A small area (1.3 km2) of nepheline syenite gneiss occurs 19 km southwest of the town of Mattawa. The gneiss contains 5-25% nepheline, sodic hastingsite, 40-60% oligoclase and sometimes garnet. A pyroxene, which is present in some rocks, is described as omphacite.
LUMBERS, S.B. 1976. Omphacite-bearing nepheline syenite in an anorthosite complex, Grenville Province of Ontario. (Abstract). Geological Association of Canada - Mineralogical Association of Canada, Annual Meeting, Edmonton, Program with Abstracts, 1: 73.