What is Slate? - Definition & Uses

Instructor: Heather Pier

Heather has taught high school and college science courses, and has a master's degree in geography-climatology.

Learn about slate, a type of fine-grained, foliated metamorphic rock. Its strong and weather-resistant nature have made it an ideal choice for many construction and industrial applications.

What is Slate?

Slate is a fine grained and foliated type of metamorphic rock. It owes its fine grained texture to its parent rock, shale. Shale, a type of sedimentary rock, consists of dominantly clay-sized particles (in geology, clay is a reference to particle size, not a reference to any resemblance to artist's clay), which create a smoother, finer grained texture when exposed to low grade metamorphic conditions. Slate can occasionally form as a result of metamorphism in volcanic ash deposits, although this is not as common as the shale-derived rocks. It also exhibits foliation, or layering, which again, is a result of the parent rock being subjected to metamorphism.

A sample of slate exhibiting its characteristic smooth surface.
Slate rock

Slate is a very strong and durable rock, due largely to its metamorphic history, and as a result has a variety of uses in construction and industrial applications. The majority of the world's slate is mined in Spain, Brazil, the northeastern United States, and in recent years, China.

A slate quarry
Slate quarry

Mineralogy and Petrology of Slate

In terms of its mineral composition, slate is comprised primarily of quartz and mica minerals, including muscovite, illite, and/or biotite. It can also contain chlorite, hematite, or pyrite.

The color of slate is determined by the color of the minerals its parent rock (shale) contained, and can vary from shades of green (chlorite-rich), to red (hematite-rich), to gray/black (biotite-rich). There are also intermediate colors that are sometimes found, including red-browns, green-blacks, and even occasionally shades of purple in certain parts of the world.

As previously mentioned, it gets its fine grained texture and foliation from a combination of its parent rock, shale, and its exposure to metamorphic conditions. The combination of shale and metamorphism leads to a unique type of foliation known as slaty cleavage, which is a fancy way of saying that slate's surfaces break (cleavage) in a way that is unique to slate. It is an identifying characteristic for slate in the field. It is this cleavage that also allows it to have such nice, flat surfaces that are so desired in the building and construction industries for use as roofing tiles or flooring.

A slate roof in Andorra
Slate roof

Construction Uses

If you have heard the term slate used before and it wasn't a specific reference to the rock itself, that is probably because over the years, slate has been used to describe many common materials that are made from slate, including most notably writing surfaces like chalkboards.

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