Geology Reference
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Fig. 6.1 Breakdown of
leucite to K-feldspar and
kalsilite under pressure
(modi ed after Fasshauer
et al. 1998)
SWd+Ky+Cs=Kfs
S W d + K y = K f s + K l s
decreases with progressive increase of pressure. Figure 6.2 shows that compared to
the
field of leucite, produced as an incongruent melting phase at 1 atmosphere, that
observed for the K-feldspar
albite join studied at 0.1 and 0.2 GPa [P(H 2 O) = P
(Total)] progressively decreases. In case of the 0.2 GPa isobaric section of the join K-
feldspar
-
albite, it is vanishingly small. Morse (1969) observed that under 0.5 GPa
water pressure, the
-
field of leucite is completely eliminated. Incongruent melting of
sanidine (Fig. 6.3 ) was studied up to 4.0 GPa under dry condition by Lindsley (1966),
who found that the
±
°
C;
above this pressure sanidine melts congruently. It can be seen from the negative slope
that the melting of leucite produces a liquid of higher density.
field of leucite + liquid, persists up to 1.9
0.1 GPa at 1,440
(b)
(c)
(a)
Leucite
+ melt
Feldspar +
melt
1200
Leucite + melt
Leucite + melt
F eldspar +
melt
M
1000
Feldspar +
Leucite +
Feldspar +
melt
melt
800
One Feldspar
One Feldspar
One Feldspar
Two
Two
Two
600
Feldspars
Feldspars
Feldspars
K
20
40
60
80 NaK
20
40
60
80 NaK20 40 60 80 Na
Wt. % NaAlSi O
3 8
Fig. 6.2 Decrease in the field of leucite formed as an incongruent melting product of sanidine
(modified after Tuttle and Bowen 1958). a Dry, b PH 2 O = 0.1 GPa, c PH 2 O = 0.2 GPa
 
 
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