『Abstract
The weathering of chlorite in hydrothermally-altered basalt was
studied with XRD, TEM and electron microprobe to determine the
type and orientation of secondary minerals. Optical examination
indicated chlorite assemblages to have altered in two distinct
microsites: one microsite near micro-fissures traversing the regolith
units, and the other away from the continuous passages. In this
paper, weathering mechanisms and products of chlorite present
in microsites distant from the micro-fissures are reported. In
all the regolith units the original chlorite grain remained intact
and was pseudomorphed by secondary products. In the saprock, chlorite
altered to corrensite with possible random interstratifications
of chlorite and corrensite and corrensite and vermiculite. In
the saprolite, corrensite altered to vermiculite. Parallelism
of two axes of the products with the host indicated topotactic
alteration. In the fine saprolite, vermiculite was found to alter
to kaolinite via a randomly interstratified kaolinite-vermiculite
stage with a high proportion of kaolinite. Goethite crystallized
in between packets of kaolinite, vermiculite and kaolinite-vermiculite.
Though the disruption of the crystal structure of vermiculite
is necessary in its alteration to kaolinite, the reaction was
such as to maintain parallelism of the c axis. The alteration
of chlorite to vermiculite was characterized by the loss of Mg
and Fe and minor Al, all ions considered to be lost from the brucite-like
sheet of chlorite. The Fe released during the alteration of vermiculite
to kaolinite is likely to have migrated to micropores to form
goethite. The presence of interstratifications of the end-members
of layer silicates involved in the reaction sequence suggests
that interstratifications are common during layer silicate weathering
in environments where space is limited and consequently solution
and ionic transport passage are restrictive.
Key Words: Chlorite; Corrensite; Goethite; Kaolinite; Transmission
electron microscopy; Vermiculite; Weathering; X-ray diffraction.』
Introduction
Experimental methods and materials
Geology
Regolith profile and landform location
Methods
Results and interpretations
Fresh rock
Saprock
Saprolite
Fine saprolite
Chemical changes
Discussion
Summary of observations and findings
Chlorite to vermiculite stage (weathered rock and coarse saprolite)
Vermiculite-kaolinite-goethite stage (fine saprolite)
Occurrence and significance of kaolinite-vermiculite interstratification
Conclusions
Acknowledgments References