Jökull - 01.12.1976, Page 40

infrared absorption bands occur at similar fre-
quencies as recorded for Fe-rich chlorites. The
minerals grouped as mixed-layer smectite/
chlorite minerals by X-ray diffraction analysis
and the swelling chlorites show closely related
DTA patterns. The infrared spectras are also
very similar and intermediate between the
smectite and the chlorite spectra.
The course of clay mineral formation by pro-
gressive hydrothermal alteration of the basaltic
rocks in Reykjanes is therefore found to be
the following: The first clay mineral to be
formed beneath the acid surface zone is an
iron-rich saponite. Interlayering of the smectite
by “chlorite” interlayers starts at fairly low
temperature. Slight rearrangement within the
silicate layers precedes this. Finally a fairly
rigid but “patchy” brucite layer is formed.
Within this interlayer a certain amount of
interlayer water can be held and the structure
can still expand. The zone where swelling
chlorite is dominant may in Reykjanes be en-
larged by the common disruption and changes
within the field (Tómasson and Kristmanns-
dóttir, 1972). At the highest temperatures reach-
ed unswelling chlorite is formed.
ACKNOWLEDGEMENTS
This work was carried out at the Department
of Geology, XJniversity of Bristol. I would like
to t.hank many members of the Department,
especially Dr. R. Bradshaw for assistance and
discussions.
1 would also like to thank for technical as-
sistance at the School of Chemistry, University
of Bristol. The work was supported by a grant
from the Icelandic Science Funcl.
REFERENCES
Grönvold, K., pers. comm.
Hayashi, H. and Oinuma, K. 1965: Relationship
between infrared absorption spectra in the
region of 450—900 cm-1 and chemical com-
position of chlorite. Amer. Mineral., 50,
476-483.
— and Oinuma, K. 1967: Si-O absorption
band near 1000 cm-1 and OH absorption
bands of chlorite. Amer. Mineral., 52, 1206
-1210.
Kristmannsdóttir, H. 1975: Clay minerals form-
38 JÖKULL 26. ÁR
ed by hydrothermal alteration of basaltic
rocks in Icelandic geothermal fields. GFF.
The Transactions of the Geological Society
of Sweden. 95, 289-292.
Mackenzie, R. C. (a) 1957 (ed.): The differential
thermal investigation of clays. Mineral. Soc.
London, pp. 456.
— (b) 1957: Saponite from Allt Ribhein Fiska-
vaig Bay, Skye. Miner. Mag., 31, 672—680.
Miyamoto, N. 1957: Iron-rich saponite from
Mazé Niigata Prefecture, Japan. Japan,
Mineral. J„ 2, 193-195.
Sudo, T. 1954: Iron-rich saponite found from
Tertiary iron sandbeds of Japan. J. Geol.
Soc. Japan, 60, 18—27.
Tómasson, J. and Kristmannsdóttir, H. 1972:
High temperature alteration minerals and
thermal brines, Reykjanes, Iceland. Contr.
Mineral. and Petrol. 36, 123—134.
Á G R I P
GERÐIR LEIRSTEINDA
í MYNDBREYTTU BASISKU BERGI
FRÁ JARÐHITASVÆÐINU
Á REYKJANESI
Hrefna Kristmannsdóttir,
Orkustofnun
Við rannsókn á ummyndun bergs frá djúp-
borun í jarðhitasvæðið á Reykjanesi fundust
rnargar mismunandi gerðir leirsteinda. Einfald-
að yfirlit, sem sýnir flokkun leirsteindanna eftir
röntgengreiningum, er sýnt í Töflu 1. Dreifing
hinna ýmsu gerða leirsteinda með dýpi í þrem
borholum er sýnd á Mynd 1 ásamt einfölduðu
jarðlagasniði.
Smektít er ráðandi leirsteind í berginu við
hitastig undir 200 °C. Við hitastig yfir 270 °C
verður venjulegt klórít ráðandi. Á hitastigsbil-
inu 200—270 °C finnast skv. röntgengreiningum
mjög margar gerðir leirsteinda. Þessar steindir
eru blandlagssteindir af smektíti og klóríti,
flestar óreglulegar, og svokallað svellandi klórít.
Ljóseiginleikar þessara steinda allra eru svip-
aðir, en þó nokkuð breytilegir og eru millistig
á milli ljóseiginleika smektíts og klóríts. Allar
leirsteindir myndbreytta bergsins eru mjög smá-
ar og yfirleitt illa kristallaðar.
Sennilegt var talið, að allar leirsteindir mynd-