Where does Sillimanite form

I would like to know where the nesosillicate Sillimanite forms, what depth and pressure, the environment (like if its a subduction zone) and what other mierals form with it. Also, how is it an index mineral?

It forms in high-temperature, high-pressure metamorphic environments.

It occurs with andalusite, kyanite, and garnets.

It is an index mineral in metamorphic rocks because there is a certain pressure-temperature stability field for it (see below).