The mineralogy of rocks changes according to the pressure and temperature conditions in which they are exposed. Consequently, at the surface where we collect them, the rocks should only consist of Low Pressure and Low Temperature minerals. This is not the case for Metamorphic Rocks , which preserve the high pressure and temperature conditions they have been exposed to. And that's lucky for me; otherwise I'd be out of work!

In fact, the mineralogiy of these rocks is preserved out of their stability field ... as the water in Lake Tilicho ... The mineralogical association is said to be metastable.

In this month of November, the diurnal temperature is about -5°C at 5000m altitude in Lake Tilicho (Nepal).

In such conditions, snow is of course the stable form of H2O. nevertheless, the lake is not frozen and the water persists, in a metastable state! This is because the rate of the freeze-melt reaction of the water-snow system is very low at this temperature.

The same is true for rocks: the reaction rate, which is responsible for the mineralogical changes, varies with temperature. When the temperature is low, as at the earth's surface, the speed becomes almost zero: the mineralogy of the metamorphic rocks is then preserved (in a metastable state).

This metastability can also be observed on the scale of the thin section, within a rock. In this metapelite (below) ...

... kyanite (Ky) and sillimanite (Sill) in its fibrous form (fibrolite) coexist without any evidence of destabilisation according to the polymorphic reaction Ky = Sill.
(the white crystals in this microphotography under crossed-polarised light are quartz and plagioclase crystals while the brown mineral is biotite mica ; photo approx. 2 mm wide)

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