Basanite
Basanite is an extrusive igneous rock, also known as volcanic rock, with a composition between basalt and tephrite. It is characterized by its dark color, fine-grained texture (aphanitic), and relatively high alkali content (Na2O + K2O) compared to typical basalt.
Composition and Mineralogy:
Basanite is defined petrologically by the presence of essential feldspathoid minerals (nepheline, leucite, or analcime) and plagioclase feldspar (typically labradorite to andesine). Clinopyroxene (augite) is a major mafic mineral, and olivine is also commonly present. Importantly, basanite lacks or has only minor amounts of modal orthopyroxene or quartz. Accessory minerals can include magnetite, ilmenite, and apatite. The normative mineralogy, calculated from the bulk chemical composition, further distinguishes basanite from other volcanic rocks. It contains normative nepheline (ne) and relatively little or no normative quartz (qz).
Formation and Occurrence:
Basanites are typically formed by partial melting of the Earth's mantle at relatively greater depths than basalts, often in the presence of carbon dioxide. They are associated with both oceanic and continental settings, including oceanic islands (e.g., Canary Islands), continental rift valleys (e.g., East African Rift), and back-arc basins.
Distinguishing Features:
- Feldspathoids: Presence of nepheline, leucite, or analcime is a key identifier.
- High Alkali Content: Relatively high Na2O + K2O content compared to basalts.
- Low Silica Content: Lower SiO2 content compared to basalt and the absence or near absence of quartz.
- Olivine Presence: Usually contains olivine.
- Lack of Orthopyroxene/Quartz: Minimal or no modal orthopyroxene or quartz.
Uses:
Basanite has limited commercial uses. It has been used as a source of aggregate for road construction and as dimension stone in some regions. Its geochemical signature is valuable in petrological studies for understanding mantle composition and melting processes.