Rit (Vísindafélag Íslendinga) - 01.06.1967, Page 189

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One olivine crystal has a reaction rim of groundmass pyrox-
ene. Titanomagnetite and apatite occur in a similar way as
in types I and II, the apatite hoth surrounding and occurring
within the titanomagnetite and the olivine. The pyroxene
occurs as small laths and occasionally as small dots.
The refractive indices and composition of the minerals and
glass are as follows:
Plagioclase: nx 1.559, nz 1.569 An 62
Olivine: — 1.733, — 1.746 Fa 35
Brown glass: — 1.578,
The lava of the third phase is classified as olivine-andesite.
In the lava of the type IY (Fig. 4) the groundmass has a
weak fluidal texture and the groundmass contains variable
amount of glass which is often more coarse grained than the
other types. The plagioclase is more basic and the olivine
more abundant than in the other three types and sometimes
it is the only mafic mineral occurring as a phenocryst. If
pyroxene phenocrysts are present they are often zoned and
with an hourglass structure.
The refractive indices and composition of the minerals and
glass for the Svínhagahraun, a Lághraun southwest of Hekla,
is as follows.
Plagioclase: nx 1.563, nz 1.576 An 70
Olivine: — 1.798, — 1.740 Fa 34
Brown glass: — 1.623.
There are no chemical analyses available of the type I
lava but one or more of the other three types. There is
only a small difference in the chemical composition of the
type II lava and type III lava. This difference will not be
discussed here. However, there is a striking difference be-
tween type II and III on the one hand (Si02 54—56%) and
type IV (Si02 45—49%) on the other. This is illustrated in
the alkali: silica diagram (Fig. 5) which shows the relation
between the alkali content and the silica content of the va-