Melilite- and Leucite-Bearing Systems - Origin of Potassium-rich Silica-deficient Igneous Rocks - page 311

10.2 Melilite- and Leucite-Bearing Ma c and Ultrama c

Rocks Containing Nepheline

10.2.1 The System Diopside Nepheline Akermanite Leucite

- - -

Compositions of many simpli ed potassium-rich nepheline-bearing rocks such as

nephelinite, melilite nephelinite, leucitite, melilite italite, melilite leucitite and

melilite - nepheline leucitite can be represented by the system diopside - nephe-

line leucite. In the system, forsterite appears as a phase because of a

reaction relationship between diopside and nepheline (Bowen 1922; Schairer et al.
1962). Study of the system is thus important to understand the paragenesis of many
other ma

akermanite

- -

nepheline
leucitite, nepheline-bearing katungite, and olivine leucitite. Gupta et al. (1973)
studied this system and found that all phases appearing in this system are solid
solutions. They established that nepheline incorporates variable amounts of kalsilite
and excess silica; melilite contains soda-melilite and gehlenite; forsterite incorpo-
rates monticellite, diopside contains Ca-Tschermak`s molecule and leucite incor-
porates nepheline in solid solutions.
The system is bounded by four limiting joins

c and ultrama c volcanic rock types such as olivine melilite

- -

1. diopside nepheline akermanite,

- -

2. diopside akermanite leucite,

- -

3. diopside nepheline leucite, and

- -

4. nepheline akermanite leucite.

- -

10.2.1.1 The Join Diopside - Nepheline - Akermanite

This join (Onuma and Yagi 1967) is a pseudoternary (Fig. 10.6 ) with two piercing
points; one is located at Di 55 Ak 6 Ne 39 and 1,212

± °

C (G), where forsterite ss ,
melilite, diopside ss , and liquid are in equilibrium, and the other at Di 38 Ak 3 Ne 59 and
1,169

3

C (H), where forsterite ss + nepheline ss + melilite + liquid coexist. The
assemblage forsterite ss , diopside ss , nepheline ss , melilite, and liquid are in equilib-
rium at 1,135

°

° C. Forsterite ss has a reaction relationship with liquid. The join

diopside liquid has already been discussed (Fig. 10.1 ). It has a piercing

point at Di 39 Ak 29 Lc 32 and 1,281

akermanite

- -

± 3 ° C.

10.2.1.2 The Join Diopside Nepheline Leucite

- -

The join was studied by Gupta and Lidiak (1973; Fig. 10.7 ), who found it to be
pseudoternary with two four-phase points. One at Di 60 Ne 8 Lc 32 and 1,275 C.

(A) where forsterite ss , diopside ss , leucite ss and liquid are in equilibrium, the second
at Di 27.5 Ne 29.5 Lc 43 , and 1,194

± 5 °

± 5 ° C (B), where nepheline ss , leucite ss and