Jökull - 01.12.1983, Síða 53

by high contents of the 7 and 14 Á phyllosilicates,
kaolinite, halloysite and nontronite-beidellite. Illite
(10 Á), pyroxene and Ca-plagioclase could not be
detected. The halloysite contains impurities of iron
and alkalies. Traces ofandesine (An 33) occur.
Characteristic mineral formulas:
Kaolinite (Feo.DfiAl3.89) (Al008Sij92) (0,0H)la
Halloysite (Fgo,30A13 79)2 (Al0 |6Si384(0,0H)18
Beidellite-nontronite
(Ca,) ]7Nao QgKfi 02) (A l2i56Fe3 33Tio 78Mg0 is)
(“'k l2.49Si5.51 )094.nH2O
Ilmenite Fe0.97Ti0.92Mn0.03O3
Titanomagnetite-goethite-limonite
f"ei .65"Fio.6o^ lo.oeMgo oeO^.nHjO
Jacobsite-galaxite: Mn,ifl.,Fc12;iAl059Ti0 [0O3
The Öshlíð section, characterized by kaolinite,
halloysite, goethite and gibbsite, corresponds to
the senile to lateritic stage of tropical weathering
(Mohr and von Baren 1960, Kuzvart 1978).
The jacobsite-galaxite and maghaemite have
only been traced in the baked red layer in contact
with the overlying lava. Maghaemite (Y-Fe203),
which is metastable with respect to hematite (°c-
Fe203), may form from lateritic goethites by grindi-
ng and heating up to 500°C (Yariv et al. 1979, Mend-
elovici et al. 1979), and oxidation from microcrystal-
line magnetite (Gallagher et al. 1968). The jacobsite-
galaxite formation probably reflects a metasomatic
Mn supply from overlying lava.
According to L. Kristjánsson (1979, pers. comm.),
a very stable magnetic remanence has been record-
ed in the baked soil below the contact. In the light of
the mineralogical analyses this may be related to
the maghaemite formation.
The presence of zeolites like mordenite and heu-
landite suggest burial temperatures of 80-100°C
(Kristmannsdóttir and Tómasson 1978, Iijima and Utada
1972). The existence of primary weathering miner-
als show the sediments in most of the profile to be
unaffectcd by the heat from the overlying lava.
The Húsavíkurkleif sediments are characterized by
presence of 7Á and 14Á phyllosilicates. Halloysite
and kaolinite are the main mineral components of
the lignite and underclay beds, while the tonsteins
(argillized tephra) are characterized by iron poor
smectites of the beidellite-montomorillonite type.
Selected analyses are as follows:
Tonstein
Montomorillonite-beidellite
(Ca<).47Na03|K0 20) (Al| 74Fej ogTij 76Mgl59)
(A1o.78SÍ7.02)(0>OH)24
Beidellite
(Cao.otNao 37Ko43) (Al4 52Feo87Tio 24MgoJ6)
(AI0.36Bi7.64) (0,0H24)
Underclay and lignite
Halloysite-kaolinite
(Cao.12Nao.31Ko.og) (Al^Feg 44Tio 4iMg0 i4)Si4J5
(0.0H),8
From thin sections and microprobe analyses it
becomes apparent that albite (An4 4) and Ca-zeolit-
es (heulandite, chabazite, mesolite) have formed at
the expense of calcic plagioclase. Traces of bytown-
ite (An72), labradorite (Ang4) and andesine (An38)
still remain. Volcanic glass is mostly replaced by
clay minerals, analcime and/or heulandite.
The composition of the beidellite corresponds to
analyses of beidellitic clay from Faeroe Island and
Mull (Sabine 1971, Sabine et al. 1972). A modern
analogue to the lignite and underclay sequences at
Húsavíkurkleif has been described for a peat form-
ing environment, South Carolina (Staub and Cohen
1978). The clays are mostly smectitic, however,
within the peat and in the rooted clay immediately
below, kaolinite dominates. The kaolinitization was
caused by leaching by acid water filtering down
from the overlying peat (Richardson and Francis
1971).
The iron/titanium oxides include hematite and
anatase. A hematite rich 5-10 cm thick bed with
traces of smectite, is recorded in the middle of the
profile. The hematite is suggested to have formed
from limonite or goethite during burial of the lava
pile. According to Langmuir (1971) the transform-
ation of goethite (a-Feö(OH) ) to hematite (a-
Fe903) and water, occurs at temperatures between
81 and 143°C at 1 and 4 bars H20 pressure, re-
spectively. This suggest the sediments to have been
heated to at least 80-90°C after deposition.
The mineralogy further place the Húsavíkurkleif
sediments within the analcime-heulandite-zone or
higher of Utada (1965), Iijima and Utada (1972),
suggesting reheating to about 120°C.
The Sámsslabir red beds are characterized by the
14 Á and 10 Á phyllosilicates; illite-vermiculite-cor-
rensite and smectite (montmorillonite), opaline sil-
ica and traces of chlorite. Neither kaolinite nor hal-
loysite could be detected.
Selected mineral analyses:
Illite (Fe-rich)-vermiculite:
JÖKULL 33. ÁR 51