alkali basalt A fine-grained, dark-coloured, volcanic rock characterized by phenocrysts of olivine, titanium-rich augite, plagioclase feldspar and iron oxides. For similar SiO2 concentrations, alkali basalts have a higher content of Na2O and K2O than other basalt types such as tholeiites. They are also characterized by the development of modal nepheline in their groundmass (only seen with the highest powered lens on a petrological microscope) and normative nepheline (Ne) in their CIPW norms. Alkali basalts are typically found on updomed and rifted continental crust, and on oceanic islands such as Hawaii and Ascension Island.
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