Melilite- and Leucite-Bearing Systems - Origin of Potassium-rich Silica-deficient Igneous Rocks - page 315

10.2.1.4 Paragenetic Relationship
in the System Diopside - Nepheline - Akermanite - Leucite under
Atmospheric Pressure

Of the two piercing points in the system diopside akermanite (Onuma and

Yagi 1967), point H (Fig. 10.6 ) has a relatively lower liquidus temperature. This point
was therefore, chosen as one of the end members to which leucite was progressively
added. The join was studied to determine the temperature of the six-phase assem-
blage, forsterite ss +diopside ss +akermanite ss +leucite ss +nepheline ss +liquidinthe
system diopside

nepheline

- -

leucite, and the phase equilibrium diagram is
presented in Fig. 10.11 . The join cuts the primary phase volumes of forsterite ss and
leucite ss . The pseudoeutectic occurs at (Di 38 Ak 3 Ne 59 ) 61 Lc 39 and 1,130

nepheline akermanite

- - -

C, where
forsterite ss ,leuc te ss and liquid are in equilibrium. Figure 10.11 shows that at
1,105

± 5 °

C, forsterite ss +diopside ss + melilite + leucite ss +nepheline ss coexist with
liquid. Forsterite starts to react with liquid and is eliminated at 1,075 o

± 5 °

± 10 ° C, and

the subsolidus assemblage is Di ss +Mel+Ne ss +Lc ss .
Experimental results on the join nepheline leucite, the pseudobinary

join (Di 38 Ak 3 Ne 59 ) 100-x Lc x and the other three bounding joins shows that the system
diopside

- akermanite -

- nepheline - akermanite - leucite is pseudoquaternary. If however, the

'

presence of small amounts of alumina as Ca-Tschermak s molecule in diopside and

the gehlenite molecule in melilite is ignored, the system can be treated as a quaternary
join of the

five component system, nepheline kalsilite CaO MgO SiO 2 .A ow

- - - -

sheet diagram of the system is shown in Fig. 10.12 , where only the ve-phase

univariant lines are shown. At A (Fig. 10.7 ), forsterite ss , diopside ss , leucite ss and liquid
are in equilibrium at 1,275 ± 5 ° C. Gupta and Lidiak (1973) found that with further

Fig. 10.11 Phase relation in
the join (Di 38 Ak 3 Ne 59 ) 100-x -
Lc x at 1 atmosphere (after
Gupta et al. 1973)

0 C

1200

1150

Fo +L

ss

Fo s +Mel
+Ne ss
+L

1100

1050

1000

10 20 30 40 50

-x

(Di +Ak +Ne )100

38 3 59

Wt.% Lc