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J. SCHETELIG
(010) in section // (001) is about 40°, and indicates that the lamellse are
orientated nearly // (120). In sections // (010) the lamellae seem to be
parallel with the axis c. The antiperthitic lamellae show a distinct lower
refringence than the albite, according to determination with the Becke«
light«line.
The antiperthitic lamellae are not met with in sections cut near the
surface of the crystals.
Chemical composition. It would seem to be of interest to know
the exact composition of this feldspar, which occurs in such good crystals,
and at my request Mr. Olaf Røer, Norsk Kemisk Bureau, Kristiania,
kindly undertook to work out a complete analysis of the feldspar, with
an exact determination also of the minor components: BaO and SrO.
Table I.
Analysis of Albite from Seiland. Analyst: Olaf Røer.
Molecular ratios:
Si02 ....... 66,42 % 1,0997
A1203 ...... 20,41 » 0,1997
Fé,0:5...... 0,28 » 0,0018
CaO........ 1,03 » 0,0184
BaO........ 0,11 » 0,0007
SrO........ 0,11 » 0,0011
MgO....... trace —
Na20 ...... 10,20 » 0,1643
K20 ....... 1,04 » 0,0110
Ign......... 0,23 » -
99,85 %
The powdered mineral dried at 110° C
(RO + R,©) : A1203 = 0,1955 : 0.1997.
Excess of A12Os = 0,43 %.
The amount of Fe203 is due to the orientated scales of haematite
producing the aventurization, and for a minor part to a few spots of
magnetite shown as inclusions, and therefore not considered in the cal«
culation of the different components of the feldspar.
126
0,1955
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