Melilite- and Leucite-Bearing Systems - Origin of Potassium-rich Silica-deficient Igneous Rocks - page 306

Fig. 10.1 Phase relation in
the join diopside -
akermanite - leucite at 1
atmosphere (after Gupta
1972)

(Diopside)
CaMgSi 26

Di ss

1302 0 Cb

1361 0 C c

1281

B

Ak ss

Lc ss

v
KAlSi 26
(Leucite)

v v v v v

v v v

a

Ca MgSi O
2 2 7
(Akermanite)

and wollastonite ss within melilite and below 1,240 ° C, wollastonite ss is absent on

the liquidus of the diopside leucite join. As the compositions of the

crystalline phases do not lie within the join, it is not ternary and point B is therefore,
a piercing point and the lines dividing the primary phase

akermanite

- -

fields are traces of

divergent surfaces cut by the diopside akermanite leucite join.

- -

10.1.1.2 The Join Forsterite Diopside Leucite

- -

Figure 10.2 shows the phase diagram (Gupta 1972) for the join forsterite diop-

-

side leucite. At point c (Fo 3 Di 60 Lc 37 ), forsterite ss + diopside ss + leucite ss coexist in

equilibrium at 1,296

-

± 3 ° C. Again the join is not ternary and the point c is a

piercing point.

10.1.1.3 The Join Forsterite - Akermanite - Leucite

Phase diagram of the join (Gupta 1972) is presented in Fig. 10.3 , which shows that
it is pseudoternary and cuts the phase volume of monticellite ss . The join has two
piercing points at Fo 17 Ak 78 Lc 5 and 1,428

C(H), where the assemblage is forste-
rite ss + akermanite ss + monticellite ss + liquid; and Fo 9 Ak 43.5 Lc 47.5 and 1,286

°

C
(G), where the assemblage is forsterite ss + akermanite ss + leucite ss . At H, montic-
ellite ss reacts with the liquid to form akermanite and is completely eliminated at
1,410

± 3 °

± 3 ° C. Therefore, monticellite ss does not coexist with leucite ss .