Jökull - 01.12.1983, Page 48

goethite, boehmite, allophane and gibbsite. Select-
ed samples of red soil were pretreated according to
the method of Mehra and Jackson (1960) to remove,
and to quantitatively estimate, the amount of free
iron and alumina hydroxides.
Comments on selected minerals
Smectites. The distinction between beidellite, non-
tronite (Fe-bearing), and montmorillonite is based
on microprobe analyses (Goodman et al. 1976).
Corrensite (Lippmann 1954) is a regular mixed
layer mineral with alternating layers of chlorite and
swelling chlorite and basal spacings 29-30 A and
14-15 Á. The term has been used to describe order-
ed stratification of chlorite with a hydrated inter-
layer clay mineral (April 1980) and to regular chlor-
ite-smectite mixed-layer minerals (Tomkins 1981).
Halloysite is distinguishable from kaolinite by ex-
pansion to 11-17 Á after ethylene glycol treatment
and by contraction to 7.2 Á after drying.
Allophane and imogolite usually coexist and are
most typically formed from tephra as weathering
products and occur together with various amounts
of volcanic glass (Yoshinga and Aomine 1962, Wada
1977). XRD ofpoorly ordered allophane give broad
enhancements at about 8.6 Á and 3.7 Á, and imogo-
lite have peaks near 7.6-7.9 Á and 5.7 Á (IVada et al.
1972, Wada 1977, Torrent et al. 1982).
RESULTS
Texture and morphology
The texture and morphology of the sediments are
illustrated by characteristic microscope- and scann-
ing electron micrographs (Figs. 3-8).
Fig. 3 A shows lateritic soil where the halloysitic
matrix has been stained red by amorphous Fe, Al-
hydroxides. Opaque oxides, like magnetite, titano-
magnetite, maghaemite and hematite, appear as
black; and the limonite-goethite as red homogenous
crystals. A tuffaceous texure may still be recognized
in central parts of argillized clasts, although the
minerals are completely altered to residual miner-
als. This is illustrated in Fig. 3 B. Argillized tephra
with relict tuffaceous texture, lignite and diagenetic
zeolites are further shown by the photomicrographs
of Fig. 4 A, B.
The matrix minerals, kaolinite-halloysite-allo-
phane, are covered by colloidal material which is
clearly displayed in the SEM micrographs (Fig. 5).
Relicts from primary minerals completely argillized
to kaolinite, have been recorded in the weathered
rhyolitic tephra (Fig. 6).
Secondary zeolite frequently occur. Clinoptilolite
and associated needles of mordenite in palagonitiz-
ed and weathered tephra, Flúsavíkurkleif (Fig. 7),
and stilbite in a smectitic matrix, Hreðavatn, are
shown in Figs. 7 and 8. Associated minerals besides
smectite, were cristobalite, plagioclase and magne-
tite. Similar associations have been reported by
others (Mumpton and Ormsby 1976, Mumpton 1977).
Densities and water contents
The weathered character of the interlava sedi-
ments is reílected in lower densities and consider-
ably higher water contents and loss on ignition
Fig. 8. Scanning electron micrographs of lignite-bearing sediments, Hreðavatn (sample Hr27). A)
Authigenic stilbite in a montmorillonitebeidellite-nontronite matrix. B) and C). Detail of A.
Mynd 8. Orgreinismyndir af syni úr seti við Hreðavatn. Einkum leirsteindir og geislasteinar.
46 JÖKULL 33. ÁR