Geology Reference
In-Depth Information
Fudali (1963) studied a selected part of the system nepheline
kalsilite
SiO
2
at
-
-
0.1 GPa P(H
2
O) and elevated temperatures. His study was con
ned to KAlSi
2
O
6
-
NaAlSi
2
O
6
join of the system in presence of excess water at 0.1 GPa. The phase
equilibrium relation studied by him is summarized in Fig. 2.1 (Chap.
2
), which
demonstrates that the join cuts through the primary phase volumes of leucite
ss
and
nepheline
ss
. In the leucite-rich section, he found that between 800 and 820
°
C there
is a
C the liquid phase is
eliminated (Fig. 2.1, Chap.
2
). In the nepheline-rich side of the join, there is a
primary phase
field of leucite
ss
+ nepheline
ss
+ L; and below 800
°
field of nepheline
ss
, which is followed at low temperature by the
appearance of the assemblage, nepheline
ss
+K
Na feldspar + liquid, and below the
curve a-b, the liquid phase is eliminated. Reference to Fig.
2.1
, it may be found that
from a liquid of composition X, leucite
ss
of composition A should crystallize at
1100
-
C, the liquid phase is eliminated, and
only leucite
ss
should appear. However, below 700
°
C, as the temperature is lowered to 815
°
C it should yield nepheline, K-
feldspar and a leucite phase should also coexist in equilibrium. Fudali (1963) con-
sidered that a pseudoleucite is formed by breakdown of a sodium-rich leucite
ss
.He
criticized the view of Bowen and Ellestad and considered that if pseudoleucite
formation is related to solid-liquid reaction, then the replacement should be from the
rims inward; however, the reverse case was actually observed by him. Based on
experimental observations, he even thought that presence of viscous liquid coexis-
ting with leucite may inhibit the formation of pseudoleucite (Fudali 1963, p. 1110).
Watkinson (1973) studied pseudoleucites from Lakner Lake and Prairie Lake in
Ontario, where they are associated with nepheline and feldspar. He supported the
hypothesis of Fudali (1963), and concluded that these pseudoleucites were pro-
duced by the breakdown of a sodium-rich leucite at low P (H
2
O).
A mixture of composition Ne
25
Ks
45
Qz
30
was studied at 840
°
C and 0.2 GPa in
presence of excess water by Taylor and Mackenzie (1975), which was cooled under
different conditions:
°
first, they cooled the pressure vessel with sample capsule in presence of
compressed air. This led to the formation of leucite with glass and vapour. The
leucites were found to have exsolved phases, which they concluded to be
analcitic in composition. Such leucites have 11 wt% soda leucite molecule in
solid solution.
ii. In the second instance, cooling of the vessel was done slowly with fall of
temperature and pressure. This yielded zoned leucite crystals in equilibrium
with glass and vapour. According to them the rim had composition similar to
that of pseudoleucite.
iii. In the last case, the vessel was cooled in two stages
i. At
first in air or keeping the
vessel inside the furnace and then cooling it from 840 to 630
°
C and then
°
holding it at 0.2 GPa and 630
4 h. In this case the rim of leucite
contained 33 wt% of sodium-rich leucite, but the core had exsolution texture
containing primarily leucite and analcite.
C for 2
-
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