The formal definition of andesite is problematic. Many authors have classified igneous rocks based upon their chemical and mineralogical compositions. However, none of these classifications are in perfect agreement.

For a fine-grained rock like andesite, these classifications are impossible to use precisely when in the field or the classroom. They require chemical or mineralogical analyses that are usually not available, affordable, or practical.

If you examine a rock that appears to be andesite, but you are not confident that it meets the mineralogical or chemical classification of andesite, you might properly call it an “andesitoid” rock. What does that mean? It means that the rock looks like andesite, but, a microscopic examination or chemical analysis might prove you wrong!