Alkali basalt

A term used for a basaltic rock containing accessory foids, typically nepheline. Such rocks generally contain calcic plagioclase, augite and olivine as their main phases. Defined as a variety of basalt within TAS field B which contains normative nepheline, or as the equivalent foids-bearing basalt in field 10’ in the QAPF diagram. With increasing silica it grades into tholeiitic basalt, and with decreasing silica into basanite or tephrite.