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Phyllite

A phyllite is a fine-grained, foliated metamorphic rock with a strong fissility. Its primary feature is that foliation planes show a lustrous sheen, caused by the presence of oriented phyllosilicates (mostly white mica). Neumann (1849) coined the term ‘phyllite’ to indicate a foliated rock with phyllosilicates, introduced as an alternative to the older term phyllade. Compared to slate, phyllite is relatively coarser and indicates a slightly higher metamorphic grade, though these rocks still occur in the low-grade (e.g. lower greenschist-facies and below). Individual metamorphic grains are still too small (in most cases) to distinguish them with the unaided eye, but they are coarse enough to reflect light efficiently. In a schist, on the other hand, metamorphic grains are visible on hand samples. The foliation in phyllites is transitional between cleavage (fine-grained foliation) and schistosity (coarse-grained foliation): some authors refer to it as phyllitic cleavage.
Phyllites derive from the low-grade metamorphism of fine-grained sedimentary rocks, as pelites (shale, mudrock) and fine-grained and matrix-rich sandstones. Major constituents are white mica, chlorite, and quartz. Other common phases present are graphite, iron oxides and sulphides, and carbonates like calcite.

Phyllite is a fine-grained metamorphic rock (like slate) but, in phyllites, foliation surfaces reflect light very well. Width: 4.5 cm. Photo by James St. John.

Phyllite from the Precambrian of Wyoming, USA (French Slate, Medicine Bow Mountains). Widht: about 9 cm. Photo by James St. John.

Phyllite from Mojave desert, California, USA. Note the evident lustrous sheen. Photo by Morrie Gasser.

Phyllite from Godrevy, Cornwall, England. This rock shows an evident, lustrous phyllitic cleavage. There are also small ‘ridges’ which are crenulation axes. Photo by Margaret W. Carruthers.

Phyllite. The brownish color is related to the presence of iron sulphides that alters to iron oxides. Anakeesta Formation, Neoproterozoic. Great Smoky Mountains, Tennessee, USA. Photo by James St. John.

 

References
De Segonzac, G. D. (1970). The transformation of clay minerals during diagenesis and low‐grade metamorphism: a review. Sedimentology15(3‐4), 281-346.
Frey, M., & Robinson, D. (Eds.). (2009). Low-grade metamorphism. John Wiley & Sons.
Kisch, H. J. (1991). Development of slaty cleavage and degree of very‐low‐grade metamorphism: a review. Journal of Metamorphic Geology9(6), 735-750.
Paterson, M. S., & Weiss, L. E. (1966). Experimental deformation and folding in phyllite. Geological Society of America Bulletin77(4), 343-374.
Sassi, F. P., & Scolari, A. (1974). The b 0 value of the potassic white micas as a barometric indicator in low-grade metamorphism of pelitic schists. Contributions to Mineralogy and Petrology45(2), 143-152.
Wood, D. S. (1974). Current views of the development of slaty cleavage. Annual Review of Earth and Planetary Sciences2(1), 369-401.

        

See also
Phyllite – Alexstrekeisen.it
Phyllite – photobook by James St. John
Schist – Sandatlas.org
Rocks and Minerals in Thin Section – a Colour Atlas

it_IT Italiano
Metamorphic Minerals
Metamorphic Structures
Metamorphic Rocks

 

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