Most slate is gray to black in color, but the rock may be red or green, depending on its mineral content. Slate is a metamorphic rock Metamorphic rock forms when heat or pressure, or both, cause changes in the "parent" rock. One of the changes is the formation of new minerals, called recrystallization. The new mineral grains often are larger than the old ones. Also, platy materials, such as mica, may crystallize in parallel planes.
When this happens, the rock breaks easily along these planes. This characteristic of metamorphic rocks is called rock cleavage. Most slate is formed below the earth's surface by changes in the makeup of shale, a sedimentary rock.
Shale primarily consists of clay minerals and of fine particles of quartz. When shale is buried in the earth's crust, heat from deep in the earth or nearby magma molten rock changes the clay minerals in shale to mica and chlorite. Downward pressure caused by burial and deformation due to crustal movement squeezes the mica and other platy flaky minerals and forms them into parallel layers.
Builders use slate for roofing and flagstones because it is weatherproof and long-lasting. Slate is also used to trim the fronts and lobbies of buildings, and for pool tables. Slate can be made into roofing slates, also called roofing shingles, installed by a slater in the USA. Slate has two lines of breakability: cleavage and grain. This makes it possible to split slate into thin sheets. Fine slate can also be used as a whetstone to hone knives.
Due to its thermal stability and chemical inertness, slate has been used for laboratory bench tops and for billiard table tops. Most slates are gray in color and range in a continuum of shades from light to dark gray.
Slate also occurs in shades of green, red, black, purple, and brown. The color of slate is often determined by the amount and type of iron and organic material that are present in the rock. The best way to learn about rocks is to have specimens available for testing and examination. The tectonic environment for producing slate is usually a former sedimentary basin that becomes involved in a convergent plate boundary.
Shales and mudstones in that basin are compressed by horizontal forces with minor heating. These forces and heat modify the clay minerals in the shale and mudstone. Foliation develops 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.
School slate: School slate used for writing practice and arithmetic. Students wrote on the slate with a "pencil" made from slate, soapstone , or clay. These slates were widely used until the late s, when wood-case pencils were easily produced and the price of paper became affordable. The word "slate" has not been used consistently over time and in some industries.
Today most geologists are careful not to use the word "slate" when talking about "shale. This confusion of terms partially arises from the fact that shale is progressively converted into slate. Imagine driving your car eastwards in Pennsylvania through areas of increasing metamorphism, starting where the rock is definitely "shale" and stopping to examine rock at each outcrop. You will have a difficult time deciding where on that route "shale" has been converted into "slate. In the coal mining industry of the Appalachian Basin, the word "slate" is still used by many miners in reference to the shale that forms the roof and floor of a mine, and for fragments of shale that are separated from the coal in preparation plants.
Experienced miners train newer miners, and archaic language is passed along. In the s, elementary school students used a small piece of slate mounted in a wooden frame for writing practice and arithmetic problems.
Writing was done with a small pencil made of slate, soapstone, or clay. The slate could be wiped clean with a soft cloth. Small slates were also used in schools and businesses to list daily events, schedules, menus, prices, and other notices. Major slate mining regions exist in areas of the UK. The Welsh slate industry has an extensive history.
It began in the Roman period, when slate was used to roof the fort at Segontium, now Caernarfon. The first large-scale slate mining in North Wales began with the opening of Penrhyn Quarry in back then known as Cae Braich y Cafn quarry. However, the first reference to slate extraction from Penrhyn is from back in , when it was mentioned in a Welsh poem.
The main pit of Penrhyn Quarry is nearly one mile long and metres deep, it was worked by nearly 3, quarrymen. Welsh Heather Blue — This beautiful slate comes in heather blue or purple, and its striking colour is incredibly rare in the industry.
Welsh Plum Slate Aggregate — These slate aggregates are a purple heather colour with tones of blue and green running through to create a stunning backdrop to plant or garden features.
It first worked as a trial pit around and produces dark blue grey slate. Today it is an open cast quarry, whereas slate was originally mined in deep caverns and pillars. Welsh Dark Blue Grey — This slate is used by architects worldwide, it is renowned for its stunning looks, reliability and ease to work with and is regarded as one of the finest slates in the world.
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