Slate

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Slate
Metamorphic rock
SlateUSGOV.jpg
Slate
Composition
Primary quartz, muscovite/illite
Secondary biotite, chlorite, hematite, pyrite Specific gravity: 2.7 – 2.8
A piece of slate (~ 6 cm long and ~ 4 cm high) Slate Macro 1.JPG
A piece of slate (~ 6 cm long and ~ 4 cm high)

Slate is a fine-grained, foliated, homogeneous metamorphic rock derived from an original shale-type sedimentary rock composed of clay or volcanic ash through low-grade regional metamorphism. It is the finest grained foliated metamorphic rock. [1] Foliation may not correspond to the original sedimentary layering, but instead is in planes perpendicular to the direction of metamorphic compression. [1]

Contents

The foliation in slate is called "slaty cleavage". [1] It is caused by strong compression causing fine grained clay flakes to regrow in planes perpendicular to the compression. [1] When expertly "cut" by striking parallel to the foliation, with a specialized tool in the quarry, many slates will display a property called fissility, forming smooth flat sheets of stone which have long been used for roofing, floor tiles, and other purposes. [1] Slate is frequently grey in color, especially when seen, en masse, covering roofs. However, slate occurs in a variety of colors even from a single locality; for example, slate from North Wales can be found in many shades of grey, from pale to dark, and may also be purple, green or cyan. Slate is not to be confused with shale, from which it may be formed, or schist.

The word "slate" is also used for certain types of object made from slate rock. It may mean a single roofing tile made of slate, or a writing slate. They were traditionally a small, smooth piece of the rock, often framed in wood, used with chalk as a notepad or notice board, and especially for recording charges in pubs and inns. The phrases "clean slate" and "blank slate" come from this usage.

Historical mining terminology

Before the mid-19th century, the terms slate, shale and schist were not sharply distinguished. [2] In the context of underground coal mining in the United States, the term slate was commonly used to refer to shale well into the 20th century. [3] For example, roof slate referred to shale above a coal seam, and draw slate referred to shale that fell from the mine roof as the coal was removed. [4]

Mineral composition

Slate is mainly composed of the minerals quartz and muscovite or illite, often along with biotite, chlorite, hematite, and pyrite and, less frequently apatite, graphite, kaolinite, magnetite, tourmaline, or zircon as well as feldspar. Occasionally, as in the purple slates of North Wales, ferrous reduction spheres form around iron nuclei, leaving a light green spotted texture. These spheres are sometimes deformed by a subsequent applied stress field to ovoids, which appear as ellipses when viewed on a cleavage plane of the specimen.

Uses

Slate roof St Fagans Tannery 7.jpg
Slate roof

Slate in buildings

Slate-faced church and homes in Wurzbach, Thuringen, Germany Kirche wurzbach.jpg
Slate-faced church and homes in Wurzbach, Thüringen, Germany
Fine slate tile work, Saint Leonhard's Church, Frankfurt am Main, Germany. St.leonhard-ffm002.jpg
Fine slate tile work, Saint Leonhard's Church, Frankfurt am Main, Germany.

Slate can be made into roofing slates, a type of roof shingle, or more specifically a type of roof tile, which are installed by a slater. Slate has two lines of breakability – cleavage and grain – which make it possible to split the stone into thin sheets. When broken, slate retains a natural appearance while remaining relatively flat and easy to stack. A series of "slate booms" occurred in Europe from the 1870s until the First World War following improvements in railway, road and waterway transportation systems. [5]

Slate is particularly suitable as a roofing material as it has an extremely low water absorption index of less than 0.4%, making the material waterproof. In fact, this natural slate, which requires only minimal processing, has the lowest embodied energy of all roofing materials. Natural slate is used by building professionals as a result of its beauty and durability. Slate is incredibly durable and can last several hundred years, often with little or no maintenance. Its low water absorption makes it very resistant to frost damage and breakage due to freezing. Natural slate is also fire resistant and energy efficient. [6]

Slates with holes for fixing, viewed from below. Photographed in Tremedda, Cornwall, a farm in England SlateTremedda.jpg
Slates with holes for fixing, viewed from below. Photographed in Tremedda, Cornwall, a farm in England

Slate roof tiles are usually fixed (fastened) either with nails, or with hooks as is common with Spanish slate. In the UK, fixing is typically with double nails onto timber battens (England and Wales) or nailed directly onto timber sarking boards (Scotland and Northern Ireland). Nails were traditionally of copper, although there are modern alloy and stainless steel alternatives. Both these methods, if used properly, provide a long-lasting weathertight roof with a lifespan of around 80–100 years.

