Andesite is an aphanitic (fine-grained) igneous rock that is intermediate in its content of silica and low in alkali metals. It has less than 10% feldspathoid by volume, with at least 65% of the rock consisting of feldspar in the form of plagioclase. This places andesite in the basalt/andesite field of the QAPF diagram. Andesite is further distinguished from basalt by its silica content of over 52%. However, it is often not possible to determine the mineral composition of volcanic rocks, due to their very fine grain size, and andesite is then defined chemically as volcanic rock with a content of 57% to 63% silica and not more than about 6% alkali metal oxides. This places andesite in the O2 field of the TAS classification. Basaltic andesite, with a content of 52% to 57% silica, is represented by the O1 field of the TAS classification but is not a recognized type in the QAPF classification.