Rit (Vísindafélag Íslendinga) - 01.06.1967, Page 188

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clase laths, pyroxene, titanomagnetite and a very variable
amount of glass. Plagioclase laths are 0.2 mm on the average
and oriented parallel to the flow. The pyroxenes are inter-
stitial and their length varies from small dots in the most
glassy samples to about 0.2 mm. The content of titanomagne-
tite varies greatly too and is highest in the samples where
the crystallization is most advanced. The average diam. is
about 0.02 mm. The amount of glass varies greatly in thin-
sections, even in samples collected at the same locality. In the
thinsection containing most glass the groundmass consists of
black glass only, hesides plagioclase laths. These are thinsec-
tions of black vesicular lava. One finds all intermediate stages
between this lava and a holocrystalline, dense lava of grey
colour. Sometimes the glass is hrown, especially in samples
containing a relatively small amount of glass.
In a second phase lava of 1845/46 — sample taken 1 km
NE of the farmruin Old Naefurholt — the refractive indices
and composition of the phenocrysts are:
Plagioclase: nx 1.549, An 45 nz 1.570 An 65
Pyroxene: — 1.698, — 1.728
Olivine: — 1.737, — 1.770 Fa 53
The composition of the phenocrysts is different from that
of the plagioclase in the groundmass. The lavas of the second
phase are andesites.
Only one sample of the third phase lava has been studied.
This sample (A 1954: 192, coll. S. Thorarinsson) was taken
from the last lava of this eruption, flowing down along Hösk-
uldsbjalli in May 1948 (Fig. 3). A thinsection of this lava
contains phenocrysts of plagioclase, olivine, titanomagnetite
and apatite and a groundmass of plagioclase laths, brown
glass and pyroxene. The groundmass has a fluidal texture,
the plagioclase laths being oriented parallel to the flow. The
plagioclase phenocrysts are 1.0—1.5 mm long and the plagio-
clase in the groundmass 0.2—0.6 mm and there is no distinct
limit hetween the two. The olivine crystals are 0.4—0.6 mm.