Igneous rocks rich in leucite but lacking or poor in alkali feldspar. Those types with essential alkali feldspar are classed as phonolites, feldspathoidal syenite, and feldspathoidal monzonite. The group includes an extremely wide assortment both chemically and mineralogically.
The rocks are generally dark-colored and aphanitic types of volcanic origin. They consist principally of pyroxene and leucite and may or may not contain calcic plagioclase or olivine. Leucite rocks are rare. They occur principally as lava flows and small intrusives (dikes and volcanic plugs). See also
