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