Melilite- and Leucite-Bearing Systems - Origin of Potassium-rich Silica-deficient Igneous Rocks - page 305

Chapter 10
Melilite- and Leucite-Bearing Systems

10.1 Melilite- and Leucite-Bearing Systems
Without Nepheline

10.1.1 The System Forsterite - Diopside - Akermanite - Leucite

The four essential minerals of many potassium-rich ma c rocks are

leucite, augite, olivine, and melilite. The bulk compositions of these simpli

c and ultrama

ed

rocks lie within the phase volumes of forsterite leucite. Phase

relation within the system is thus applicable to the genesis of potassium-rich ma

diopside akermanite

- - -

c

and ultrama c volcanic rocks. A detailed study of the system was made by Gupta

(1972), who showed that all phases in this system are solid solutions. Forsterite
may contain monticellite; and diopside and akermanite possibly incorporate
Ca-Tschermak s molecule and gehlenite in solid solution, respectively. Likewise,

leucite may include K 2 O

'

5SiO 2 and forsterite probably contains monticellite.
The system is bounded by four ternary joins:

· MgO ·

(1) diopside akermanite leucite,

- -

(2) forsterite diopside leucite,

- -

(3) forsterite - akermanite - leucite, and

(4) forsterite - diopside - akermanite.

Phase relations in these joins are described below.

10.1.1.1 The Join Diopside Akermanite Leucite

- -

Phase equilibria diagram of the join (Gupta 1972) is presented in Fig. 10.1 . At point
B (Di 39 Ak 29 Lc 32 ), diopside ss + akermanite ss + leucite ss + liquid coexist in equi-
librium at 1,281

C. In their study of the thermal behaviour of pure synthetic
akermanite, Schairer et al. (1967) found well-distributed inclusions of diopside ss

± 3 °