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?
A) temperature must increase
B) temperature must decrease
C) temperature must stay the same
D) It is impossible for it to continue to have stable andalusite and sillimanite.

Question

Quizplus · Accepted Answer

Andalusite and sillimanite are both polymorphs of the mineral Al2SiO5, meaning they have the same chemical composition but different crystal structures. As pressure increases on the rock, it will want to change into a denser structure to accommodate the pressure. Sillimanite is the densest of the three polymorphs (including kyanite), and at high enough pressure, andalusite will transform into sillimanite. However, such a transformation would require breaking and forming many new chemical bonds within the mineral, which is an endothermic process (it requires heat/energy). Therefore, in order for the rock to maintain stable andalusite and sillimanite as pressure increases, the temperature must decrease to provide the necessary energy for the transformations to occur.

If a Rock Containing Andalusite and Sillimanite Were to Undergo