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

Abstract

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 characteristics and formation in the northern Golan Heights

“…Halloysite ''nuclei'' grow into spheroidal or ellipsoidal forms, which join with each other along some planar surfaces. A similar process had been described by Jeong (1998) in weathering profiles from Korea. Larger aggregates, with shapes seen in Fig.…”

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

“…Their relative abundances were known to change within one weathering profile, or from one to another profile (Parham, 1969). Transformations between the two minerals after their early formation may change their relative abundances, as suggested by thermodynamic and kinetic simulation (Steefel and Van Cappellen, 1990), by microscopic observations (Churchman and Gilkes, 1989;Robertson and Eggleton, 1991;Singh and Gilkes, 1992;Singh and Mackinnon, 1996;Jeong, 1998a), and by structural modeling (Singh, 1996). The abundances of kaolin minerals in the relatively young weathering profile appear to be largely controlled by the different formation processes because transformations require time.…”

A new microenvironment for the formation of clay minerals: the example of authigenic halloysite-7Å and gibbsite in a stalactite from Agios Georgios Cave, Kilkis, north Greece

“…A reaction that describes the formation of halloysite from muscovite, which is illustrated in Figures 7 As it was mentioned above there is an uncertainty for the exclusive occurrence or coexistence of platy kaolinite in the sample, hence kaolinite may coexist with halloysite, formed also by the above reaction. These two minerals can also be structurally related to each other via solid-state transformations (Jeong, 1998;Papoulis et al, 2004;Churchman et al, 2010). Apart from low pH, kaolinite formation is generally favoured from halloysite at higher alumina and lower Speleology, 44 (3), 327-340.…”