Jökull - 01.12.1976, Page 33

INTRODUCTION
The alteration and the overall mineralogy
of the hydrothermally altered basaltic rocks in
a 1750 m deep section through the Reykjanes
geothermal area has been described in a pre-
vious paper (Tómasson and Kristmannsdóttir,
1972). In Fig. 1 the distribution of clay minerals
with depth in three of these drillholes is shown.
The clay minerals formed were by X-ray identi-
fication found to be smectites in the uppermost
cooler parts (at rock temperature below 200
°C) and chlorites in the lower part of the
section. An intermediate zone of randomly
mixed-layer smectite/chlorite minerals bridges
the two main zones (in the temperature interval
200—270 °C). Normal non-swelling chlorite is
found beneath this depth, but the swelling
chlorite is found together with it to the deepest
levels reached by drilling. In Table 1 is shown
the classification of the clay minerals from X-
ray diffraction analyses. The mixed-layer min-
erals of chlorite and smectite are irregularly
interstratified and their diffraction peaks are
often broad and without a distinct maxima.
The swelling chlorites are divided into four
main types, but minerals showing intermediate
characteristics have been found. The thermally
instable chlorite 3 and chlorite 5 were classified
mainly on basis of their swelling abilities being
similar to the two thermally stable types and
their occurrence together with them. The struc-
ture of those swelling, chlorite like minerals
and their relation to the other clay minerals
was uncertain after the first study by X-ray
diffraction methods.
Optical observations suggest that the clay
minerals are all Fe-rich. The optical properties
of the mixed-layer minerals and the swelling
chlorites are mostly rather similar but very
variable from one sample to another. No clear
correlation between microscopic observation
and results from X-ray diffraction analyses could
be obtained for those minerals.
Due to these features it was suggested that
a process of gradual and continuous chloritiza-
tion of the smectites did happen by increasing
temperature without any major chemical re-
arrangement of the silicate layers themselves.
The aim of the present study was to investigate
the clay minerals, mainly the mixed-layer
minerals and swelling chlorites by differential
thermal analysis and infrared spectroscopic
methods and to find out whether these features
changed abruptly or gradually and study their
relations to the smectite and normal chlorite
minerals.
A main difficulty in preparing the samples
for this study was to obtain nearly pure clay
mineral fractions from the rock. Some of the
clay mineral types occur only together with one
or two of the other types. In some cases a
mixture of two or three minerals was there-
fore obtained in the clay fraction. A sample
containing only one single clay mineral, and
large enough for the differential thermal ana-
lyser was obtained of the smectite, a mixed-
layer smectite/chlorite mineral and the swelling
chlorite of group 2 (see Table 1). A sample of
nearly pure swelling chlorite 5 was also pre-
pared. All the samples contained small quanti-
ties of non clay minerals, mainly plagioclase,
calcite, quartz and opaques. The quantity of
the non clay minerals was in most samples 1—
5%. The purity of the samples was checked by
X-ray diffraction scanning.
RESULTS OF THE DIFFERENTIAL
THERMAL ANALYSIS (DTA)
In Fig. 2 are shown some characteristic DTA
patterns of the samples. A striking feature is
the similarity of the curves for swelling chlorite
and the mixed-layer minerals. Mixtures of
smectite and swelling chlorite, mixed-layer
minerals and swelling chlorite also show closely
similar patterns.
The main peaks appear at practically the
same temperatures in all instances, only their
relative intensities are different. The differ-
ential thermal analyses show that the “chlori-
tic” minerals all contain structural bound
water in varying degree. A rather strong and
well defined dehydroxylation peak occurs at
590—600 °C. An endothermic peak at about
830 °C, which is probably due to dehydration
of the mica layer, has lower intensity than the
dehydroxylation peak, but is quite well defined.
It is followed immediately by an exothermic
peak at about 860 °C. The DTA curves for the
chloritic minerals are quite similar to curves of
“corrensite” (Mackenzie, 1957a). The similarity
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