Incompatible Mineral Pairs in K-Rich Rocks - Origin of Potassium-rich Silica-deficient Igneous Rocks - page 273

Chapter 8
Incompatible Mineral Pairs in K-Rich
Rocks

The mineral pairs orthopyroxene - leucite, leucite - albite, and melilite - plagioclase

are not observed together in natural K-rich silica-de cient igneous rocks formed

under volcanic to subvolcanic conditions. The probable cause of incompatible
relationship for these pairs are described in this chapter.

8.1 Incompatibility Between Leucite and Orthopyroxene

Wendlandt and Eggler (1980b) studied the reaction, sanidine + forsterite leu-

cite + enstatite. They observed that the univariant equilibrium curve is stable at the
following pressure

≈

temperature conditions: (0.5 GPa, 1,150 ° C), (0.25 GPa,

-

1,040 ° C) and (0.1 GPa, 1,040 ° C). They also established that the reaction leu-

cite + enstatite forsterite + liquid may set the upper limit of coexistence of leucite

and enstatite. This univariant reaction occurs at following pressures and tempera-
tures: (0.25 GPa, 1,175

≈

C). Thus, under volatile-free
condition, orthopyroxene and leucite may coexist in an extremely narrow

° C) and (0.5 GPa, 1,150 °

field, but

investigation of Wendlandt and Eggler shows that below 1,000 C leucite and

enstatite reacts to form sanidine and forsterite; and this reaction may inhibit the
presence of leucite and enstatite in volcanic rocks crystallised at temperature below
1,000

°

C, when the lava cools down slowly to room temperature.
In their study of the system forsterite

°

CO 2 , Wendlandt and
Eggler (1980b) observed that in presence of volatile, following reactions would take
place:

- kalsilite - SiO 2 - H 2 O -

1. phlogopite + enstatite + sanidine + vapour forsterite + liquid (0.2 GPa and

→

C),
2. phlogopite + sanidine + vapour

950 ° C; 0.5 GPa and 975 °

forsterite + (leucite + liquid (0.5 GPa and

→

1,000 ° C and 0.2 GPa and 975 ° C).

Thus, in presence of H 2 O and CO 2 the above two reactions are stable and leucite
and orthopyroxene should not coexist under equilibrium condition. On the basis of
the data for the plane forsterite

kalsilite quartz provided by Wendlandt and Eggler

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