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melteigite An undersaturated, intrusive igneous rock containing essential nepheline and alkali pyroxene (aegirine or aegerine-augite) and displaying a colour index of 70–90. With decreasing mafic mineral content and increasing nepheline content melteigites pass into ijolites and then into urtites. All three rock types are essentially feldspar-free, undersaturated syenites (see SILICA SATURATION) and are well displayed in the Fen intrusive complex of southern Norway.