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nephelinite A fine-grained, often porphyritic, ultra-alkaline extrusive igneous rock consisting of essential titanaugite (see ESSENTIAL MINERAL), nepheline, and titanomagnetite, with accessory apatite, sphene, and perovskite (see ACCESSORY MINERAL). Where present, phenocrysts usually consist of the titanaugite, and occasionally nepheline. Sodalite and analcite may also be found in the groundmass. Where olivine is an additional component, the rock is termed an ‘olivine nephelinite’. Nephelinites are essentially super-undersaturated basalts (see SILICA SATURATION) with nepheline taking the place of all the plagioclase feldspar.