Here's why:
* Garnet is a relatively high-grade metamorphic mineral. This means it forms under higher temperatures and pressures than the minerals found in slate and phyllite.
* Schist and Gneiss form under higher metamorphic grades than slate and phyllite. The intense heat and pressure allow for the formation of minerals like garnet, which are not stable at lower grades.
* Slate and Phyllite are lower-grade metamorphic rocks. They are formed from shale and mudstone under relatively low temperatures and pressures. Minerals like chlorite, mica, and quartz are more common in these rocks.
Therefore, garnet is a good indicator of the metamorphic grade of a rock. Its presence signifies that the rock has been subjected to higher temperatures and pressures than slate or phyllite.
