Melilite- and Leucite-Bearing Systems - Origin of Potassium-rich Silica-deficient Igneous Rocks - page 310

10.1.1.7 Course of Crystallization of Liquid in the System
Forsterite - Diopside - Akermanite - Leucite

The system is pseudoquaternary, being a part of the quinary system K 2 O

-

Al 2 O 3 - SiO 2 . The composition of all crystalline phases lie outside the

tetrahedron. However, the range of solid solution in these phases is small, and the
system can thus be treated almost as quaternary. The complete course of
crystallization of liquid in this system is summarised by a

CaO MgO

- -

flow sheet diagram,
shown in Fig. 10.5 , in which rock nomenclatures are also given, It shows that an
olivine leucitite can be produced from a melilite leucitite, an olivine

leucitite, a katungite or an olivine melilitite. Reference to Fig. 10.3 suggests that a
katungite (point G) can be produced from an alnoite (point H). Schairer and Yoder
(1964), in their study on the system nepheline

melilite

- -

silica, showed that
a parental liquid for the sodium-rich undersaturated part may be represented by
either an olivine melilitite or an olivine nephelinite. Olivine

forsterite larnite

- - -

melilite nephelinite is
produced from liquids of these compositions. It appears that rocks called olivine
melilitite can differentiate towards either an olivine

-

- melilite nephelinite or an

olivine melilite leucitite. The Na 2 O/K 2 O ratio of the parental olivine melilitite melt

probably controls subsequent crystallization. If the liquid is potassium-rich, leucite-
bearing ma

-

c rocks would be produced, and if sodium-rich,
nepheline-bearing mafic and ultramafic rocks would crystallise.

c and ultrama

o

B 1281 C (Cuts Di-Ak-Lc)

1 280 o

L

Fo ss
Ak ss
Lc ss

Fo ss
Di ss
Lc ss

(Cuts Fo-Di- Lc)
1296 o

K (Cuts Fo-Ak-Lc)

o

1286 C

1295 o

o

C Katungite

(Fo -Di

1274 C G

BM 1255 o

-Lc -Ak )

ss ss ss ss
(Olivine-Melilite Leucitite)

Olivine Leucitite

1357 o

(Cuts Fo-Di-Ak)

Fig. 10.5 The flow-sheet and rock-nomenclature diagram of the system forsterite - diopside - ak-
ermanite - leucite (after Gupta 1972)