Within increasing pressure of metamorphism, the pore spaces in which the fluid resides is reduced, and thus the fluid is driven off. Thus, no fluid will be present when pressure and temperature decrease and, as discussed earlier retrograde metamorphism will be inhibited. The chemical reactions involved in metamorphism, along with recrystallization, and growth of new minerals are extremely slow processes. Laboratory experiments suggest that the longer the time available for metamorphism, the larger are the sizes of the mineral grains produced. Thus, coarse grained metamorphic rocks involve long times of metamorphism.
Experiments suggest that the time involved is millions of years.
The preferred orientation of these sheet silicates causes the rock to easily break along the planes parallel to the sheet silicates, causing a slatey cleavage. Note that in the case shown here, the maximum stress is applied at an angle to the original bedding planes, so that the slatey cleavage has developed at an angle to the original bedding. Quartz and Feldspar grains, however show no preferred orientation. The irregular planar foliation at this stage is called schistosity . As metamorphic grade increases, the sheet silicates become unstable and dark colored minerals like hornblende and pyroxene start to grow.