SLATE
Slate is a foliated metamorphic rock. It forms by low-grade metamorphism of
shales and non-fissile mudrocks. Slates typically weather into relatively
thin, platy pieces. They superficially resemble fissile shales.
Slate's foliation is often not conformable to the original bedding.
Sometimes, the original bedding (& fossils) can be discerned on foliation
planes of slate. The color of slate
is generally blackish, grayish, greenish-gray, or brick-red.
Distinguishing slate from shale is a bit tricky at
first, but slates are harder and heavier than shales. The broken edges of
shales & slates also have different geometries - slate has a more
consistently stepped broken edges, while shale has blocky broken edges.
When tapped together, two pieces of shale make a relatively soft
“clicking” sound. Two
pieces of slate tapped together make a louder “ringing” sound, or
resonance. With practice,
distinguishing shale and slate samples is fairly straightforward.
Traditionally, slate was often used as roofing
material, due to its impermeability (it keeps out the rain). Slate roofs
do weather eventually, but the physical degradation rate of slate slabs is
relatively slow.
Slates sometimes contain fossils, but they are usually
only recoverable if the metamorphic foliation & the original shale bedding
are congruent, or if the original bedding planes are readily split. This
doesn’t happen at many localities.
Black slate (7.9 cm across) showing original shale bedding (dark stripes).
Red slate (5.8 cm across)