Nyman, Matthew W. Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico.
- Additional Reading
A type of metamorphic rock formed during low-grade metamorphism of clay-rich sediments called pelites. Phyllites are very fine grained rocks with a grain size barely visible in a hand specimen. They have a well-developed planar element called cleavage defined by alignment of mica grains and interlayering of quartz-rich and mica-rich domains. Typically, mica grains show the greater alignment, although other mineral components (quartz, carbonate, and feldspars) may show a preferred shape orientation. Where all minerals of a particular type show the same degree of alignment and the fabric is well developed throughout the rock, the fabric is termed a penetrative fabric. Cleavage surfaces in phyllites have a glittery, lustrous sheen due to light reflecting off grains of chlorite and muscovite. The mineralogy of phyllites is dependent on chemical composition; typical minerals in phyllites are chlorite, muscovite, and quartz. Other minerals that may be present in phyllites formed during low-grade metamorphism include chlorotoid, garnet (rarely), sodium-mica, and sulfide minerals. These minerals are typically millimeter in scale and resolvable in a hand specimen by using a 10× hand lens. See also: Chlorite; Muscovite; Quartz
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