The concept of non-equilibrium phase diagrams is .explored and an example presented for the quartz-coesite reaction. The equilibrium phase boundary is relevant for systems undergoing mineral reactions only for very high temperatures and very low strain rates. The influence of strain rate is to modify the equilibrium Clausius-Clapeyron slope by a factor that is similar in magnitude to that slope, Increases in strain rate remove the non-equilibrium phase boundary further from the equilibrium phase boundary. This explains the common observation that mineral reactions proceed to completion in shear zones rather than in adjacent undeformed material.

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