If a rock containing andalusite and sillimanite were to undergo an increase in pressure what must happen to temperature in order for that rock to continue to have stable andalusite and sillimanite?

The temperature must decrease

Explanation:

The three minerals namely the andalusite, kyanite, and sillimanite are characterized by the following ways-

Andalusite is a mineral that forms at low pressure and low-temperature conditions. Kyanite is a mineral that forms at high pressure and low-temperature conditions. Sillimanite is a mineral that forms at high pressure and high-temperature conditions.

In the given condition, when a rock comprising of andalusite and sillimanite mineral is allowed to experience high and increasing pressure, and in order to continue to have these minerals stable in it, the temperature factor must be decreased. This way, the minerals can be consistently present in the rock, otherwise, metamorphism of rocks will take place and will alter it to a high-grade metamorphic rock.