Phyllite

Phyllite is between slate and schist in the spectrum of metamorphic rocks. Geologists tell them apart by their surfaces: slate has flat cleavage faces and dull colors, phyllite has flat or crinkled cleavage faces and shiny colors, and schist has intricately wavy cleavage (schistosity) and glittering colors. Phyllite is "leaf-stone" in scientific Latin; the name may refer as much to phyllite's color, which is often greenish, as to its ability to cleave into thin sheets. Phyllite generally is in the pelitic series—rocks that are derived from clay sediments—but sometimes other rock types can take on the characteristics of phyllite too. That is, phyllite is a textural rock type, not a compositional one. The sheen of phyllite is from microscopic grains of mica, graphite, chlorite and similar minerals that form under moderate pressure.

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