Final answer:
The grain size in metamorphic rocks typically increases with metamorphic grade. High-grade metamorphic rocks like gneiss showcase how intense heat and pressure cause minerals to recrystallize into larger grains and self-segregate into distinct bands.
Step-by-step explanation:
The grain size in metamorphic rocks typically increases as the metamorphic grade increases. As rocks are subjected to higher temperatures and pressures, minerals recrystallize and grow larger. For example, slate can metamorphose into phyllite, and then into schist as the grade of metamorphism increases, each stage showing larger mineral grains than the previous.
In the case of high-grade metamorphic rocks such as gneiss, metamorphism has been so intense that minerals have self-segregated into discrete zones, exhibiting a banded appearance due to the alternation of light and dark minerals. This reflects a significant increase in grain size compared to lower-grade metamorphic rocks. The progression from slate to phyllite to schist to gneiss with increasing metamorphic grade illustrates the direct relationship between metamorphic grade and grain size.