Rit (Vísindafélag Íslendinga) - 01.06.1967, Síða 186

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mass has a fluidal texture, plagioclase needles being the only
discernible mineral and these needles are all oriented parallel
to the flow. Conspicuous is a colour banding due to different
content of glass in the groundmass, which results in diffe-
rent shades of brown to nearly black (Fig. 1). The bands fol-
low the direction of the flow. The fluidal texture curves
around the phenocrysts, especially the pyroxenes. The amount
of phenocrysts is usually less than 2%. The length of the
plagioclase crystals is 2= 1.5 mm, of the pyroxenes ^ 1 mm,
of the titanomagnetites 0.3—0.5 mm. The olivines are some-
what smaller and there are some apatite crystals grouped
near or within the pyroxene and titanomagnetite crystals.
In the first phase of the 1845/46 lava — sample collected
high up in the mountain slopes — we find the following re-
fractive indices and composition of the phenocrysts:
Plagioclase: nx 1.558, nz 1.566 An 60
Pyroxene: — 1.701, — 1.723 2V = 45°
Olivine: - 1.730, - 1.779 Fa 50-53
In the first phase of the 1947 lava — sample collected from
a lava tongue from the first day of eruption — we find the
following indices and composition of phenocrysts:
Plagioclase: nx 1.558, nz 1.564 An 57
Pyroxene: — 1.701, — 1723
Olivine: — 1.730, — 1.781 Fa 55
In spite of the absence of quartz the type I lava is classi-
fied as dacite.
The groundmass of the lava of the second phase (type II
lava, Fig. 2) is considerably more coarse-grained than in the
lavas of the first phase, but contains the same minerals as
phenocrysts, although more of olivine and in bigger crystals.
The content of phenocrysts varies a lot, from nearly nothing
to about 3%. The titanomagnetites are most conspicuous in
samples containing the smallest amount of apatite and often
surrounded by the latter. The groundmass consists of plagio-