Texture In Igneous Rocks The isolated large grains are phenocrysts sf a porphyry is an igneous rock in which 50 or more of the rock is coarse mineral grains scattered through a mixture of fine mineral grainsTexture In Igneous Rocks ◼ The isolated large grains are phenocrysts 斑 晶. ◼ A porphyry is an igneous rock in which 50% or more of the rock is coarse mineral grains scattered through a mixture of fine mineral grains