Some mainland European slate suppliers suggest that using hook fixing means that: [7]

The metal hooks are, however, visible and may be unsuitable for historic properties.

Slate tiles are often used for interior and exterior flooring, stairs, walkways and wall cladding. Tiles are installed and set on mortar and grouted along the edges. Chemical sealants are often used on tiles to improve durability and appearance, increase stain resistance, reduce efflorescence, and increase or reduce surface smoothness. Tiles are often sold gauged, meaning that the back surface is ground for ease of installation. Slate flooring can be slippery when used in external locations subject to rain. Slate tiles were used in 19th century UK building construction (apart from roofs) and in slate quarrying areas such as Blaenau Ffestiniog and Bethesda, Wales there are still many buildings wholly constructed of slate. Slates can also be set into walls to provide a rudimentary damp-proof membrane. Small offcuts are used as shims to level floor joists. In areas where slate is plentiful it is also used in pieces of various sizes for building walls and hedges, sometimes combined with other kinds of stone. In modern homes slate is often used as table coasters.

Other uses

Because it is a good electrical insulator and fireproof, it was used to construct early-20th-century electric switchboards and relay controls for large electric motors. 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. In 18th- and 19th-century schools, slate was extensively used for blackboards and individual writing slates, for which slate or chalk pencils were used.

In areas where it is available, high-quality slate is used for tombstones and commemorative tablets. In some cases slate was used by the ancient Maya civilization to fashion stelae.

Slate was the traditional material of choice for black Go stones in Japan. It is now considered to be a luxury.

Slate extraction

Exhibition Slate mine at Fell, Germany Bergwerk-fell-hoffnung.jpg
Exhibition Slate mine at Fell, Germany
Historical Pit Vogelsberg 1 at Fell Bergwerk-fell-stollen2.jpg
Historical Pit Vogelsberg 1 at Fell
Mules carrying slate roof tiles on their backs, Dharamshala, India, 1993 Mules carrying slate. Dharamsala.jpg
Mules carrying slate roof tiles on their backs, Dharamshala, India, 1993

Europe

Most slate in Europe today comes from Spain, the world's largest producer and exporter of natural slate, and 90 percent of Europe's natural slate used for roofing originates from the slate industry there.

Lesser slate-producing regions in Europe include Wales (with a museum at Llanberis), Cornwall (famously the village of Delabole), Cumbria (see Burlington Slate Quarries, Honister Slate Mine and Skiddaw Slate) and, formerly in the West Highlands of Scotland, around Ballachulish and the Slate Islands in the United Kingdom. Parts of France (Anjou, Loire Valley, Ardennes, Brittany, Savoie) and Belgium (Ardennes), Liguria in northern Italy, especially between the town of Lavagna (which means chalkboard in Italian) and Fontanabuona valley; Portugal especially around Valongo in the north of the country.

Germany's Moselle River region, Hunsrück (with a former mine open as a museum at Fell), Eifel, Westerwald, Thuringia and north Bavaria; and Alta, Norway (actually schist, not a true slate). Some of the slate from Wales and Cumbria is colored slate (non-blue): purple and formerly green in Wales and green in Cumbria.

Americas

Slate is abundant in Brazil, the world's second-biggest producer of slate, around Papagaios in Minas Gerais, which extracts 95 percent of Brazil's slate. However, not all "slate" products from Brazil are entitled to bear the CE mark. [8]

Slate is produced on the east coast of Newfoundland, in Eastern Pennsylvania, Buckingham County, Virginia, and the Slate Valley of Vermont and New York, where colored slate is mined in the Granville, New York area. Pennsylvania slate is widely used in the manufacture of turkey calls used for hunting turkeys in the U.S. The tones produced from the slate (when scratched with various species of wood strikers) imitates almost exactly the calls of all four species of wild turkey in North America: eastern, Rio Grande, Osceola and Merriam's.

