Mineralogical and physical properties of lateritic pallid zone materials developed from granite and dolerite

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).

Section: Morphologymentioning

confidence: 93%

Section: Morphologymentioning

confidence: 99%

Section: Morphologymentioning

confidence: 99%

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An Electron Optical Investigation of the Alteration of Kaolinite to Halloysite

Singh¹,

1992

103

“…Such variability in kaolinite morphology is common and widespread in natural deposits. Similarly, the defect density and probably chemical composition of kaolinite may differ among occurrences and this may influence the surface chemical and physical properties of clay materials (Robson and Gilkes, 1981;McCrea et al, 1990).…”

Section: Introductionmentioning

confidence: 99%

The Relationships between Kaolinite Crystal Properties and the Origin of Materials for a Brazilian Kaolin Deposit

Varajão

Hart

2001

The clay particles in a kaolin deposit from Brazil were investigated by X-ray diffraction (XRD), differential thermal analysis (DTA), analytical transmission electron microscopy (ATEM), and electron paramagnetic resonance (EPR) to examine the relationships between morphological and chemical properties of the crystals and to relate these properties to formation conditions. The XRD patterns show the dominant presence of kaolinite with minor amounts of gibbsite, illite, quartz, goethite, hematite, and anatase. ATEM observations show two discontinuities in the deposit as indicated by changes in morphology and size of the kaolinite crystals. At the base of the deposit, hexagonal platy and lath-shaped particles (mean area of 001 face = 0.26 p,m 2) maintain the original fabric of the parent rock which characterizes an in situ evolution. In the middle of the deposit a bimodal population of large (mean area of 001 face > 0.05 ixm:) and small (mean area of 001 face < 0.05 p.m 2) sub-hexagonal platy kaolinite crystals occurs. This zone defines the boundary between the saprolitic kaolinite and the pedogenic kaolinite. Near the top of the profile, laths and irregular plates of kaolinite, together with sub-hexagonal particles, define two different depositional sources in the history of formation of the deposit. Crystal thickness as derived from the width of basal reflections and the Hinckley index are compatible with the morphological results, but show only one discontinuity. At the base of the deposit, kaolinite has a lowdefect density whereas in the middle and at the top of the profile, kaolinite has a high-defect density. Likewise, EPR spectroscopy shows typical spectra of low-defect kaolinite for the bottom of the deposit and typical spectra of high-defect kaolinite for the other portions of the deposit. Despite the morphological changes observed through the profile, the elemental composition of individual kaolinite crystals did not show systematic variations. These results are consistent with the deposit consisting of a transported pedogenic kaolinite over saprolite consisting of in situ kaolinized phyllite.

“…XRD of the bulk material and oriented clay samples showed that quartz and kaolinite are the major crystalline components of the saprolite, which is consistent with the mineralogy ofsaprolites on felsic rocks in the region. The pseudomorphs of the feldspars are commonly isotropic, either because they consist of randomly oriented arrays of kaolinite crystals (McCrea et al, 1990) or, more probably, because they are partly infused by amorphous material. This material is intimately associated with isotropic aluminosilicate cement in adjacent veinlets and is assumed to be similar in composition.…”

Section: Microfabric and Composition Of Saprolitementioning

confidence: 99%

An Electron Optical investigation of Aluminosilicate Cements in Silcretes

Singh

Butt

1992

Abstract--Silcretes developed within the in situ regolith in the Barr Smith Range, Western Australia, were investigated using optical and electron-beam techniques. One of the cementing agents in these silcretes showed geMike optical properties and had a variable aluminosilicate chemical composition at the scale of electron microprobe analysis so that it might he considered as allophane-like material High resolution transmission electron microscopy demonstrated that the material consists of a fine-grained and poorly ordered kaolinite embedded in a matrix of amorphous silica.