Slate is the first stage in the sequence of rocks
that develop when mudrock is progressively
metamorphosedon a regional scale.
Moderate heat and pressure on
mudrock alters it to slate, but if the
pressure increases it develops into
phyllite. Even more intense pressure
turns phyllite to schist. Just like
sedimentary and igneous rocks, slate
can also be cooked and altered by
contact with a hot granite intrusion.
Right next to the intrusion, slates lose
their distinctive cleavage, and develop
intosplintery, tough
hornfels. Further away, they retain their
cleavage but develop dark or light round
spots of altered minerals, typically white mica
or chlorite. Spotted slates such as these
usually contain minerals such as andalusite,
garnet or, more rarely, cordierite. Andalusite
inthe form of chiastolite is especially
characteristic of these spotted slates. Chiastolite
slates contain distinctive large porphyroblasts
of andalusite with dark crosses embedded in
a light crystal. These crystals can often be
7-8cml2.3-3.1in long.Commonly, in
exposed slates, though, they are
weathered to white mica or kaolin