A major slating operation existed in Monson, Maine during the late 19th and early 20th centuries, where the slate is usually dark purple to blackish, and many local structures are still roofed with slate tiles. The roof of St. Patrick's Cathedral, New York was made of Monson slate, as is the headstone of John F. Kennedy. [9]

Slate is found in the Arctic, and was used by Inuit to make the blades for ulus.

Workers mining slate at Mintaro quarry, c. 1880 Mintaro Slate Quarry 1880.jpeg
Workers mining slate at Mintaro quarry, c. 1880

Asia

China has vast slate deposits; in recent years its export of finished and unfinished slate has increased.

Australia

Deposits of slate exist throughout the Australian continent, with large reserves quarried in the Adelaide Hills (Willunga and Kanmantoo) and the Mid North (Mintaro and Spalding).

Fossils

Because slate was formed in low heat and pressure, compared to a number of other metamorphic rocks, some fossils can be found in slate; sometimes even microscopic remains of delicate organisms can be found in slate. [10]

Related Research Articles

Roof Top covering of a building

A roof is the top covering of a building, including all materials and constructions necessary to support it on the walls of the building or on uprights, providing protection against rain, snow, sunlight, extremes of temperature, and wind. A roof is part of the building envelope.

Schist Medium grade metamorphic rock with lamellar grain

Schist is a medium-grade metamorphic rock formed from mudstone or shale. Schist has medium to large, flat, sheet-like grains in a preferred orientation. It is defined by having more than 50% platy and elongated minerals, often finely interleaved with quartz and feldspar. These lamellar minerals include micas, chlorite, talc, hornblende, graphite, and others. Quartz often occurs in drawn-out grains to such an extent that a particular form called quartz schist is produced. Schist is often garnetiferous. Schist forms at a higher temperature and has larger grains than phyllite. Geological foliation with medium to large grained flakes in a preferred sheetlike orientation is called schistosity.

Shale A fine-grained, clastic sedimentary rock

Shale is a fine-grained, clastic sedimentary rock, composed of mud that is a mix of flakes of clay minerals and tiny fragments of other minerals, especially quartz and calcite. Shale is characterized by breaks along thin laminae or parallel layering or bedding less than one centimeter in thickness, called fissility. It is the most common sedimentary rock.

Metamorphic rock Rock that was subjected to heat and pressure

Metamorphic rocks arise from the transformation of existing rock types, in a process called metamorphism, which means "change in form". The original rock (protolith) is subjected to heat and pressure, causing profound physical or chemical change. The protolith may be a sedimentary, igneous, or existing metamorphic rock.

Quartzite Hard, non-foliated metamorphic rock which was originally pure quartz sandstone

Quartzite is a hard, non-foliated metamorphic rock which was originally pure quartz sandstone. Sandstone is converted into quartzite through heating and pressure usually related to tectonic compression within orogenic belts. Pure quartzite is usually white to grey, though quartzites often occur in various shades of pink and red due to varying amounts of hematite. Other colors, such as yellow, green, blue and orange, are due to other minerals.

Phyllite

Phyllite is a type of foliated metamorphic rock created from slate that is further metamorphosed so that very fine grained white mica achieves a preferred orientation. It is primarily composed of quartz, sericite mica, and chlorite.

Slate (writing)

A slate is a thin piece of hard flat material, such as the rock also called slate, that is used as a medium for writing. The rock is "a metamorphic rock created by the recrystallization of the minerals in shale from clay to parallel-aligned, flat, flake-like minerals such as mica".

Dimension stone Natural stone that has been finished to specific sizes and shapes

Dimension stone is natural stone or rock that has been selected and finished to specific sizes or shapes. Color, texture and pattern, and surface finish of the stone are also normal requirements. Another important selection criterion is durability: the time measure of the ability of dimension stone to endure and to maintain its essential and distinctive characteristics of strength, resistance to decay, and appearance.

