How is a slate different from a phyllite?

Answer 1

Slate is the low-pressure, low-temperature metamorphic rock whose parent rock (protolith) was shale. Clay minerals which were present in the shale have started to change into mica, a platy mineral, which give slate its cleavability. Because the transformation of the clay minerals is not complete, slate generally appears dull in light.

With exposure to higher temperatures and pressures, shale will become the metamorphic rock phyllite. In phyllite, the transformation of the clay minerals to nearly visible micas and other minerals is nearly complete, giving the rock a very shiny appearance. It may also appear wavy and foliated.

With higher temperature and pressure, phyllite will become the metamorphic rock schist.

Answer 2

Slate is a fine-grained metamorphic rock that forms from the low-grade metamorphism of shale or mudstone, while phyllite is a slightly higher-grade metamorphic rock that forms from the metamorphism of slate. Phyllite has a more shiny, reflective surface due to the growth of tiny mica minerals within it, whereas slate has a duller appearance.