ENE Exam 2 Practice

When rocks undergo metamorphism, the minerals present are driven by the conditions of temperature and pressure. As the grade increases, minerals that were stable at low temperatures and pressures react and transform into new minerals that are stable under higher temperatures and pressures. This neocrystallization means the high-grade rock typically develops a different mineral assemblage from its low-grade counterpart, even though they started from the same protolith. Some minerals from the low-grade rock may persist, but many are consumed and replaced by minerals characteristic of higher-grade conditions, so they do not share all the same minerals. Fluid content can influence the exact minerals that form, but the main reason for different mineral sets is the change in stability with increasing metamorphic grade.