. Surface layers are unprotective (USL) when the volume of the products formed by replacement is less than the volume of the reactants replaced. Under such circumstances, reaction kinetics at the interface between the garnet and the replacing mineral are interface controlled and the detachment of ions or molecules from the mineral surface is rate limiting. Almandine garnet surfaces beneath USLs exhibit crystallographically oriented etch pits. However, contrary to expectations, etch pits occur on almandine garnet grains beneath some layers consisting of mineral assemblages consistent with PSLs.

Based on the Pilling-Bedworth criterion, surface layers are more likely to be protective over a broad range of reactant-mineral compositions when they contain goethite, kaolinite, and pyrolusite. However, this combination requires specific ranges of Fe and Al content of the natural reacting almandine garnet. To form a PSL of goethite and kaolinite, an almandine garnet must have a minimum Al stoichiometric coefficient of ~3.75 a.p.f.u., and a minimum Fe stoichiometric coefficient of ~2.7 a.p.f.u.

Product minerals also influence the mobility of the least-mobile major rock-forming elements. A PSL consisting of goethite, gibbsite, and kaolinite yields excess Al for export during almandine garnet weathering. As the quantity of kaolinite present in the PSL decreases, the amounts of Al available for export increases.

Type
Research Article
Copyright
Copyright © The Clay Minerals Society 2013

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