P-T Stability of Phlogopite, K-Richterite and Phengite, as a Source of Potassium in the Mantle - Origin of Potassium-rich Silica-deficient Igneous Rocks - page 353

Fig. 12.5 P-T projection of the reactions investigated in the join KAlSiO 4 SiO 2 .

Reactions associated with I 1 are from Lindsley (1966). The 1 atm results are from Schairer (1954)
and Schairer and Bowen (1955). Because the diagram is a projection, the data points must also be
projected, and there may be numerous runs involving different compositions corresponding to a
pressure and temperature. Dashed lines are inferred. En enstatite, Fo forsterite, Ks kalsilite, Lc
leucite, Sa sanidine, L liquid, I invariant point, S singular point. (after Wendlandt and Eggler 1980)

Mg 2 SiO 4

- -

vapour-saturated assemblages because of the following subsolidus reactions that
passes through I 8 .
The reaction enstatite + magnesite forsterite + vapour (24), has been studied

by Newton and Sharp (1975). According to them at pressures higher than I 8 the
CO 2 -saturated phase assemblage melts by the following reaction:
1. enstatite + kalsilite + magnesite + vapour

⇌

⇌ liquid, and the vapour absent

assemblage by the reaction,
2. enstatite + magnesite + kalsilite ⇌ forsterite + liquid.
Liquids at I 8 and higher pressures are carbonate-rich, but the exact composition
of the melts are unknown.
They noted that near 1.4 GPa, for CO 2 saturated conditions and near 1.9 GPa for
volatile-absent conditions, the primary phase field of enstatite crosses the sanidine-