Temperature control

For doing a project I face some unknowns.

The one is that I want to simulate crack propagation in shale (a type of
sand) and investigate crack pattern in shale if I decrease the temperature by LIGGGHTS. I need to know how I can decrease it (temperature of the simulation box)?

Second is that do you know the material properties of shale?

The easiest way is to fix the temperature of some particles, I need to know how I can set it to a certain temperature every few time-steps?

Thanks a lot for your attention in advance.

Sincerely,
Rasoul