Slate

Lore & Background

Slate forms from mudrock through low-grade regional metamorphism, involving a volume loss of up to 50% as clay minerals rotate and recrystallize into parallel layers perpendicular to compression. This process creates slaty cleavage, which is continuous and dominates the rock's texture. Under a microscope, slate consists of thin lenses of quartz and feldspar separated by mica layers, typically less than 100 microns thick. Organic carbon in the rock is converted to graphite during metamorphism. Slate occurs in various colors, including grey, purple, green, and cyan, even from a single locality such as North Wales. The purple slates of North Wales sometimes contain ferrous reduction spheres that form around iron nuclei, leaving a light-green spotted texture. These spheres may be deformed into ovoids by stress, but caution is required in using them to estimate deformation, as they can also form after deformation. Historically, the terms 'slate', 'shale', and 'schist' were not sharply distinguished until the mid-19th century. In underground coal mining in the United States, 'slate' was commonly used to refer to shale well into the 20th century. The British Geological Survey recommends using 'slate' in scientific writings only when little else is known about the rock; if its origin is certain, it should be described as a metashale or pelite.

Reader's Guide

Slate's significance lies in its unique combination of properties: fine grain, slaty cleavage, durability, low water absorption (less than 0.4%), fire resistance, and electrical insulation. These made it a preferred material for roofing, flooring, and industrial applications such as early-20th-century electric switchboards and relay controls. Its ability to be split into thin, flat sheets allowed for widespread use in construction, particularly in Europe during slate booms from the 1870s to World War I, following improvements in transportation. Slate also played a cultural role as writing slates in 18th- and 19th-century schools, giving rise to phrases like 'clean slate' and 'blank slate.' Its use for tombstones and commemorative tablets in areas where high-quality slate is available further underscores its historical importance. The rock's metamorphic origin and the preservation of fossils, including microscopic remains, provide valuable geological information. However, the article notes that caution is needed in interpreting deformation features like reduction ellipsoids, preserving scientific uncertainty.

Did You Know?

• Slate can be split into thin sheets due to its slaty cleavage, which is perpendicular to the direction of metamorphic compression.
• Slate from North Wales can be found in shades of grey, purple, green, or cyan.
• The phrases 'clean slate' and 'blank slate' originate from the use of small, smooth pieces of slate as writing notepads.
• Slate has an extremely low water absorption index of less than 0.4%, making it resistant to frost damage.