Igneous rocks that have visible mineral grains (phaneritic rocks) are classified as intrusive, while those that are glassy or very fine-grained (aphanitic) are classified as extrusive igneous rock. Intrusive igneous rocks are usually classified using the QAPF classification, which is based on the relative content of quartz, alkali feldspar, plagioclase, and feldspathoid. Special classifications exist for igneous rock of unusual compositions, such as ultramafic rock or carbonatites. Where possible, extrusive igneous rocks are also classified by mineral content using the extrusive QAPF classification, but when determining the mineral composition is impractical, they may be classified chemically using the TAS classification. This is based on the total content of silica and alkali metal oxides and other chemical criteria.