Most metamorphic reactions are very slow or sluggish, especially at low temperatures.
This is evidenced by the preservation of relict minerals and textures in low grade rocks. It is therefore very important to identify the scale at which minerals have equilibrated in very low grade metamorphic rocks, as such scales may be much smaller than those of a thin section. In terms of the relative rates of metamorphic reactions, prograde reactions are faster than retrograde ones. This is evidenced by the common preservation of mineral assemblages that form under peak metamorphic conditions, as opposed to the rarity of assemblages that formed prior to peak conditions. Higher rates of diffusion of elements will lead to faster reaction rates.