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J. SCHETELIG
Excess of Si02 = 0,47 %
- » A1203 = 0,45 »
0,92% + 0,92%
100,00 %
If the small amounts of Ba* and Sr=feldspar as usual are regarded
as components of the Or^constituent, the composition of the albite from
Seiland is as follows:
A b : 87,9 °/0
An: 5,2 »
Or + Ba + Sr: 6,9 »
100,0 %
The antiperthitic lamellae, observed in the orientated sections of the
analysed albite seem to indicate that orthoclase with the Ba* and Sr*
components is unmixed in the solid state.
The composition of the albite computed from the determined angles
of extinction in orientated sections shows a little higher percentage of
anorthite (medium An7,5).
The difference is due either to small errors in the exact orientation
of the sections and in the determination of the angles of extinction, or to
the hitherto not exactly known influence of the relatively large amount
of the Or*component on the optical properties of the plagioclases.
The tables of F. Becke for computation of the compositing ?>f the
plagioclases are based upon careful investigations of pure mixtures of
the Ab* and An*components, but the influence of the Or*component are
not taken into consideration.
Calcite occurs only as granular masses, chiefly crystalized later
than the albite and the other minerals, but in part perhaps simultaneous
with the albite. The two minerals sometimes show intergrowth. The
rounded grains of calcite are generally from 3 mm. to 5 mm. in size.
Minerals from the pegmatitic dyke in Ørnetind south of
Skarvevand, Seiland.
The specimens collected by Mr. Hoel show the following minerals,
also partly collected as loose crystals, arranged according to the order of
crystallisation:
128
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