The Main factor that determines the texture of an igneous rock is the cooling rate other factors that are involved are:

• The Rate of nucleation of new crystals - The rate at which enough of the chemical consitituents of a crystal can come together in one place without dissolving

• The Diffusion Rate - The rate at which atoms or molecules can move (diffuse) through the liquid.

• The rate of growth of crystals - The rate at which new constituents can arrive at the surface of the growing crystals. This depends largely on the diffusion rate of the molecules of concern.