Incompatible Mineral Pairs in K-Rich Rocks - Origin of Potassium-rich Silica-deficient Igneous Rocks - page 275

Fig. 8.2 Joins in the
Ks - Fo - Qz system as a
function of pressure for
T = 1,100 ° C. Deduced by
Yoder (1989) from data of
Schairer (1954), Luth (1967)
and Wendlandt and Eggler
(1980)

(a) (b)

Lc Sa Sa

Ks Lc Qz

Ks Qz

En

En
P < 3 Kb

P = 3-20 Kb

Fo

Fo

(c) (d)

(Lc)

Sa (Lc)

Ks Qz Ks Sa Qz

En En

P > 20 -23 Kb

P > 23 Kb

Fo Fo

8.2 Incompatible Relation Between Leucite
and Sodic-Plagioclase

Feldspar coexisting with leucite is either calcium-rich plagioclase or a potassium-
rich feldspar (Shand 1943). Albite and leucite are not found to coexist in equilib-
rium in ultrapotassic rocks (Cross et al. 1902).
On , however, Miyashiro (1960) postulated that the

apparent incompatibility between leucite and albite might not exist at high tem-
peratures. MacKenzie and Rahman (1968) also described veins of leucite and
sodium-rich feldspar in a basanite from the Massif Central, France, but they did not
con

' thermochemical grounds '

rm this as a stable assemblage.
Gupta and Edgar (1975) studied the phase relations in the system leucite

albite
and leucite - albite - anorthite at atmospheric pressure. The following discussion
should highlight the causes of their incompatibility.

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8.2.1 Phase Relations in the Join Leucite - Albite
under Atmospheric Pressure

Phase relations in the leucite albite join (Gupta and Edgar 1975) is summarised in

Fig. 8.3 , which indicates that the join is a pseudo-binary, and is part of the ternary
system nepheline

-

C) represents a
pseudoeutectic point, where leucite and feldspar coexist with liquid. The temper-
ature of the pseudoeutectic point is in good agreement with that of Schairer (1950)

kalsilite SiO 2 . Point A (Lc 41 Ab 59 , 1,068 ± 5 °

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