Scan Electron Micrographs of Kaolins Collected from Diverse Environments of Origin—IV. Georgia Kaolin and Kaolinizing Source Rocks

Self Cite

Abstract--"Kaolinites" from classic, large deposits of kaolin are shown commonly in scanning electron micrographs to be mixtures, at least in part, in microdimensions, of kaolin-mineral polymorphs.Artificial mixtures of selected kaolin polymorphs simulating the natural mixtures, also micrographed, show crystallinities intermediate between the crystallinities of the end members in X-ray powder diffractograms. Thus, apparent crystallinity interpreted from diffractograms of a kaolin specimen may be a product of kaolin-mineral mixture in microdimensions as well as from ordering in the crystals. Evaluation of the crystallinity of a kaolin from powder diffraction may be suspect if independent means, such as SEM, are not used to assess the monomineralic character of that kaolin specimen.Analogous to the apparent crystallinity of a "kaolinite" being the product of a mixture, so may other widely ranging properties of"kaolinite" be products of kaolin-mineral mixtures in microdimensions. These properties include DTA, IR, chemical composition, free energies of formation, and industrial applications.Specifications for a monomineralic, nearly ideal kaolinite are considered the Keokuk geode variety possesses desired crystallographic and chemical properties.

Section: Observations From Scanning Electron Micrographsmentioning

confidence: 99%

Effects of Micro-Sized Mixtures of Kaolin Minerals on Properties of Kaolinites

Keller

1978

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“…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%

“…A genetic relation between the 2 minerals in the weathering environment is rarely established, despite many reports describing their coexistence. Thermodynamic studies predict that halloysites would convert into kaolinite with aging (Huang 1974;Tsuzuki and Kawabe 1983), but while this sequence is currently accepted by most investigators (Parham 1969;Keller 1977;Nagasawa 1978;Churchman and Gilkes 1989;Steefel and Van Cappellen 1990), there is little direct microtextural evidence to support it. Banfield (1985), from SEM observations, suggested that halloysite tubes merged into platy kaolinite.…”

Section: Introductionmentioning

confidence: 99%

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.