Characteristics

Slate has a dominant foliation and due to this fact slate falls apart in thin platy pieces. Slate originate from low-grade regional metamorphism at a former sedimentary basin involved in convergent plate boundaries, which means it forms under horizontal forces and minor heating. Slate can be distinguished from phyllite by its grain size. The grain size of slate is much smaller than that from phyllite.

Composition

Slate is mainly composed of quartz and some metamorphosed clay minerals. Most slates are grey in color and range from light to dark. The minerals in slate form at right angles to the compressive forces of the convergent plate boundary to yield a vertical foliation that usually crosses the bedding planes that existed in the shale.

References

• NEN-EN-ISO 14688-1:2019
• https://geology.com/