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NEPHELINsBEARING PEGMATITIC DYKES ON SEILAND
zircon and apatite,
magnetite and ilmenite(?),
euxenite«polycrase,
corundum,
muscovite,
plagioclase (oligoclase An13—14),
Nepheline is not known from this dyke.
Zircon. The crystals in colour and habit similar to the crystals
from the well«known locality at Fredriksvaern in southern Norway
are generally small, and show the following forms:
(100), (111), (311), and (110).
Apatite is only met with as small grains of a bluish«green colour in
the larger specimens of the pegmatite. Crystals were not collected by Mr.
Hoel from the locality.
Magnetite occurs as irregular masses together with biotite along
the borders of the dyke.
Ilmenite is shown as fragments of euhedral crystals in the plagio«
clase.
The mineral of the euxenite*polycrase group is met with in the habit
of polycrase, tabular // (010), but the material collected is not sufficient
for an exact determination of the mineral.
Corundum. A large specimen from the dyke, consisting chiefly
of oligoclase and muscovite, shows a rough crystal of corundum, em«
bedded in muscovite and with a coating of this mineral. — The colour
is light brownish. Parting // (10H) = R is well marked. — The mi«
neral shows in thin sections the optical properties of corundum: refring«
ence high, birefringence law, uniaxial, sometimes with anomalies. Opt. neg.
The occurence of corundum here is of much interest, as hitherto this
mineral in Norway is known only from the iron ore from Rødsand Mine,
Søndmør, as pointed out by J. H. L. Vogt.1
Mr. Adolf Hoel has thus discovered the second known occurence
of corundum in Norway.
Muscovite. As mentioned, the dyke is mined for muscovite, which
occurs in large crystals. In thick tables the colour of the muscovite is
peculiarly reddish, similar to the colour of phlogopite.
Plagioclase occurs only as coarse«grained crystalline masses. Cry«
stals are not met with. The colour is white.
1 Z. f. prakt. Geol. XVIII, 1910, p. 59.
9 — Festskrift Helland.
129
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