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Earth and Space Sciences · Glossary

What is Invariant point?

Definition 10.8 University Earth and Space Sciences — Year 2 · Chapter 10 — Metamorphic Reactions and Facies

An invariant point is a point of the pressure–temperature plane where three or more phases of a system coexist in equilibrium and every univariant line among them meets; no variable is free there.

The Al2SiO5 diagram computed from the thermodynamic tables of Robie and Hemingway (1995) with temperature-dependent enthalpy and entropy (blue; dashed: metastable extensions), and the experimental diagram of Holdaway and Mukhopadhyay (1993) (grey, their tabulated points). Orange: the muscovite + quartz breakdown measured by Day (1973), which bends.
The AlX2SiOX5\ce{Al2SiO5} diagram computed from the thermodynamic tables of Robie and Hemingway (1995) with temperature-dependent enthalpy and entropy (blue; dashed: metastable extensions), and the experimental diagram of Holdaway and Mukhopadhyay (1993) (grey, their tabulated points). Orange: the muscovite + quartz breakdown measured by Day (1973), which bends.
Gibbs energies of andalusite and sillimanite relative to kyanite at 6\, kbar, from the same tables. Kyanite is stable while both curves are positive; sillimanite takes over at 624\, C. Andalusite crosses zero only at 692\, C, where sillimanite is already lower: above the pressure of the triple point, andalusite is never the most stable.
Gibbs energies of andalusite and sillimanite relative to kyanite at 6 kbar6\,\mathrm{kbar}, from the same tables. Kyanite is stable while both curves are positive; sillimanite takes over at 624 ∘C624\,{}^{\circ}\mathrm{C}. Andalusite crosses zero only at 692 ∘C692\,{}^{\circ}\mathrm{C}, where sillimanite is already lower: above the pressure of the triple point, andalusite is never the most stable.
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