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)

 


 

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