Chemical and Physical Constraints for Crystallization of Feldspathoids and Melilite in Potassium-Rich Rocks - Origin of Potassium-rich Silica-deficient Igneous Rocks - page 248

Fig. 6.9 Study of the
composition Di 40 Ne 60 under
variable P-T - T condition in
presence of excess water
(after Singh et al. 2000)

1.0

I G

A

Di+Amph
+L+V

0.8

Di
+
Amph
+
Ne
+
V

L+V

Di+L+V

Di+Ne+
Amph+L+V

0.6

H

0.4

Di+Ne+Mel
+Fo+L+V
Ne+L+V
Ne+Fo+Mel
+L+V
Fo+Mel+L+V
Fo+L+V

Di+Ne+L+
V

0.2

D

Di+Mel+Ne
+Amph
+L+V

B E

F

Di+Mel+Ne+V C

0
600

800

J

1200

1000

°

Temperature ( C)

appearance of nepheline at a lower temperature. At high pressure it may be
observed that the equilibrium assemblage is diopside ss + amphibole + nepheline ss
below the solidus. At low pressures between 0.1 and 0.15 GPa the assemblages
obtained progressively at lower temperatures are as follows:

(1) forsterite ss +L,
(2) forsterite ss + melilite + L,
(3) nepheline ss + forsterite ss + melilite + L,
(4) diopside ss + melilite + forsterite ss + nepheline ss + L, and
(5) diopside ss + melilite + nepheline ss .
Below 0.57 GPa as the temperature is systematically lowered the assemblages
are:

1. diopside ss +L,
2. diopside ss + nepheline ss +L,
3. diopside ss + nepheline ss + amphibole + L, and
4. diopside ss + amphibole + nepheline ss .
It is interesting to note that whereas forsterite appears as a primary phase below
0.1 GPa, diopside becomes the liquidus phase above this pressure. But there is a
small

E line at high temperatures at a
limited range of pressure. Melilite and forsterite are totally eliminated at
0.15

field of Diopside + nepheline + L above the D

-

0.05 GPa. The assemblage, characteristic of nephelinites (diopside ss +
nepheline ss + L), is restricted to a pressure below 0.57 but above 0.15 GPa and at a
temperature range of 880 and 805

±

° C.