Mineral Thin Sections Flashcards | QuizletOccurrence: gneous and metamorphic rocks. In igneous rocks, it is found more commonly in silicic and alkalic rocks, e.g. granties, diorites, gabbros and peridotites. It is important in metamorphic rocks including schists, gneisses, phyllites, and hornfels. Also found in immature sedimentary rocks, but will alter to clay minerals when weathered.