The weathering of almandine garnet in the oxidized, vadose zone of saprolite near Otto, North Carolina, begins at grain boundaries and along fractures traversing the garnet grains. Radially oriented fibrous intergrowths of gibbsite and goethite form layers of uniform thickness, which grow by centripetal replacement as weathering proceeds. The contact between the garnet surface and the layer of weathering products is sharp and smooth, and garnet corners are rounded. Large, well-defined etch pits are absent on the underlying garnet surface. These observations suggest that diffusion (transport) of reactants and/or products through the gibbsite-goethite layer is the rate-limiting step in the weathering of almandine garnet in the oxidizing environment of the saprolite.
In soils overlying the saprolite, garnet surfaces are directly exposed to weathering solutions without the intervening surface layer. Such “unprotected” grains in soils (and stream sediments) exhibit numerous large, well-developed etch pits. Surface-reaction control, rather than transport control, prevails during weathering of almandine garnet in the absence of protective surface layers of weathering products. Biochemical or biological processes in the soil apparently prevent the gibbsite-goethite layer from forming or persisting. Soil solutions can therefore react directly with the garnet surface, rather than requiring reactants or products to diffuse through the gibbsite-goethite coating as in the saprolite. The mechanism, and rate-limiting step, of almandine garnet weathering is apparently strongly dependent on the chemical environment in which weathering occurs.