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Porphyritic Granite Porphyritic Granite Porphyritic granite Porphyritic granite is an igneous rock, which consists of orthoclase feldspar, quartz and orthoclase. This type of rock is made up of two different grain sizes, larger crystals referred to as phenocrysts, and smaller granite sized grains. The name for porophyritic originated from the Latin word porphyry which is.