Acta naturalia Islandica - 01.07.1964, Síða 24

22
SIGURDUR STEINTHORSSON
Rock I. The granular rock in the quarry (See Table I).
Phenocrysts occupy 30% of the volume.
The rock is dense, light grey or dark in colour in the hand specimen, depen-
ding on which side of the colour boundaiy it was taken. The groundmass is
granular, medium- to fine-grained. It is of note that with increasing modal
pyroxene content in the groundmass the rock becomes (a) more granular and
(b) finer grained.
The plagioclases are aligned in parallelism, suggesting movement during
crystallization. The olivines in the groundmass are rounded; in the dark variety
they are heavily serpentinized, but completely unaltered in the light variety.
The ore occurs in discrete grains or clusters, showing sieve structure. Massive
core with poikilitic margins is common. (Pl. Va is taken from this rock).
In the dark variety, a halo of hematite staining is present around the magne-
tite grains.
Phenocrysts of olivine and pyroxene carry a discontinuous outer zone. It
is conceivable that some resorption took place before the rim was deposited,
because the outline of the core is slightly rounded. Otherwise both olivines
and pyroxenes have good outline. The pyroxene rim is granular in appear-
ance, but shows perfect optical continuity with the core. A few olivine crystals
show exsolution of dendritic magnetite. A plagioclase-rich zone is often seen
around the pyroxene phenocrysts (Pl. II b is from this rock).
A few small plagioclase phenocrysts occur. They are very rounded in shape
due to resorption. A broad outer margin shows sieve structure due to numerous
small and rounded pyroxene grains. The feldspar phenocrysts are surrounded
by a zone enriched in pyroxene, which presumably is a mirror image of the
feldspar zone round the pyroxene phenocrysts. Occasional aragonite crystals
are present filling vesicles.
Rock II. The sub-ophitic rock in the quarry.
Phenocrysts occupy about 30% of the volume. Feldspar laths, up to 0.5
mm long form the main substance of the groundmass. The crystallization
of the plagioclase seems to have been well under way when the pyroxene
started to crystallize, because quite large laths of plagioclase are incorporated
in the outer rim of the pyroxene phenocrysts. The magnetite may incorporate
all three, plagioclase, olivine and pyroxene in ophitic intergrowth. The pheno-
crysts have poor outline. They may reach large size (Max. olivine 7.5 mm,
average size about 2—2.5 mm. The pyroxenes are generally smaller).
Reaction rim (outer discontinuous zone) is pronounced in the pyroxene,
but in the olivine it is hardly visible though probably present. A few of the