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NEPHEI.INsBEARING PEGMATITIC DYKES ON SEILAND
The albite as a rule has solidified before the calcite, but in part both minerals
show simultaneous crystallisation. The fluid rest*magma, chiefly consisting of
calcite, has produced phenomena of corrosion on the faces of the crystals,
indicated by a brib
liant glassy lustre
on (010), (110) and
(110) and by stria*
tion on the other
faces due to placing
into relief of the
twinning lamellae
according to theab
bite«law. The edges
and corners be*
tween the faces are
generally rounded,
also due to corro*
Fig. 6. Crystal of albite, tabular // (010), in granular calcite.
sion.
The size of the
crystals varies from
small to relatively
large; the dimensions of the largest broken crystal are as follows: // a 23
cm., // b 13 cm., and // c 27 cm. This crystal is thick tabular // (010).
Four different habits of the crystals could
be distinguished:
1. thick tabular // (010),
2. thin tabular // (010),
3. elongated // the axis a, with (010) and
(001) nearly equally developed,
4. elongated // the axis c.
Besides one crystal, thick tabular // (001),
is met with.
The following forms are observed:
always developed: b (010), c (001), m (110), M. (lTO), f (130), z (130),
p (111), o (111), (Fig. 7)
commonly —»— : y (201),
rarely " (021), x (TOl).
The form (221) is frequently developed as corrosion-form probably
together with x (101).
123
Fig .7. Crystal of albite Nat. size.
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