Jökull - 01.12.1983, Page 51

TABLE 5. Major elemental composition (weight
%) of lignite, weathered tephra and clay, Húsavík-
urkleif profile.
TAFLA 5. Niðurstöður efnagreininga á sýnum úr Húsa-
víkurkleif.
Interbasalt soil
Weathered Lignite Underclay
tephra and H12 H14
clay H 9
Gjóska og leir Surtarbr. Leir
Si02 58.4 14.3 47.7
tío2 1.78 1.02 3.86
ai2o3 19.3 6.85 24.0
Fe203(tot.) 8.66 2.36 8.53
MnO 0.04 0.02 0.05
MgO 2.50 0.91 1.08
CaO 2.00 3.89 2.91
Na20 0.51 0.09 0.89
k2o 0.32 0.09 0.49
p2o5 0.22 1.25 0.30
h2o+ 4.14 43.2 8.31
C 1.7 26.8 4.0
s 0.03 1.06 0.04
Total 99.6 101.84 102.16
h2o- 25 18 24
Density 2.30 0.64 2.04
sío2/ai2o3 3.0 0.60 2.0
2MgO+CaO-t-Na2 o
5.0 4.3 4.9
%
(up to 58%) relative to the lateritic Óshlíð soil. The
kaolinite bearing underclay (H14) are characteriz-
ed by high A1203 (24.0%) and Ti02 (3.9%) con-
tents. The chemical composition varies consider-
ably within the profile mainly due to a varying
organic matter content.
Mineralogy
The mineralogical composition of the interbasalt
sediments as determined on the basis of microscop-
ic, microprobe, X-ray diffraction and IR-analyses,
are included in Table 6.
The following minerals are identified:
l) As major constituents:
Halloysite, kaolinite, smectite (beidellite, non-
tronite, montmorillonite), allophane-imogolite,
amorphous Fe, A1 hydroxides, illite-vermiculite,
corrensite, sepiolite, pyroxene, plagioclase, lign-
ite.
2) As minor constituents
Gibbsite, diaspor, boehmite, limonite, goethite,
hematite, maghaemite, jacobsite-galaxite, cha-
bazite, mesolite, clinoptilolite-heulandite, stil-
bite, epistilbite, laumontite, intermediate to acid
plagioclase, cristobalite, tridymite, anatase/ru-
tile, phosphate, illite, quartz, amphibole.
The fine silt and clay fraction, which is enriched
in halloysite, smectite allophane-imogolite and
amorphous material, amounts to 50-70% of the
sample. The coarser material is characterized by
the Fe- and Ti- oxides/hydroxides, authigenic zeo-
lites and primary pyroxene and plagioclase.
The upper parts of each sediment profile have
been subjected to thermal metamorphism, partly
also metasomatism, by the overlying lava. As the
heat mainly has escaped to the lava surface, the
contact metamorphic impact on the underlying
sediments have been limited to the upper 20-40 cm
of the sediment. Maghaemite and jacobsite-galaxite
may have formed in response to this thermal im-
pact. The sediments have later been subjected to
burial diagenesis and low-grade metamorphism to
zeolite facies.
The progressive variation in zeolite mineralogy is
in consistence with zeolite zoning described from
tephra and clastic sediments, Japan (Iijima and
Utada 1972, Mumþton 1977) rather than with zones
described from East Iceland and Icelandic geo-
thermal fields (IValker 1960, Kristmannsdóttir and
Tómasson 1978).
The presence of gibbsite is interesting as this
mineral usually is considered as a typical productof
tropical weathering. It may occasionally form in
other environments provided low silica activity of
the aqueous phase (Schwedtmann 1979).
The Fe, Ti-oxides/hydroxides of the interlava
sediments range in composition within the limon-
ite-goethite-magnetite and maghaemite-ulvöspin-
el-ilmenite series. Most oxides have weathering
rims grading from pure oxide in the central parts to
limonite in the outer. The oxides have occasionally
been completely hydrated to limonite (Fig. 3 A).
Anatase (Ti02) and Fe, Ti-oxides may occur as
products of surface weathering in association with
kaolinite (Weaver 1976, Schwerdtmann 1979) while
rutile is considered to be mainly detrital.
The oldest sediments from Oshlíð are characterized
JÖKULL 33. ÁR 49