A Study of a Deep Weathering Profile on Granite in Peninsular Malaysia: III. Alteration of Feldspars

Abstract--Alteration products of andesite cobbles from wet soils formed in volcanic colluvial material were studied using petrographic, electron microscope, X-ray powder diffraction, and thermal techniques. Augite phenocrysts altered by congruent dissolution leaving voids which were subsequently filled with smectite. Plagioclase also altered to produce micrometer-size spheroidal aggregates of smectite. Halloysite was not observed within the altered cobbles, although it was abundant in the soil matrix. The formation of smectite in the altered cobbles was probably favored by the restrictive drainage of the microenvironment in combination with wet soil conditions.

Section: Resultscontrasting

confidence: 52%

Alteration of Andesite in Wet, Unstable Soils of Oregon's Western Cascades

Glasmann

1982

“…They found that the abundance of secondary minerals within the grains increased towards the surface of the soil although grains at different stages of alteration were present in each horizon. The mineralogy of these weathering products is similar to that of the altered biotite described by Eswaran and Heng (1976) and Eswaran and Bin (1978), but little information is available in the literature on the mechanism of these transformations.…”

Section: Introductionmentioning

confidence: 90%

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

Gilkes

1979

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.

“…Eswaran and co-workers (Eswaran and Heng, 1976;Eswaran and Bin, 1978) described pseudomorphs of biotite from tropical soils which consisted of mixtures of mixed-layer minerals, tubular halloysite, kaolinite, and goethite. In the present work, however, tubular halloysite was not found to be a weathering product of biotite despite its presence as the major weathering product of associated feldspars.…”

Section: Morphological Propertiesmentioning

confidence: 99%

Biotite Alteration in Deeply Weathered Granite. I. Morphological, Mineralogical, and Chemical Properties

Gilkes

Suddhiprakarn

1979

Abstract--Biotite in deeply weathered granitic rocks in southwestern Australia has altered to exfoliated grains composed of biotite, mixed-layer clay minerals, kaolinite, vermiculite, gibbsite, goethite, and hematite. Discrete vermiculite and vermiculite-dominant mixed-layer clay minerals are not major weathering products. Oxidation of octahedrai iron in biotite is associated with ejection of octahedrai cations, loss of interlayer K, and a contraction of the b-dimension of the biotite sheet. Si, Mg, Ca, Mn, K, and Na are lost from biotite during weathering, and Ti, Al, Ni, and Cr are retained. Fe and water have been added to the grains during weathering. Much Fe occurs as aggregates of microcystailine, aluminum-rich goethite particles on flake surfaces and within etchpits, with smaller amounts occurring as hexagonal arrangements of lath-shaped crystals of goethite on flake surfaces.