Emi Koussi has erupted phonolite, trachyandesite and trachyte, as well as mafic rocks like basanite and tephrite. The erupted rocks define two alkaline suites. Phenocryst chemistry and content varies between the various rocks; among the minerals are alkali feldspar, amphibole, biotite, clinopyroxene, olivine, oxides and plagioclase. Alkali feldspar, apatite, clinopyroxene, olivine, magnetite, mica, nepheline, oxides, plagioclase, quartz, sodalite, titanite and zircon also form the groundmass of microliths in erupted rocks. Magma genesis mainly involved fractional crystallization processes. Rocks from Emi Koussi were used as raw material by Neolithic societies of the region.