Geology Reference
In-Depth Information
7.3 Genesis of Pseudoleucite with Reference
to Nepheline
Kalsilite
Silica System
-
-
The name pseudoleucite refers to an intergrowth of nepheline and K-feldspar within
a trapezohedron crystal in the shape of leucite. Bowen and Ellestad (1937)
explained the genesis of pseudoleucite, with reference to the system, nepheline-
kalsilite-SiO
2
(Fig.
7.10
). They plotted the composition of a typical leucite with 1.5
wt% Na
2
O (point X, Fig.
7.10
). They thought that after its formation, if a leucite of
composition X, reacts with the liquid of composition R (Fig.
7.10
), it should yield a
solid product of composition similar to the pseudoleucite as its composition lies
close to the line XR near point Y, the ratio of the two reactants, liquid and leucite
where should be equal to the ratio of XY/YR. Nepheline and K-feldspar produced
by the reaction should have a composition given by points U and V, respectively. If
from a liquid of composition X some leucite is subtracted as cooling continues the
composition of the liquid may not reach point R, but may reach either curve WR or
else curve MR. If the liquid reaches WR, leucite should be transformed by reaction
to K-feldspar only, and if it reaches the curve MR it would react with the liquid to
form nepheline.
Larsen and Buie (1938) studied pseudoleucite crystals from Highwood Moun-
tains), Montana. They thought that the hypothesis of Bowen and Ellestad should
involve a slow reaction process; but in the Highwood Mountains, some of the
dykes, where pseudoleucites are found, the groundmass is very
fine- grained, and
indicated rapid crystallization. They further thought that in such a reaction process
between leucite and liquid, the trapezohedral form with such a sharp outline will be
completely destroyed.
Fig. 7.10 Plot of the
compositions of pseudoleucite
in the system
nepheline
SiO
2
Cristobalite
SiO
2
(after Bowen and Ellestad
1937)
kalsilite
-
-
Tridymite
W
50
50
V
Ab
Or
X
R
Y
Lc
Leucite
M
Na, K Nepheline
Orthorhombic
U
Carnegieite
ss
NaAlSiO
4
[(K, Na)AlSiO
4ss
KAlSiO
4
V
50
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