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NEPHEI.INsBEARING PEGMATITIC DYKES ON SEILAND
are rfiet with as yellow to reddish coloured lines, forming an angle of
17V» ° with the trace of (010).
In sections // (010) the scales // (112) and (112) are shown as lines
forming an angle of 18° with the trace of (001). A few scales // (150)
and (150) with generally sixsided outlines are met with.
Scales of haematite orientated // other faces e. g. (001) and (010)
occur but very scarcely. Small inclusions of magnetite opaque and black
are met with in small amounts.
The crystals of albite generally show a pretty bluish or greenish
play of colour on (010) — (labradorization) — best developed if the albite
is entirely fresh (translucent or subtranslucent). The slightly altered,
nearly opaque crystals show instead of the play of colour, a silky nacrous
lustre, visible in the same position of lightreflection as the play of colour.
The position of the play of colour seems to be a crystahplane near (010)
in the zone [010 : 112].
Optical properties. The angles of extinction in orientated see«
tions // (001), (010) and normal to the axis a are determined under
the microscope.
1. Sections of the large crystal, used for the chemical analysis (inner
part of the crystal).
c (001): extinction : + 3 ° . . . An7
b (010): -»- : + 16° . . . An9
axis a: —»— : — 13’/a0. . An6,s
2. Sections of another crystal (inner part).
c (001): extinction : + 3° . . . An7
b (010) : -»- : + 167a °. , An8,5
axis a : —»— : — 121/» °. . An8
3. Sections of a small crystal, cut near the surface.
c (001): extinction : + 3 ° . . . An7
b (010): -»- : + 17° ... An«
axis a: —»— : — 14ü . . . An6,r,
The composition of the albite according to the determined angles
of extinction is:
Min.: Ab94 Ane . . . Max.: Ab92 An8.
In the sections from the inner part of the large crystals are shown
antiperthitic lamellae of potash«feldspar. The angle: lamellae to trace of
125
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