An Electron Optical Investigation of the Alteration of Kaolinite to Halloysite

Abstract

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).

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

“…These authors describe bundles of laths originating from a parallel fracturing of a larger plate at regular intervals. These parallel kaolinite laths may be an early stage of formation of halloysite tubes by rolling or folding (Brindley and Comer, 1956;Singh and Gilkes, 1992b). In the present work, we did not observe this last stage either because it requires an appropriate environment or because the laths are unrolled tubes.…”

“…Both are typical morphologies of pseudomorphous kaolinite occurring in weathering profiles developed by in situ evolution. Laths are described by Singh and Gilkes (1992b) as a common morphological feature of kaolinite pseudomorphs after mica. These authors describe bundles of laths originating from a parallel fracturing of a larger plate at regular intervals.…”

Kaolinite to Halloysite-7 Å Transformation in the Kaolin Deposit of São Vicente De Pereira, Portugal

“…Topotactic or epitactic alteration of mica or feldspar to halloysite has not previously been identified in nature (Gilkes et aL, 1986;Singh and Gilkes, 1992), whereas kaolinite formed from mica by topotactic alteration can subsequently produce halloysite tubes . Nevertheless, halloysite may also be formed through alteration of feldspar via solution or a non-crystalline intermediate stage (Eswaran and Bin, 1978;Singh and Gilkes, 1992). Robertson and Eggleton (1991) explained the transformation of platy kaolinite into spiral halloysite rods by a loss of structural rigidity at points along the kaolinite crystal due to hydration of kaolinite.…”

“…Robertson and Eggleton (1991) explained the transformation of platy kaolinite into spiral halloysite rods by a loss of structural rigidity at points along the kaolinite crystal due to hydration of kaolinite. Also, Singh and Gilkes (1992) showed a development of parallel halloysite tubes and laths through deformation of platy kaolinite pseudomorphs after mica. These authors described the tranformation of kaolinite plates by fragmentation into laths that rolled or folded to form halloysite tubes.…”

“…It may form from feldspars and/or micas by weathering or hydrothermal processes or it may crystallize direct from solution (Stoch and Sikora, 1976;Keller, 1978;Wilke et al, 1978;Meunier and Velde, 1979;Anand et al, 1985;Banfield and Eggleton, 1990;Robertson and Eggleton, 1991;Jiang and Peacor, 1991;Singh and Gilkes, 1992). Topotactic or epitactic alteration of mica or feldspar to halloysite has not previously been identified in nature (Gilkes et aL, 1986;Singh and Gilkes, 1992), whereas kaolinite formed from mica by topotactic alteration can subsequently produce halloysite tubes . Nevertheless, halloysite may also be formed through alteration of feldspar via solution or a non-crystalline intermediate stage (Eswaran and Bin, 1978;Singh and Gilkes, 1992).…”