Phyllite is a foliated
Like other foliated
metamorphic rocks, it forms when other rock (mudstone,
shale) is subjected to heat and pressure.
The amount of heat and
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
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.
minerals are quartz, feldspar, graphite and chlorite, as well as
smaller amounts of magnetite, pyrite, cordierite, staurolite and
The minor minerals often occur as porphyroblasts.
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.