Biotite Alteration in Deeply Weathered Granite. II. The Oriented Growth of Secondary Minerals

Abstract

Abstract--Single grain X-ray and electron diffraction patterns of weathered biotite flakes exhibit groupings of 001 and hk reflections of biotite, vermiculite, mixed-layer clay minerals, and kaolinite indicating that the secondary minerals are in parallel crystallographic orientation to the parent biotite. Asterism of biotite reflections is enhanced by weathering. Gibbsite crystals developed in parallel basal orientation to biotite flakes. Most goethite in weathered biotite occurs as aggregates of randomly oriented crystals in cleavages and on grain surfaces. Some goethite is present on micaceous fragments as 0.05-/~m size, lathlike crystals in a hexagonal arrangement with their (100) face resting on the (001) biotite face. Selected area electron diffraction patterns of aggregates of lathlike goethite crystals contain 0k~, lk~, and 2k( reflections due to undulation of the aggregates and the extreme thinness of the crystals. These patterns indicate that the close packed anion layers in goethite coincide with the brucite-like layer of the micaceous minerals.

An Electron Optical Investigation of the Alteration of Kaolinite to Halloysite

“…(Keller, 1978). Many studies of the weathering of biotite and muscovite have reported the oriented growth of kaolinite to form pseudomorphs (Gilkes and Suddhiprakarn, 1979;Ahn and Peacor, 1987;Eggleton, 1988, 1990). The bundles of tubes ( Figure 4A) show clear boundaries and appear to be more densely packed than halloysite masses that are considered to have crystallized from solution (Keller, 1978;Banfield and Eggleton, 1990).…”

“…Halloysite tubes crystallized from solution can also achieve some degree of parallel orientation (Keller, 1978), but they mostly radiate outwards from the substrate and are less densely packed. Micas exhibit a perfect cleavage along (001) and commonly exfoliate along this plane in the initial stages of weathering thereby creating an array of parallel slits (Gilkes and Suddhiprakarn, 1979). Such parallel slits provide an environment that would generate a planar (two dimensional) mat of halloysite tubes but would not lead to a high degree of orientation of tubes in three dimensions.…”

“…Consequently, topotaxial or epitaxial alteration is not possible and there is no consistent orientation of the secondary mineral relative to the parent mineral (Gilkes el al., 1986). The structure and composition of micas on the other hand are closely related to these properties of kaolin-group minerals and oriented growth of secondary kaolinite with crystallographic axes parallel to those of the parent mica is common (i.e., topotaxial alteration) (Gilkes and Suddhiprakarn, 1979;Ahn and Peacor, 1987;Banfield and Eggleton, 1988). However, the alteration of platy mica to tubular halloysite cannot occur by simple topotactic alteration.…”

Weathering of a Chromian Muscovite to Kaolinite

“…There is also a very weak ring pattern for kaolinite in Figure 1B, which indicates that although the majority of the kaolinite crystals are in parallel orientation, some kaolinite crystals, or possibly halloysite crystals, are distributed at random orientations about the c'axis. A similar, highly-oriented growth of secondary clay minerals in weathered biotite extending from micrometer to millimeter scale was observed by Gilkes and Suddhiprakarn (1979). Oriented secondary minerals develop if the parent and secondary minerals contain similar structural units (groups, chains, sheets), and the parent mineral contains adequate amounts of elements of a residual nature (Gilkes et al, 1986).…”

“…Weathering of mica in soils has been investigated by many workers, with most research being directed towards biotite (Meunier and Velde, 1979;Gilkes and Suddhiprakarn, 1979;Ahn and Peacor, 1987;Banfield and Eggleton, 1988). In contrast, weathering of muscovite, which is a common constituent of soils, has received relatively little attention (Banfield and Eggleton, 1990).…”