Facts about Slate

Slate is a durable, attractive and useful type of rock that is mined in countries all over the world, including the US, Brazil, and China, as well as in Europe and most recently, in Australia. Slate is a metamorphic rock, which means it was formed, or “morphed”, from another type of rock into its present state.

• How Slate is Formed
Slate is formed through Barrovian metamorphism, which occurs in areas of intense tectonic pressure. Slate is the first metamorphic rock resulting from the Barrovian metamorphism of its parent rock, shale. Shale, the most plentiful type of sedimentary rock in the world, is made up of a mix of silt, clay and tiny fragments of other minerals.  When processes deep in the earth cause shale to be subjected to intense heat and pressure, changes to the minerals occur and slate is formed.  

• Composition
During the metamorphic process, the clay minerals in shale become micas. The micas in slate are minute, giving slate a fine-grained appearance. Slate can also contain quartz and small amounts of feldspar, calcite, pyrite, hematite and other trace minerals. Slate is a foliated rock, meaning that there is a parallel alignment of the minerals which allows slate to be cut into smooth slabs. This makes it possible for slate to be used in many practical applications.

• Color
The most common color of slate is gray, although it may be any shade from very light gray to black.  More colorful shades of purple, red, green and brown also occur, as the color of slate depends much on the minerals in the shale in the location where the slate was formed.

• Uses of Slate
Slate has historically been a popular material for roofing, as it is fireproof and when properly set, makes for a waterproof roof covering. Slate does not absorb much moisture and therefore is not susceptible to freezing temperatures. A roof constructed of high quality slate may last 150 years. It is not used as often in new construction, due to the high cost of the slate tiles and the skill required for proper installation, although it is a popular choice in more expensive buildings and for restoration work on historical buildings. Slate may also be used for interior flooring as well as outdoor patios and for decoration in the garden.  Slate is the preferred material for making pool tables, as it is renowned for its smooth, stable and fast playing surface.

• Slate in popular culture
Slate tiles were once widely used as the first chalkboards in schools. In the 1800s, each student had his own wood-framed slate at his desk. Many expressions still used today, such as “start with a clean slate” or to “wipe the slate clean”, originated as a result of these early school work surfaces made of stone.