8.3.1: Grain size and Porphyroblasts
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Textural changes take place as rocks undergo prograde metamorphism, and rocks may develop metamorphic fabrics. A general coarsening of grain size is typical as small mineral grains recrystallize to form larger ones. This is Ostwald ripening in action (refer to the discussion in Section 4.4.2, Chapter 4). While minerals that are already present recrystallize, new metamorphic minerals may grow and modify rock texture. If minerals develop into large crystals that contrast in size with other minerals in a rock, we call the large crystals porphyroblasts. Fine-grained material around the porphyroblasts is the groundmass. The garnets in Figure 8.23 are good examples of porphyroblasts surrounded by groundmass.