8.3.4: Phyllite

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8.27.png — Figure 8.27: Example of a phyllite

Figure 8.27 shows a sample of phyllite, a shiny foliated rock created by further metamorphism of slates. The foliation is due to parallel alignment of very small – mostly microscopic – muscovite, chlorite, or other micas, sometimes with graphite. Phyllites, which form at higher metamorphic grades than slates, sparkle because clay minerals have metamorphosed to produce small grains of reflective mica. Thus, foliation of phyllites is different from the foliation in slates that stems from clay mineral alignment, and different from foliation in schists because schists always contain visible mica grains. Like slates, phyllites exhibit fissility.

Phyllites are typically black, gray, or green, and the fine-grained micas and graphite, which are too small to see without a microscope, give phyllites their silky/shiny appearance, or sheen, called a phyllitic luster. It is this luster – which is absent from slate and schist – that really defines a phyllite. Additionally, although not seen in Figure 8.27, layering in some phyllites is deformed, giving the rocks a sort of wavy or crinkly appearance.