Feldspar Weathering in Lateritic Saprolite

Abstract

Abstract--Feldspars in granitic saprolite in southwestern Australia have altered to halloysite, kaolinite, and gibbsite with no evidence of noncrystalline material. The secondary minerals are commonly present as intimate mixtures within altered feldspar grains, but discrete zones of gibbsite or halloysite-kaolinite also are present. Variations in the chemical microenvironment within micrometer-size zones in grains apparently controlled the type and distribution of secondary minerals. The alteration of both plagioclase and alkali feldspars involved congruent dissolution by soil solution and crystallization of halloysite, kaolinite, and gibbsite from this solution. Highly altered feldspar grains consist of etched feldspar fragments embedded within a highly porous framework of subhedral to euhedral platy crystals of kaolinite and gibbsite, or of spherical and felted aggregates of halloysite.

An Electron Optical investigation of Aluminosilicate Cements in Silcretes

“…The silica grains are anhedral, about 100/%, in diameter and are combined into rod-shaped particles. This occurrence of silica is consistent with many other studies of regolith in arid environments in Australia (e.g., Langford-Smith, 1978) and is in contrast to findings for more humid (> 800 m m rainfall) regions of Western Australia where similar granitic saprolites usually contain no secondary silica (Anand et al, 1985;Gilkes and Suddhiprakarn, 1979). The presence of secondary silica is considered to reflect a change from the leaching conditions that prevailed during laterization in the Tertiary to present-day semi-arid to arid regimes (Ollier, 1978;Butt, 1981Butt, , 1985.…”

“…The pseudornorphs are very porous and preserve only the external morphology of the parent feldspar grains. Because of the dissimilarity between the structures of the primary feldspar and the secondary kaolinite, topotactic or epitactic alteration is not possible and the kaolinite crystals simply grow (precipitate) in a random orientation from solution (Anand et al, 1985). In contrast pseudomorphs of mica grains are composed of parallel arrays of kaolinite plates, which may be as large as 10 tzm, that have formed by topotactic or epitactic replacement of the mica (Gilkes and Suddhiprakarn, 1979;Banfield and Eggleton, 1988;Singh and Gilkes, 1991).…”

Características dos óxidos de ferro e de alumínio de diferentes classes de solos

“…A pobreza do material que deu origem aos sedimentos em minerais ferromagnesianos (granito e gnaisse leuco e mesocrático), a abundância dessas rochas em feldspatos, a possibilidade de transformação direta desse mineral em Ct sob clima tropical (Grant, 1964;Anand et al, 1985), o alto grau de intemperismo sofrido antes da deposição e as condições úmidas dos sedimentos e dos Tabuleiros Costeiros favoreceram a concentração de Ct e a remoção de minerais, principalmente óxidos de ferro.…”

Mineralogia das frações areia, silte e argila de sedimentos do grupo barreiras no município de Aracruz, estado do Espírito Santo

“…O baixo teor de minerais ferromagnesianos no material, granito e gnaisse leuco e mesocrático, que deu origem aos sedimentos, a abundância dessas rochas em feldspatos e a possibilidade de transformação direta deste mineral em Ct sob clima tropical (Grant, 1964;Anand et al, 1985), o alto grau de intemperismo ocorrido antes da deposição e as condições úmidas dos Tabuleiros Costeiros favoreceram a concentração de Ct e a remoção de minerais, principalmente, óxidos de Fe (Melo et al 2001a). O teor de Ct na fração argila dos sedimentos atingiu 953 g kg -1 a 14 m de profundidade (Quadro 4).…”