Gluing or using mortar for laying granite tiles on countertop?

I am in the process of using granite tiles for a counter top. I am using 2 layers of 3/4 " plywood under the granite but was wondering if I should be using a glue under the granite instead of the thin set mortar. Is there a special glue to use or can I just use the carpenter glue on top of plywood. I was thinking that if I used mortar it may dry out over time if warm items are placed on countertop.