『Abstract
Weathering of chlorite adjacent to macro- and micro-passages
(fissures) in regolith units has been studied to determine the
alteration mechanisms and products and compare them to those that
prevail at a distance from the open passages. The micro-fissures
in a the saprolith units evolve as a function of weathering with
an increase in micro-fissure density and their infilling with
ferriargillans corresponding to an increase in weathering intensity.
In the saprock and saprolite units, goethite-rich bands with surrounding
reddish haloes invade the chlorite adjoining the micro-fissures.
In the reddish haloes surrounding the goethite bands and fissures,
the phyllosilicates alter to an intimate mixture of goethite,
kaolinite, feroxyhyte and halloysite with the possible nanoscale
presence of other fine-grained iron aluminosilicates such as hisingerite
and smectite. In the fine saprolite, the weathering products after
original chlorite adjoining the micro-fissures bearing ferriargillans,
alter directly to ferriargillan products - kaolinite and goethite.
The lack of orientation of the products with the parent phyllosilicates
indicates the operation of a dissolution-precipitation mechanism
which is in contrast to the topotactic alteration mechanism functioning
at a distance within the phyllosilicate grain assemblages. The
differences in alteration mechanisms and products of chlorite
weathering in different microsites suggest the rate of weathering
of chlorite can differ in microsites within individual regolith
units. The presence of fine-grained metastable products in the
form of feroxyhyte and halloysite adjoining the fissures suggests
an Ostwald Step Rule sequence during alteration of phyllosilicates
with rapid oxidation of Fe and the presence of Si in the microsite
considered the main factor favoring fine-grained metastable product.
Key Words: Chlorite; Feroxyhyte; Goethite; Hisingerite; Kaolinite;
Transmission; Electron microscopy; Weathering.』
Introduction
Experimental methods
Samples
Methods
Results and interpretation
Saprock
TEM results
Coarse saprolite
Petrographic observations
TEM results
Fine saprolite
Petrographic observations
TEM results
Discussion
Alteration sequence and mechanisms
Significance of metastable phases
Comparisons between two microsites - spatial differences in pathways
and products
Conclusions
Acknowledgments
References