I don't know if this helps, but if water has specific heat 1.0 cal/(g*K), then grain is about 0.4 cal/(g*K). These are the numbers you use in the mash.
If you're looking for wort specific heat, this is a pretty good approximation...Assume the wort is just water and sucrose (this is what the Brix/Plato scale assumes). Specific heat of sucrose is 0.3 cal/(g*K). So, the specific heat of a solution that is P degrees Plato is 1-0.7*(P/100) cal/(g*K).