Lateritic Deep Weathering of Granite

Suddhiprakarn

Armitage

1980

Self Cite

Abstract--Laterite profiles developed from granite in southwestern Australia were studied by scanning electron microscopy. The morphology of soil materials reflects the mineralogy of secondary minerals formed from feldspar. In the saprolite, etched feldspar surfaces are coated with kaolinite or radiating, spherical aggregates of tubular halloysite. In the lower pallid zone these minerals have replaced most of the feldspar. In the upper pallid zone a porous framework has developed consisting mainly of quartz and gibbsite with 5-/xm euhedral gibbsite crystals in voids. Halloysite crystals in the upper pallid zone are partly unrolled and have splayed ends. Differences in mineralogy and morphology between profiles are thought to be due to variations in the intensity of leaching.

Section: Methodsmentioning

confidence: 99%

Scanning Electron Microscope Morphology of Deeply Weathered Granite

Suddhiprakarn

Armitage

1980

Self Cite

“…The profile is exposed in a railway cutting at Merredin, 265 km east of Perth, Western Australia. The profile exhibits a sequence of zones typical of in situ deeply-weathered lateritic profiles in the region (Gilkes et al, 1973). The sample was taken from the pallid zone, which is a product of extreme chemical weathering and leaching.…”

Section: Methodsmentioning

confidence: 99%

An Electron Optical Investigation of the Alteration of Kaolinite to Halloysite

Singh¹,

1992

103

Abstract--Parallel-oriented and exceptionally long (> 10 #m) tubes of halloysite occur in the pallid zone of a deeply-weathered lateritic profile on granite in southwest Australia.Transmission electron microscopy and selected-area electron diffraction of ultrathin sections showed that kaolinite plates within pseudomorphs of mica crystals had fractured at irregular intervals along the a crystallographic axis to produce laths elongated along the b axis. The laths near the edges of the pseudomorphs were less constrained by the pseudomorph and had roiled to produce halloysite tubes. The tubes varied in diameter and degree of roundness. Some tubes were polyhedral rather than cylindrical in cross section. The length and number of planar faces in a tube and the angle between faces varied, exhibiting no consistent pattern.Tubes in dispersed clay samples showed two types of twinning. In the first type, tubes and associated laths were joined together side by side. In the second type, single tubes bifurcated into two individual tubes. It is proposed that the first type of twinning occurred by folding of adjacent laths that remained joined together while the second type occurred due to exfoliation of a thick lath followed by folding of the exfoliated lath fragments into tubes.Analytical electron microscopy showed that the chemical compositions of halloysite tubes, laths, and kaolinite plates were similar with the average cation exchange capacity of single tubes being small (4.5 meq/100 g) but higher than values for laths (2.8 meq/100 g) and plates (1.9 meq/100 g).

“…Detailed profile descriptions and general weathering patterns for these and similar deeply weathered materials were given by Sadleir and Gilkes (1976) and Gilkes et al (1973), respectively. Quartz veins and dolerite dykes which occur as intrusions in the granitic rocks are preserved within the weathered profiles, demonstrating that the profiles were formed by in situ chemical weathering processes.…”

Section: Methodsmentioning

confidence: 99%

Feldspar Weathering in Lateritic Saprolite

Anand

Armitage

et al. 1985

Abstract--Feldspars in granitic saprolite in southwestern Australia have altered to halloysite, kaolinite, and gibbsite with no evidence of noncrystalline material. The secondary minerals are commonly present as intimate mixtures within altered feldspar grains, but discrete zones of gibbsite or halloysite-kaolinite also are present. Variations in the chemical microenvironment within micrometer-size zones in grains apparently controlled the type and distribution of secondary minerals. The alteration of both plagioclase and alkali feldspars involved congruent dissolution by soil solution and crystallization of halloysite, kaolinite, and gibbsite from this solution. Highly altered feldspar grains consist of etched feldspar fragments embedded within a highly porous framework of subhedral to euhedral platy crystals of kaolinite and gibbsite, or of spherical and felted aggregates of halloysite.