Incompatible Mineral Pairs in K-Rich Rocks - Origin of Potassium-rich Silica-deficient Igneous Rocks - page 276

1700

1600

1500

Lc +

+L

OKs

L

1400

ss ss

Lc +L

ss

1300

Lc +Ne +L

ss ss

1200

Lc + OKs

ss ss

11 00

A

Felds + L

B Lc

s ++

Ne Felds+L

ss

1000

Lc

s ++

Ne Felds

Felds+Ne

ss +

L

C

ss

Lc + Felds+OKs

Felds + Ne ss ss ss

900

10 20 30 40 50 60 70 80 90

NaAlSi O 8
3 KAlSi 26

(Leucite)

Wt. %

(Albite)

Fig. 8.3 Phase diagram of the Join leucite albite at 1 atm (after Gupta and Edgar 1975)

-

for the same chemical composition in the NaAlSiO 4 - SiO 2 system, where

leucite, nepheline, and feldspar coexist with liquid. Point C (990

KAlSiO 4 -

± 5 ° C) is the

intersection of solidus in the system and the boundary curve of the field, where

leucite and feldspar are naturally stable phases. Reference to Fig. 8.3 shows that
nepheline ss and alkali feldspar coexist in equilibrium in the compositional range of
Ab 100 and Ab 59 Lc 41 , and leucite does not coexist with albite; but leucite ss , feldspar
and nepheline ss are in equilibrium between Ab 59 Lc 41 and Ab 17 Lc 83 . Calibration of
alkali feldspar composition as a function of cell volume was determined using
X-ray crystallographic technique (Orville 1967); it is observed that feldspars
coexisting with leucite ss are monoclinic, and maximum amount of albite in the
feldspars, crystallizing from a mixture Ab 59 Lc 41 , is 55 mol%. With progressive
increase in leucite content in the bulk compositions, albite content of these feldspars
further decreases. Thus, in the compositional range, where leucite and feldspar are
in equilibrium the feldspar composition varies between Or 55 Ab 45 .
As shown in Fig. 8.3 , the extent of solid solution of albite in leucite is very
small, but increases with increasing temperature up to Lc 83 Ab 17 and 1,100 C,

where leucite ss coexist with orthorhombic kalsilite ss . A slight increase in the
interplanar spacings (211, 004, 400) of leucite, crystallised from bulk composition
Lc 95 Ab 5 and Lc 90 Ab 10 compared to those of its pure synthetic equivalent, and a
decrease in those of (132 and 202) in kalsilite from the same compositions relative

°