Slate is a fine-grained, homogenous, and foliated metamorphic rock that forms due to regional metamorphism. Its parent rock is shale or mudstone, and the conversion process occurs due to low-grade regional metamorphism. The rock has a slaty cleavage, breaking easily along the foliation lines.
“Slate” is derived from the German word “schleissen,” meaning to split. Slate is usually gray, but it is also found in different colors. For example, slate mined from North Wales, England, is in many shades of gray. Some specimens are also found in purple, green, or cyan colors. The color is usually determined by the iron content, its type, and the organic materials in the rock.
Deciphering the word “Slate”
The terms slate, shale (sedimentary rock), and schist (metamorphic rock) were not distinguished until the mid-19th century. The term slate was referred to shale in terms of underground coal mining till the end of the 20th century.
The word slate is still used to define shale that makes the roof and floor of a mine. It is more common in the Appalachian Basin coal mining industry. The language and terminology are passed from seniors and trained miners to beginners.
The British Geological Survey recommends that the word “slate” only be used in scientific writings when little is known about the rock.
Slate consists of clay minerals (micas) depending on the exposure level of metamorphism during the formation process.
The clay minerals in shale or mudstone change to micas with increasing levels of heat and pressure. The more heat and pressure it receives, the more aligned and crystallized the rocks become.
Slate contains large amounts of quartz, chlorite, and illite. These three minerals make up 95% of the rock. The trace minerals include iron oxides (hematite and magnetite), iron sulfides (pyrite), and carbonate minerals. Some specimens may also have feldspars. The purple slate mined from North Wales contains ferrous iron.
Slate forms in sedimentary basin environments at convergent plate boundaries. The parent rocks, shale, and mudstones are compressed by horizontal forces and minor heating, resulting in modification of the clay minerals in these rocks.
The resulting foliation forms at 90-degree to the compressive forces of the convergent plate boundary and produces a vertical foliation. This vertical foliation usually crosses already existing bedding planes in shale.
The low-level metamorphism produces slaty cleavage with microscopic individual mineral crystals. It is different from phyllite, which has a silky cleavage. Phyllite is the next and higher grade of metamorphic derived from mudstone and shale.
The conversion process (mudstone/shale to slate) involves a 50% volume loss of the original mudstone.
Where is Slate Found?
Slate is found across the world. Big reserves exist in Europe, the Americas, Asia, and Australia.
- Spain is the world’s largest slate producer. Most of the slate used today around the world comes from Spain, and 90% of the slate used in Europe’s roofing comes from Spain.
- England has lesser-known slate reserves in Wales, Cornwall, and Cumbria. Other European countries with slate reserves are France, Belgium, Italy, Germany, and Portugal.
- Brazil has huge slate reserves. It is the world’s second-largest slate producer, and most come from Papagaios in Minas Gerais.
- US slate reserves are in Pennsylvania, Virginia, Vermont, and New York.
- Asian reserves are located mainly in China, and Australian reserves are located in Adelaide Hills and the Mid-North.
Slate’s biggest and most economical use is roofing slates. It is used for roofing because it can be easily cut into thin sheets and withstands moisture and extreme temperatures. However, its installation is costly compared to other roofing materials.
Slat is an excellent electric insulator, and it is fireproof. It was used to make electric switchboards and relay controls for large electric motors in the early 20th century.
Slate was also used for blackboards and writing slates in the 18th and 19th centuries. People used to write with chalk on them. Slate is used to making high-end luxury GO game stones in Japan.
Slate is also used as indoor and outdoor flooring options in areas where it is available in bulk. Some people also use decorative stone in gardens and wall cladding.