Acta naturalia Islandica - 01.07.1964, Blaðsíða 14

12
SIGURDUR STEINTHÓRSSON
The two zones represent two phases of crystallization. The core crystallized
in a magma chamber at depth or under way to the present site. Subsequently,
the mush, liquid with phenocrysts, was emplaced and the crystallization of
the ouitier rim took place.
The pyroxene phenocrysts show usually some sort of crystial faces. Signs
of resorption are not always evident, but blunt edges of the core suggest some
resorption between the two phases of erystallization. In certain of the rocks,
however, the crystals are highly rounded. A number of times the outer rim was
seen to follow embayments in the crystal indicating that the embayment was
there prior to the emplacement of the magma. Embayments, which are quite
common in the phenocrysts, may be due to either rapid or faulty crystalliza-
tion, „accidents" during flow, or resorption. A probable instance of „skeletal
growth" is seen in the pyroxene in Pl. II b. Cavities in the crystals, filled with
groundmass, occur as well.
Olivine is the most abundant phenocryst of these rocks but subordinate to
pyroxene and plagioclase in the groundmass. The phenocrysts range in size
up to 6—7 mm in diameter; the average size is similar to that of the pyroxenes.
They are usually extremely fresh but show some deuteric alteration to serpen-
tine or iddingsite on the edges and in cracks. The olivine phenocrysts show
discontinuous zoning in the same way as the pyroxenes. The rim is, however, not
easily distinguished because there is no change in colour or extinction angle.
Inclusions sometimes emphasize the boundary line, but the outer zone is most
conclusively seen when selective alteration has taken place, i. e. the crystals
of the groundmass and the rim are altered while the core is fresh and unaltered
(Pl. III a). Finally the rim may be defined by a curved crack.
A considerable variation of 2V and R.I. was found in the olivines, or from
+ 2V = 93, n= 1.663 (Fa7) to +2V = 87, n = 1.687 (Fa20)- An X-ray
measurement of a fresh and clear crystal obtained Fa17. The most forsteritic
values are those of the „black olivines" (cf. below). Suitable sections showed
slight zoning of the core.
In many sections the olivine crystals are euhedral. Their habit is then
short-columnar with elongation parallel to the „c“ crystallographic axis. More
commonly, however, the crystals are more or less rounded or irregular in shape.
The roundness would generally be due to resorption, but attrition may play
some part too. Cataclastic effects are, however, not evident.
Holes and embayments in the phenocrysts provide evidence of rapid or
imperfect crystal growth. In certain rock types a circular or semi-circular
cavity, fulled with groundmass, is commonly seen in the middle of the olivine