『Abstract
Halloysite and kaolinite occur in gneissic weathering profiles
developed under a temperate climate in the Leucogia area of NE
Greece. X-ray diffraction analyses of samples from three profiles
of partially weathered gneisses have revealed that kaolinite becomes
more abundant than halloysite with increasing intensity of the
decomposition processes. To resolve the genetic relationship between
halloysite and kaolinite, microtextures were examined by scanning
electron microscopy using polished thin-sections and freshly exposed
fracture surfaces, and their chemical compositions were determined
using an analytical scanning electron microscope fitted with energy
dispersive and wavelength dispersive spectrometers. Different
morphological forms of halloysite and kaolinite were detected
within the same rock mass. In the earliest stage of weathering,
spheroidal aggregates consisting of microcrystalline halloysite
are formed on the plagioclase surface. With progressive weathering,
spheroidal halloysite converts to tubular halloysite. As weathering
advances, tubular halloysite converts to platy halloysite, which
in turn converts to kaolinite. Halloysite and kaolinite may coexist
in the upper parts of the profiles. Electron microprobe analyses
of spheroidal, tubular and platy halloysite and kaolinite show
that the chemical composition of the various forms indicate a
progressive Fe enrichment and Al depletion with advancing kaolinization,
from VIFe0.08VIAl3.70 a.p.f.u. in spheroidal halloysite through
VIFe0.29VIAl3.50
in platy halloysite to VIFe0.53VIAl3.32 in newly formed kaolinite. Final-stage, book-type
kaolinite approaches its ideal chemical composition (VIFe0.04VIAl3.95)
. The mineralogy and composition of the kaolin minerals and the
development of the various morphological forms of halloysite and
kaolinite in different parts of the profiles are attributed to
the chemistry of the ambient solutions. However, other physical
and microenvironmental conditions (e.g. time and space available)
also seem to have strong influence on the precipitation processes
producing the morphological variations.
Key words: Gneiss; Greece; Halloysite; Kaolinite; Leucogia; Microtextures;
Plagioclase; Weathering』
Introduction
Experimental methods
Mineralogy and bulk-rock chemistry
Microtextures and kaolinization processes
First stage
Second stage: coalescence of halloysite spheres and formation
of tubular halloysite
Third stage
Fourth stage: formation of book-type kaolinite
Chemical composition of the various halloysite and kaolinite morphological
forms
Discussion
Conclusions
Acknowledgments
References