Stability of phlogopite and quartz

KMg AlSi 0 q(0H)2 + (3+n) SiO xKAlSi O + 3MgSiO phlogopite quartz sanidine 

Abbreviations as given in Table 3. Projection from H O onto KAlSiO -Mg SiO -SiO plane. Reaction labels are given in text. Solid lines represent stable reactions dotted lines are metastable extensions of the stable portions of reactions the dashed line represents a condensed reaction in which no vapor participates. Reaction labels are given in text. 

The negative slope of curve (Sa) indicates that the H2O content of the melt (in mole percent) is greater than that in the solid assemblage phlogopite + 3 quartz. This is in agreement with Shaw s (1963) estimate of the H2O contents of the melt. 

Two other univariant reactions which intersect the invariant point are  

Si02 + MgSiO + H O J melt quartz enstatite gas liquid 

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