Geologists use the term porphyry only with a word in front of it describing the composition of the groundmass. This image, for instance, shows an andesite porphyry. The fine-grained part is andesite and the phenocrysts are light alkali feldspar and dark biotite. Geologists also may call this an andesite with porphyritic texture. That is, "porphyry" refers to a texture, not a composition, just as "satin" refers to a type of fabric rather than the fiber it's made from (see the various igneous rock textures).
The phenocryst gallery shows some of the different minerals that are found as phenocrysts. See other examples of porphyritic texture in the volcanic rocks gallery. A porphyry may be plutonic, intrusive or extrusive.
Other galleries:
Fossils
Geologic Features and Processes
Glaciers and Ice
Landforms
Minerals
Rocks
Geology and Society
