Jökull - 01.12.1976, Page 37

Fig. 4. The OH-region of the infra-red spectra
of smectite and smectite-like samples from
Reykjanes.
a) Sample dried at 90 °C.
b) Sample heated at 400 °C.
Minerals
1 Smectite................
2 Smectite................
3 Smectite and “swelling
chlorite 5” ...........
4 Smectite and swelling
chlorite 4 ............
Drill- Depth
hole m
4 218
8 20
8 950
4 776
Mynd 4. Gleypni nokkurra smektitsýna í inn-
rauða litrófinu.
chlorite (Hayashi and Oinuma, 1967). The
peaks are however often poorly defined. Heat-
ing of the chloritic samples up to 400 °C has
either no effect on the spectra or it rather
improves the absorption peaks. After heating
at 550 °C the peaks have disappeared, but a
low, broad and poorly defined peak at slightly
lower frequency can be distinguished. The
samples investigated contained mainly “therm-
ally stable” chloritic minerals. Where tlie
samples contain “thermally instable chlorites”
those are in minor amounts. The similarities
of the spectra show that the crystallographic
structure of all the chloritic minerals and their
chemical composition are very similar. This is
in accordance with the results from the DTA
analyses. In the spectral region 400—1300 cm4
only one of the investigated samples shows a
typical smectite pattern. The others show pat-
terns intermediate between smectite and chlo-
rite. Comparison with DTA shows those samples
to be more interlayered by chloritic layers.
“Chlorite 5” is similar to the interlayered
smectite samples. The spectra of the mixed-
layer minerals are indiscernable from the
chlorites. As found for the OH-region the
spectra of the chlorites are fairly similar to
spectra of Fe-rich chlorites (Hayashi and Oin-
uma, 1965). Where the chlorites are admixed
to smectites or “chlorite 5” the intensity of the
640—660 cm-1 absorption band tends to be
lower and the pattern more similar to the inter-
layered smectites.
THE CHEMICAL COMPOSITION
OF THE ALTERED ROCKS
The clay minerals make up 50—80% of the
rocks drilled into. Compositional variations of
the clay minerals woulcl therefore certainly be
reflected in the total chemistry of the rocks.
Similarly major changes in the rock composi-
tion would certainly affect the clay minerals.
Some few chemical analyses of the altered
basaltic rocks are shown in Table 2. As seen
from the table the major difference between
the hyaloclastic rocks and the basalts is that
the hyaloclastites are highly hydrated. The
altered basalts are also more hydrated than
fresh basalts normally are. The FeO has not
been determined and only total iron is given.
JÖKULL 26. ÁR 35