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Crystal-Chemical Changes in an Oxidative Weathering Front in a Georgia Kaolin Deposit

Published online by Cambridge University Press:  01 January 2024

Paul A. Schroeder ,
Robert J. Pruett  and
Nathan D. Melear
Paul A. Schroeder*
Affiliation:
Department of Geology, University of Georgia, Athens, GA 30602-2501, USA
Robert J. Pruett
Affiliation:
IMERYS, Sandersville, GA 31082, USA
Nathan D. Melear
Affiliation:
Department of Geology, University of Georgia, Athens, GA 30602-2501, USA
*
*E-mail address of corresponding author: schroe@uga.edu
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Abstract

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The Eocene Huber Formation, exposed in the CFI Hall mine south of Irwinton, in Wilkinson County, Georgia, displays colored zones that are a consequence of an oxidative weathering front. These zones vary from very light gray (gray) to pinkish white (pink) to very pale yellow (cream) (Munsell N8, 5YR 8/2 and 2.5Y 8/2, respectively) with increasing degree of oxidation. A representative sample from each zone was collected and analyzed for its chemical, crystallographic and Raman spectroscopic properties. The comparison of these genetically related samples allowed for a quantitatively accurate assessment of anatase’s contribution to the total TiO2 content. All samples contain ∼1.5 wt.% TiO2. Progressing from gray to pink to cream, anatase contents range from half to nearly all the TiO2. The a lattice parameter for anatase is constant in all three kaolins (3.7916 Å), suggesting a constant 4.6 mol.% isomorphous substitution of Fe for Ti.

Phase characterization and mass-balance considerations of the gray, pink and cream kaolins indicate that ilmenite and pyrite are precursor Ti- and Fe-bearing phases in the gray kaolin that undergo oxidation. Pseudorutile is a proposed intermediate phase resulting from the anodic corrosion of ilmenite. Hematite, goethite and anatase are the dominant end-products via dissolution-precipitation from the reactants pyrite and pseudorutile. The removal of Fe sulfides and organic matter and addition of hematite and goethite causes kaolin colors to change from gray to pink. Oxygen diffusing from groundwater in permeable overlying and underlying formations facilitates a process that probably involves aerobic and anaerobic bacteria that utilize Fe from pyrite, hematite and goethite. We postulate that the end result is the mobilization of Fe by siderophores and a kaolin color change from pink to cream.

Keywords

AnataseCrandalliteHematiteIlmeniteKaolinitePseudorutileRutileXRDRaman
Type
Research Article
Information
Clays and Clay Minerals , Volume 52 , Issue 2 , April 2004 , pp. 211 - 220
Copyright
Copyright © 2004, The Clay Minerals Society

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