Which mineral is commonly used as an indicator of metamorphic conditions?

Garnet is commonly used as an indicator of metamorphic conditions due to its stability and specific growth conditions during metamorphism. When rocks undergo metamorphic processes, minerals form under varying temperatures and pressures. Garnet, in particular, can help geologists determine the metamorphic grade of a rock because it forms at higher temperatures and pressures.

The presence of garnet can indicate the specific conditions under which the rock formed, making it an effective marker for understanding the thermal and pressure history of metamorphic rocks. This is in contrast to the other options; while minerals such as quartz, calcite, and feldspar are important in various geological processes and can be found in metamorphic rocks, they do not provide the same level of detail about the metamorphic conditions. They either form under a wider range of conditions or are more commonly associated with different types of rocks, rather than serving as reliable indicators of metamorphic grade.