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Foyaite igneous rock. Foyaite, a type of nepheline syenite, is an igneous rock that forms when volcanic magma solidifies under pressure in the Earth's crust. It is coarse- grained, and consists mostly of microcline, nepheline and aegirine minerals. This sample measures 120 millimetres across. (KEYSTONE/SCIENCE PHOTO LIBRARY/CHR 366 DIRK WIERSMA/SCIENCE PHOTO LIBRARY) === ===