Basanite


Basanite (IPA: ) is an igneous, volcanic (extrusive) rock with aphanitic to porphyritic texture. The mineral assembly is usually abundant feldspathoids (nepheline or leucite), plagioclase, and augite, together with olivine and lesser iron-titanium oxides such as ilmenite and magnetite-ulvospinel; minor alkali feldspar may be present, as illustrated by the position of the field for basanite in the QAPF diagram. Clinopyroxene (augite) and olivine are common as phenocrysts and in the matrix. The augite contains significantly greater titanium, aluminium and sodium than that in typical tholeiitic basalt. Quartz is absent, as are orthopyroxene and pigeonite. Chemically basanites are low in silica (42 to 45% SiO<sub>2</sub>) and high in alkalis (3 to 5.5% Na<sub>2</sub>O and K<sub>2</sub>O) compared to basalt, which typically contains more SiO<sub>2</sub>, as evident on the diagram used for TAS classification. Nephelinite is yet richer in Na<sub>2</sub>O plus K<sub>2</sub>O compared to SiO<sub>2</sub>.