Metastability in Near-Surface Rocks of Minerals in The System Al 2 O 3 -SiO 2 -H 2 O

Abstract--Formation processes of weathering products of pumice collected from a rhyolitic pyroclastic flow deposit were investigated by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy dispersive X-ray analysis (EDX). SEM clearly showed the presence of some weathering products adhering to the surface of pumice. XRD showed that the products were composed mainly of noncrystalline materials with a relatively small amount of halloysite (I0 ~k). TEM and EDX revealed texture transformation sequences from the earliest weathering product to a final product of halloysite as follows: 1) precipitation of very thin flaky or film-like noncrystalline Al-hydroxide; 2) transformation into irregularly aggregated A1-Fe-Si-rich fibrous material; 3) morphological changes to rounded aggregates; 4) development of halloysite-tike curled domains with successive decrease in Fe content; and 5) further development of curled domains in rounded aggregates. These materials must be metastable phases at early reaction stages with kinetics limiting formation of the stable equilibrium phase of kaolinite.

Section: Discussionmentioning

confidence: 94%

Analytical Electron Microscopic Study of the Noncrystalline Products Formed at Early Weathering Stages of Volcanic Glass

Kawano

Tomita

Shinohara³

1997

“…In thermodynamic terms, the Gibbs free energy of formation of halloysite is slightly higher than that of kaolinite (Robie and Waldbaum, 1968;Robie et al, 1978;Anovitz, et al, 1991), so halloysite is metastable with respect to kaolinite. In low-temperature environments, metastable phases commonly precipitate in advance of stable minerals, because of the lower activation energy of nucleation of the former (Morse and Casey, 1988;Steefel and Van Cappellen, 1990;Stumm, 1992).…”

Section: Discussionmentioning

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.

“…Conversion of halloysite to kaolinite is consistent with thermodynamic considerations. The Gibbs free energy of formation of halloysite is about 4 kcaYmol higher than that of kaolinite (Robie and Waldbaum 1968;Robie et al 1978;Anovitz et al 1991). Therefore, halloysite is metastable compared to coexisting kaolinite.…”

Section: Bulk Chemistry and Mineral Composition Of Weathered Anorthosmentioning

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.