Final answer:
Granite and rhyolite are the two igneous rocks with the same composition but differing crystal sizes, with granite being coarse-grained and rhyolite being fine-grained.
Step-by-step explanation:
Igneous rocks are categorized by their chemical composition and texture, related to their mineral content and grain size, respectively. The two igneous rocks that have the same composition but differ only in terms of crystal size are granite and rhyolite.
Granite is a coarse-grained, intrusive igneous rock, which means it cooled slowly beneath the Earth's surface. Rhyolite, on the other hand, is a fine-grained, extrusive igneous rock formed from lava that cooled quickly on the surface. Despite their differing textures, both granite and rhyolite share a felsic (or silicic) mineral composition predominantly made up of quartz, muscovite, potassium feldspar, and occasionally biotite and Na-rich plagioclase.
The reason for their different crystal sizes is the rate of cooling they experienced. Granite formed intrusively with a slow cooling process allowing large crystals to grow, while rhyolite cooled rapidly after erupting as lava, leading to the formation of much smaller crystals.