Basanite

Basanite (pronounced /ˈbæsənaɪt/) 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% SiO₂) and high in alkalis (3 to 5.5% Na₂O and K₂O) compared to basalt, which typically contains more SiO₂, as evident on the diagram used for TAS classification. Nephelinite is yet richer in Na₂O plus K₂O compared to SiO₂.

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Basanites occur both on continents and on ocean islands. For example, together with basalts, they are produced by hotspot volcanism in the Hawaiian and the Comores Islands. (e.g.,[1])

The term basanite has been used occasionally to refer to the mineral jasper, for example a black flinty jasper found in several New England states of the U. S. A. Such varieties of jasper are also informally known as Lydian stone, lydite and touchstone and have been used to test for the purity of precious metal alloys.

References

• Blatt, Harvey and Robert J. Tracy (1996) Petrology, 2nd ed., W. H. Freeman ISBN 0-7167-2438-3
• Buchwaldt, Robert (2003) Geochemistry and Petrology of a Miocene Trachyte-Basanite Suite from Mt. Tsaratanana, Northern Madagascar, Abstract.
• Mindat data