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Reactant-product textures, volume relations, and implications for major-element mobility during natural weathering of hornblende, Tallulah Falls Formation, Georgia Blue Ridge, U.S.A

Michael A. Velbel^*,, Angela R. Donatelle^* and Michael J. Formolo^**

^* Department of Geological Sciences, 206 Natural Science Building, Michigan State University, East Lansing, Michigan 48824-1115 U.S.A
^** Max-Planck-Institut für Marine Mikrobiologie, Celsiusstraβe 1, D-28359 Bremen, Germany

Corresponding author: velbel{at}msu.edu

Natural weathering of hornblende from the Tallulah Falls Formation (Georgia, U.S.A.) takes place under thoroughly leached, oxidizing conditions. At the earliest stage of weathering, corrosion of hornblende forms denticulated terminations, with only minimal porosity apparent within the partially pseudomorphic kaolin-group weathering product or between it and the reactant mineral. Isovolumetric replacement of hornblende by kaolin-group clay or precursor products implies inheritance of parent-mineral silica tetrahedra by the weathering products and largely immobile behavior of Si during early weathering. Aluminum, normally considered immobile or minimally mobile during weathering, must be imported into early-stage weathering products. More advanced weathering of hornblende forms denticulated terminations, separated from the earlier-formed products by a large, continuous peripheral void; porosity is much more conspicuous at this stage. Late-stage ferruginous products precipitate from Fe mobilized in solution. Dissolved Fe to form the late-stage goethite can be entirely derived from the parent hornblende, requiring only short transport distances for aqueous Fe between the source parent mineral and the location of product precipitation. Textural and volumetric relationships between naturally weathered hornblende and its partially pseudomorphic weathering products suggest structurally controlled immobility of much parent-mineral Si and considerable mobilization of Al at early stages of weathering, but at more advanced stages of weathering the textural and volumetric relationships are consistent with immobility of Al and Fe and leaching of substantial Si and all base cations.

Key Words: Weathering • Kinetics • Mafic-ultramafic • Amphibole • Hornblende