Jökull - 01.12.1963, Blaðsíða 16

TABLE 1
Chernical composition of precipitation, monthly
averages.
Rjúpnahæð Vegatunga
Jan. May July Oct. Jan. May July Oct.
Na 10.3 7.2 1.6 4.0 8.1 1.5 0.7 2.1
Iv 0.4 0.4 0.1 0.2 0.3 0.4 0.1 0.1
Ca 0.5 0.5 0.5 0.3 1.3 0.3 0.7 0.3
Mg 1.2 - 0.3 0.6 1.0 - 0.1 0.3
C1 11.6 13.6 2.3 6.7 9.0 4.0 0.9 3.7
S04 3.1 3.2 2.2 3.5 1.5 1.9
The range of silica concentration is 0.9 to 17
ppm SÍO2, the highest value found in a river
draining andesitic volcanic rocks, the lowest
value from a gianitic region. Other elements
such as the alkalies show relatively low con-
centrations, the highest Na being 3.05 ppm
from andesitic terrain and the highest K 1.72
ppm frorn granite. The authors conclude that
the compositions of the country rock and the
rocks suceptibilitv to hydrolysis control the
amount of dissolved solicls in the glacier river
water.
From studies on hydrothermal alteration in
active thermal nreas it is evident, that under
such conditions andesites and basalts are the
mots readily attacked rock types. This depends
principally on the fact that the femic minerals
are more readilv dissolvecl than the feldspars
and quarz on the one hand and on the other
because calcic plagioclase is less resistant to
hydrothermal att.ack than sodic plagioclase or
the alkalifeldspars (Sigvaldason and White 1961).
Although these results apply specifically to
hydrothermal systems, a similar pattern appears
to hold for the low temperature systems of the
glacier rivers.
The mineralogy of rock debris carried in
suspension by the rivers is therefore of major
importance with regard to interpretation of the
results of the chemical analysis of the water. For
the study of mineral composition fine grained
sand from the nver bed was sampled. Further-
more the finest material carried in suspension
was filtered off and x-ray diffraction photo-
graphs obtained. The three rivers clealt with in
this paper are Múlakvísl and Skálm, draining
the outlet glacier Höfðabrekkujökull, and Fúli-
leekur draining the Sólheimajökull glacier.
Although the three rivers drain an area, which
in general terms is cotnposed of basaltic mate-
rial, soine clifferences in the mineralogical com-
position of the rocks were observed. Skálm and
Múlakvisl are the main streams draining the
southern part of the glacier. The sandur plain
formed by the rivers, and by the glacial bursts,
is composed principally of volcanic ash material
originating from Katla. The main component
of the sand at Skálm is brown translucent glass
with subordinate amounts of a basic plagio-
clase and olivine. The glass is fresh and does
not show any sign of hydration or alteration.
X-ray diffraction photographs of the finest sus-
pension carried by the river show the strongest
feldspar lines along with the 001-reflection of
an expandable montmorin mineral at 14 A in
the air dry state. After treatment with ethylene-
glykol the lattice expands to 17 Á. The 060 re-
flection lies at 1.544 Á.
At Múlakvísl the sandfraction is composed of
similar material except for some fragments of
altered glass derived from the palagonite hills to
the west of the river. The finest suspended mate-
rial is composed of glass, plagioclase and some
expandable montmorin mineral with the same
x-ray properties as the material from Skábn.
The sandur plain formed by the Fúlilœkur
outwash does not contain as much fresh ash as
does Mýrdalssandur. The sandfraction of the
river load is composed chiefly by altered glass,
feldspar and olivinegrains, some calcite and zeo-
lites(?). The finest fraction is here composed
chiefly of the same expandable montmorin mi-
neral as found before, the main difference being
a much higher cjuantity of this mineral as com-
parecl to Skálm and Múlakvisl.
CHEMICAL ANALYSIS OF
WATER SAMPLES.
The water was samplecl in polyethylene bot-
tles and analysis was usually started within two
days from sampling. Alkalinity ancl pH were
determined immediately upon arrival in the la-
boratory. Before determination of other com-
ponents the suspended particles had to be fil-
tered off in order to obtain clear solution. Be-
12
JOKULL 1963