The blueschist facies metamorphism has always been one of the most difficult to quantify. This is for two important reasons. Firstly, the temperatures and pressures of the facies tend to be inaccessible to experimental analogues of the blueschist indicator minerals: aragonite, glaucophane, jadeite and lawsonite. Secondly, the conditions of blueschist metamorphism are not typical of the earth's crust. For a rock to be metamorphosed under lower temperatures at a given pressure than those of normal geothermal gradients (considered here as negative deviation metamorphism) requires ‘rapid’ downwarping and reuplifting. The movement during metamorphism, combined with hydration rather than dehydration (a feature of this environment) fails to preserve a single metamorphic culmination and hence the mineralogy and textures of blueschist assemblages may fail to match the conventional phase equilibrium metamorphic model. Thus the simple phase equilibrium interpretative basis of metamorphic petrology is recognized as only a special case of a more general theory of metamorphic chemical/mineral interaction.