Scanning Electron Microscope Morphology of Deeply Weathered Granite

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.

Section: Morphology and Mineralogy Of Alteration Productsmentioning

confidence: 93%

Feldspar Weathering in Lateritic Saprolite

Anand

Gilkes

Armitage

et al. 1985

“…Previous electron microscopic studies frequently reported the coexistence of both minerals in weathering profiles (Keller 1977;Gilkes et al 1980;Keller et al 1980;Banfield 1985;Banfield and Eggleton 1990). Their coexistence is of interest because they have similar chemistry and structure.…”

Section: Introductionmentioning

confidence: 97%

Formation of Vermicular Kaolinite from Halloysite Aggregates in the Weathering of Plagioclase

Jeong

1998

Abstract--Halloysite and kaolinite coexist in anorthosite weathering profiles in the Sancheong district, Korea. X-ray diffraction (XRD) analysis on a transect of partially weathered anorthosites reveals an increasing amount of kaolinite development with weathering age. Microtextnres were examined by scanning electron microscopy (SEM) of thin sections and raw samples in an attempt to resolve the genetic relation between halloysite and kaolinite. In the earliest stage of weathering, halloysite ellipsoids and short tubes form on the plagioclase surfaces. The successive formation of new grains from the early grains results in globular aggregates where hailoysite grains are partially interconnected. With the further progress of weathering, the hatloysite grains coalesce from the inner part of the aggregate outward, and convert into stacked kaolinite plates. Continuous addition of halloysite grains on existing plates and their subsequent conversion into plates form long vermicular kaolinite. Solid-state transformation is suggested as a major conversion mechanism. Concurrently with its successive transformation into kaolinite, hatloysite also continuously precipitates, giving rise to overgrowths on vermicular kaolinite. Halloysite forms as a kinetically favored metastable precursor and is subsequently transformed into thermodynamically stable kaolinite. Halloysite and kaolinite coexist temporarily in the weathering profile far from equilibrium.

“…Halloysite and kaolinite are the most common kaolin minerals in these weathering environments (Keller, 1977;Gilkes et al, 1980;Keller et aL, 1980;Banfield and Eggleton, 1990;Schroeder et al, 1997). The common coexistence of these two minerals in the weathering profile attracts much interest because of their chemical and structural similarity.…”

Section: Introductionmentioning

confidence: 99%

The Dependence of Localized Crystallization of Halloysite and Kaolinite on Primary Minerals in the Weathering Profile of Granite

Jeong

2000

Abstract~The formation of kaolin-group minerals in the weathering profile of granite, under the humid, temperate climate as found in Korea, was studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), and electron microprobe analysis (EMA). The granite was gradually weathered to saprolite. K-rich feldspar was not weathered in the profile, but plagioclase partially weathered to halloysite septa (i.e., wail-like masses). At the bottom of the profile, biotite had weathered to regularly interstratified biotite-vermiculite (B-V), and subsequently to kaolinite, with a considerable increase in grain volume. In the upper part of the profile, loose aggregates of transported clays, including halloysite and kaolinite, coated the preformed halloysite septa in the weathered plagioclase. Halloysite had precipitated as a metastable phase in the microfissures of partially weathered plagioclase. Kaolinite had precipitated heavily in the weathered biotite, where surfaces supply abundant templates facilitating the nucleation of kaolinite. The localized crystallization of halloysite and kaolinite, depending on the distribution of primary minerals, strongly influenced the kaolin mineralogy of the granite weathering profile.