Phyllite

Like other foliated metamorphic rocks, it forms when other rock (mudstone, shale) is subjected to heat and pressure.



The amount of heat and pressure matters - on the scale phyllite is between slate and schist (less pressure forms slate and more pressure forms schist).

Like slate and schist, it is a layered type of rock, but unlike slate it is almost glittery (it is said it has phyllitic lustre).

 
 By Ian Geoffrey Stimpson via Flickr.com

That is thanks to its mineral composition, which includes more mica (created by more pressure) than slate.

Other minerals are quartz, feldspar, graphite and chlorite, as well as smaller amounts of magnetite, pyrite, cordierite, staurolite and tourmaline.

The minor minerals often occur as porphyroblasts.

The layers of phyllite are also different - they are stronger together than those in slate (that are well known to break apart very easily).

Also while the layers in slate are flat, in phyllite they are wrinkled.

It is fine grained, however slate is even finer.