Jökull - 01.12.1976, Síða 39

%
MgO
CaO
9 II 13
Si02 Ti02 A1203 Fe203
42 44 46 48 50 52 54 2 4 10 12 14 16 7 9 II 13 5 7 9 II
Na20 K20
2 4 2 4
H-3 ------ H-6 ....- H-8
Fig. 5. The change by depth in the chemical composition of the hydrothermally altered rocks
from Reykjanes. Only the change in main components is shown.
Mynd 5. Breytingar á magni nokkurra aðaloxíða í myndbreyttu bergi frá Reyk]anesi með dýpi.
SUMMARY
Chemical analyses of altered basaltic rocks
from Reykjanes (Table 2) show that except for
hydration the chemistry has not changed signi-
ficantly by the alteration. Between 50—80% of
the rocks are recrystallized to clay minerals,
calcite, quartz, zeolites, prehnite and epidote.
Clay minerals are dominant by quantity and
according to the chemical analyses they contain
a considerable amount of Fe and Mg. Opticai
and X-ray diffraction results have also shown
that the clay minerals are all Fe-rich. As the clay
fractions always contain a small arnount of
other minerals as calcite, quartz, plagioclase
and opaques exact mineral formula cannot be
calculated for the clay minerals. The minerals
are also often poorly and irregularly crystalliz-
ed making interpretation of results from the
X-ray diffraction difficult and giving broad
absorption peaks in the infrared spectra. This
study has shown that the classification of the
clay minerals into seven different groups based
on the X-ray results is a fictive one. The clay
minerals are here shown to be smectite and
chiorite and all the others can be grouped as
irregularly “mixed-layer” minerals of those two.
The identity of the smectites was well based
before this recent study. The new evidence
found for the smectites is the beginning inter-
stratification by chloritic layers discovered by
means of DTA and infrared analysis long be-
fore it can be traced by X-ray methods. The
results of DTA and the study of the infrared
spectra of “chlorite 5” (of Table 1) group it
as a smectite-like structure. All the “swelling
chlorites” show similar thermal features in
spite of the different swelling abilities. Their
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