Goethitas na interface solo-rocha em amostras do Rio Grande do Sul e Minas Gerais

Goethites from 23 yellowish (6YR - 2.5Y) saprolite materials representing the soil-rock interface of migmatite, andesite, rhyolite, basalt, sandstone, argillite and limestone, were analysed. These materials are differentiated by weathering stage, as indicated by their silicate mineral composition: halloysitic (0.7 and 1.0 nm halloysite); kaolinitic; gibbsitic; and mixed (kaolinite, mica, vermiculite and/or smectite). Iron content varies from 16 to 354 g kg -1 Fed, consisting mainly of goethite, which was concentrated as much as 1.4 to 8.9 times with a 5 mol L -1 NaOH boiling treatment, thus enhancing the XRD characterization of goethite. The goethites contain 1 to 27 cmol mol -1 Al, where samples > 20 cmol mol -1 Al are associated to gibbsitic saprolites, and the lower ones to reductive environments or to the presence of smectite. Their WHH(110) and (111) ranges, respectively, from 0.32 to 0.87°2θ and from 0.28 to 0.74°2θ; calculated SSA show a broad range from 56 to 153 m 2 g -1 . A WHH(110) > (111), and the intensity ratio I(110)/(111) > 1.2 are suggestive of goethites with an anisotropic crystal morphology, whose calculated MCD values in the a, b, and c directions, range from 10-28 nm, 5-11 nm, and 11-30 nm, respectively. Dehydroxilation temperatures T(DTA) range from 257 to 313°C, with lower values (257-265°C) for goethites with WHH(110) > 0.72°2θ. The six samples from basaltic saprolites show significant relation between WHH(110) and Al substitution (r = -0.960), and of Al with T(DTA) (r = 0.985), indicating that Al substitution increases crystallinity (= coherent domains), and consequently the thermal stability of goethite. Different parent materials, and their degree of weathering, represent different environments, thus affecting goethite properties. Established goethite properties indicative of pedoenvironments were useful to identify the environment and weathering processes of saprolites, thus confirming the continuity ofpedogenesis at the soil-rock interface.