A plagioclase feldspar with composition Ab₇₀An₃₀ to Ab₅₀An₅₀ (Ab = NaAlSi₃O₈; An = CaAl₂Si₂O₈). Andesine occurs primarily in igneous rocks, often in a glassy matrix as small, chemically zoned, lathlike crystals known as microlites. The rock types may be called andesinites (if dominantly feldspar), andesites (see illus.), andesitic basalts (or olivine-bearing andesites, as in Hawaiian lava flows), or pyroxene-, hornblende- or biotite-andesites (depending on the second most dominant mineral—all are volcanic).