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nepheline-syenite A medium- to coarse-grained igneous rock consisting of essential alkali feldspar (see ESSENTIAL MINERAL), nepheline, pyroxene (aegirine-augite or sodic hedenbergite) and amphibole (ferrohastingsite, NaCa2(Fe2+)4AlAl2Di6O22(OH)2, which is essentially a sodic, iron-rich hornblende, barkevikite and/or arfvedsonite) with accessory sphene, apatite, titanomagnetite, ilmenite, and zircon (see ACCESSORY MINERAL). The rock is, in effect, a syenite which is undersaturated in silica (see SILICA SATURATION).