Discussion
Undersaturated alkaline lavas with abundant felspathoids sanidine or anorthoclase, aegine or aegirine augite is termed as.....................
Answer: B
Such a lava is a phonolite.
Phonolite is the fine-grained, extrusive rock of silica-undersaturated alkaline magma. It consists of alkali feldspar (sanidine or anorthoclase) with abundant feldspathoids such as nepheline or leucite and sodic pyroxene such as aegirine or aegirine-augite.
Syenite has the feldspar mineralogy but is a coarse-grained plutonic rock, not a lava. Keratophyre is an altered sodic lava without abundant feldspathoids, and limburgite is a glassy ultramafic lava of olivine and augite with no feldspar.
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