Acta naturalia Islandica - 01.07.1964, Page 13

THE ANKARAMITES OF HVAMMSMÚLI, EYJAFJÖLL, SOUTHERN ICELAND 11
steel yard. The unaltered rocks showed a variation in density about the mean
of 3.06.
The plagioclase oecurs mainly as an important constituent of the ground-
mass. Phenocrysts do occur in some of the rocks, and may then reach up to
3 mm in length. Albite and Carlsbad twinning is the rule but pericline twinning
occurs as well. Both phenocrysts and groundmass crystals show zoning. The
phenocrysts have a central portion of fairly uniform sodic bytownite, which, how-
ever, shows a certain amount of direct continuous zoning. Then follows a sharp
break and an outer strongly zoned rim of more albitic composition. The core
is rounded by resorption. The groundmass crystals show zoning from An77
(bytownite) to An40 (andesine). They vary greatly in size according to the
rock types, but in the coarsest grained varieties their average length is about
0,4 mm.
The plagioclase phenocrysts commonly contain a number of magnetite crys-
tals in the core. These inclusions are not seen in the groundmass plagioclase or in
the outer zone of the phenocrysts. The inclusions, which are small but euhedral
with rectangular outline, are orientated in the phenocryst with their faces
parallel to a cleavage (001), (010), (cf. Pl. Ila). These inclusions seem to
indicate parallel growth of plagioclase and magnetite in the magma prior to
emplacement to the present position.
Diopsidic augite occupies the greatest modal volume in these rocks, both
as phenocrysts, (where it, however, is frequently subordinate to olivine),
and in the groundmass. The phenocrysts are variable in size, commonly 3—4 mm
across and may reach up to 6 mm. Occasional crystals, 2—3 cm in diameter,
were seen. The size of a crystal in thin section depends of course on two factors,
i. e. the actual sizt, and the way in which the crystal was cut. The shape of
the crystal in thin section is also dependent on this second faetor.
The phenocrysts are composed of two discontinuous zones; the core, which
is sligthly zoned, is clear and colourless and devoid of inclusions. The com-
position is that of diopsidic augite (+ 2V = 50—54°, extinction angle ZAc
= 46—48°, and average n = 1.675 giving 52% MgSiOg, 41% CaSiOg, 7%
FeSi03.) (Tröger; Hess 1949, Muir 1951). The outer zone shows a distinctly
darker brownish tint than does the core, that of the groundmass pyroxene.
Both zones are unpleochroic. The boundary between the two is further emphasized
by the presence of abundant small, rounded inclusions, mainly plagioclase. The
outer zone shows sieve structure and may exhibit ophitic intergrowth with
the plagioclases of the groundmass, The two zones, however, show physical
(cleavage and crystal faces) and optical continuity, but he rim extinguishes
at a slightly greater angle than the core.