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2.5
Cpx
+
L
+
V
Cpx
+
Ph
+
L
+
V
2.0
L
+
V
Cpx
+
Ph
+
Ga
+
V
Cpx + Ph + Ga + L + V
1.5
1.0
600
700
800
900
1000
1100
1200
T
o
Cpx + Ph + Ga + V
Cpx + L + V
L + V
Cpx + Ph + Ga + L + V
Cpx + Ph + L + V
Fig. 13.1 P(H
2
O)-T diagram of a synthetic leucite basanite (Fo
20
Di
30
Lc
30
An
20
) (after Gupta et al.
1976) Abbreviation same as in Table
13.1
; Ga: garnet, L: liquid, V: vapour
13.2.2 Investigation on a Synthetic Melilite Nepheline
Leucitite in Presence of Excess Water
A synthetic melilite-nepheline leucitite (Di
28
Ne
29
Lc
43
) was heated at different
temperatures (650
0.5 GPa in presence of excess water)
by Gupta et al. (1976). The starting material was crystallized from a glass of its
composition at 900
850
°
C) and pressures (0.1
-
-
C under 1 atmospheric pressure. Appearance of melilite and
olivine in this composition is related to the reaction between nepheline and diopside
(Bowen 1922a; Schairer et al. 1962). With the mixture Di
28
Ne
29
Lc
43
, studied under
1 atm, leucite was the liquidus phase at 1,197
°
°
C under atmospheric pressure and
was followed by the appearance of forsterite
ss
(1,194
°
°
C), nepheline
ss
(1,185
C),
°
°
melilite (1,178
C). Forsterite
ss
disappeared at
1,160
±
10
°
C, and the final assemblage corresponds to a melililite nepheline
leucitite. The composition of melilites and nepheline
ss
are as follows: Ak
70
Sm
30
and Ne
68
Ks
29.8
Si
2.2
, respectively.
Experimental study of Gupta et al. (1976) on this composition is summarized in
Fig.
13.2
, which shows that melilite disappears at or above 0.3 GPa (AB). Break-
down of leucite occurs at slightly higher pressure (CD, Fig.
13.2
). Phlogopite and
pyroxene coexist with liquid above DEF, below which phlogopite disappears at low
P(H
2
O) by reaction with a liquid; thus, supporting the
C) and
finally diopside
ss
(1,100
findings of Luth (1967). The
reaction, leucite
kalsilite + K-feldspar (CE, Fig.
13.2
), has been studied by Scarfe
et al. (1966; see Fig. 6.1). The results of this investigation suggest that leucite and
melilite-bearing rocks are strictly con
→
ned to volcanic and subvolcanic conditions.
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