Acta naturalia Islandica - 01.07.1964, Page 17

THE ANKARAMITES OF HVAMMSMÚLI, EYJAFJÖLL, SOUTHERN ICELAND 15
It is thought that the conditions in this vesicular zone were such as to cause
the exsolution of iron from the olivine, which was then oxidized to magnetite.
It appears that the magnetite rim is actually on the crystal rather than
inside it. In that case the clear zone seems to represent a mantle of composi-
tion different from that of the core, which did not suffer exsolution. A later
phase of crystallization would, however, not be expected, considering the very
fine grained groundmass. In all the other rocks the pyroxene shows greater
tendency than the olivine to develop a mantle, but as seen in Pl. IVa the py-
roxene does not carry any such rim here. This problem must await further
research and methods allowing the exact analysis of the various parts of the
crystals.
As stated, some of the crystals are almost completely blackened with ore.
It seems that all this great quantity of iron cannot be accounted for by ex-
solution alone, because the change from Fosn to Fa93 (i. e. 8% Fo) would
not give rise to all that much magnetite. This fact, and the presence of a magne-
tite rim on the crystals, appears to indicate the introduetion of certain amount
of iron which may have been concentrated by the volatiles (to account for
the absence of all these phenomena further down).
Magnetite forms the fourth primary constituent of the rocks. It usually
occurs in discrete grains, clusters or ,,streaks“ in the groundmass, and frequent-
ly shows poikilitic intergrowth with plagioclase; occasionally it is seen en-
closing pyroxene or olivine. Pl. Va shows magnetite with a dense core and a
poikilitic margin. The magnetite started to crystallize fairly early on and later
proceeded to do so, together with the plagioclase, to form the intergrowth. In the
finer-grained rocks, the ore forms small euhedral grains of rectangular outline
(crystals). In weathered rocks it is partly oxidized to hematite. Magnetite also'
occurs as inclusions and exsolutions in plagioclase and olivine (above).
Aragonite is found in some of the rocks, filling vesicles. A large well formed
crystal from a vesicle, and a tiny carbonate spot from a thin section, were deter-
mined by means of X-ray analysis. Both proved to be aragonite.
Small biotite crystals were seen in the aragonite in one thin section. Both
the aragonite and the biotite are late stage products, deuteric or post-crystal-
lization. The aragonite occurs both in the dark and the light coloured rock
types (cf. G. P. L. Walker 1960).