This is an uncommon mafic intrusive igneous rock that falls into the foid gabbro classification. It forms deep underground from magmas that are highly deficient in silica, meaning quartz is entirely absent and feldspathoid minerals like nepheline form instead. It is closely related to essexite and teschenite but is distinguished by its specific mineral assemblage under a petrographic microscope.
Heard on the job
The thin section confirms the intrusion is theralite, given the abundance of nepheline alongside the calcic plagioclase.