Roof shingle Overlapping plates for covering a roof

Roof shingles are a roof covering consisting of individual overlapping elements. These elements are typically flat, rectangular shapes laid in courses from the bottom edge of the roof up, with each successive course overlapping the joints below. Shingles are held by the roof rafters and are made of various materials such as wood, slate, flagstone, metal, plastic, and composite materials such as fibre cement and asphalt shingles. Ceramic roof tiles, which still dominate in Europe and some parts of Asia, are still usually called tiles. Roof shingles may deteriorate faster and need to repel more water than wall shingles. They are a very common roofing material in the United States.

Rock microstructure includes the texture of a rock and the small scale rock structures. The words "texture" and "microstructure" are interchangeable, with the latter preferred in modern geological literature. However, texture is still acceptable because it is a useful means of identifying the origin of rocks, how they formed, and their appearance.

Foliation (geology)

Foliation in geology refers to repetitive layering in metamorphic rocks. Each layer can be as thin as a sheet of paper, or over a meter in thickness. The word comes from the Latin folium, meaning "leaf", and refers to the sheet-like planar structure. It is caused by shearing forces, or differential pressure. The layers form parallel to the direction of the shear, or perpendicular to the direction of higher pressure. Nonfoliated metamorphic rocks are typically formed in the absence of significant differential pressure or shear. Foliation is common in rocks affected by the regional metamorphic compression typical of areas of mountain belt formation.

Slate industry

The slate industry is the industry related to the extraction and processing of slate. Slate is either quarried from a slate quarry or reached by tunneling in a slate mine. Common uses for slate include as a roofing material, a flooring material, gravestones and memorial tablets, and for electrical insulation.

Slate industry in Wales Tentative World Heritage site in the United Kingdom

The existence of a slate industry in Wales is attested since the Roman period, when slate was used to roof the fort at Segontium, now Caernarfon. The slate industry grew slowly until the early 18th century, then expanded rapidly until the late 19th century, at which time the most important slate producing areas were in northwest Wales, including the Penrhyn Quarry near Bethesda, the Dinorwic Quarry near Llanberis, the Nantlle Valley quarries, and Blaenau Ffestiniog, where the slate was mined rather than quarried. Penrhyn and Dinorwig were the two largest slate quarries in the world, and the Oakeley mine at Blaenau Ffestiniog was the largest slate mine in the world. Slate is mainly used for roofing, but is also produced as thicker slab for a variety of uses including flooring, worktops and headstones.

Roofer Profession

A roofer, roof mechanic, or roofing contractor is a tradesperson who specializes in roof construction. Roofers replace, repair, and install the roofs of buildings, using a variety of materials, including shingles, bitumen, and metal. Roofing work can be physically demanding because it involves heavy lifting, as well as climbing, bending, and kneeling, often in extreme weather conditions.

Yorkstone Variety of sandstone from Yorkshire, England

Yorkstone or York stone is a variety of sandstone, specifically from quarries in Yorkshire that have been worked since mediaeval times. Yorkstone is a tight grained, Carboniferous sedimentary rock. The stone consists of quartz, mica, feldspar, clay and iron oxides.

Cleavage (geology)

Cleavage, in structural geology and petrology, describes a type of planar rock feature that develops as a result of deformation and metamorphism. The degree of deformation and metamorphism along with rock type determines the kind of cleavage feature that develops. Generally these structures are formed in fine grained rocks composed of minerals affected by pressure solution.

90% of Europes's natural slate used for roofing originates from the slate industry in Spain, with the region of Galicia being the primary source of production.

Roof tiles

Roof tiles are designed mainly to keep out rain, and are traditionally made from locally available materials such as terracotta or slate. Modern materials such as concrete, metal and plastic are also used and some clay tiles have a waterproof glaze.

Flexible stone veneer

Flexible stone veneer is a veneer with a layer of stone 1 to 5 mm thick. Flexible stone veneers should not be confused with traditional stone veneer. It is used for both interior and exterior and especially where bending to a curved surface is required. Flexible stone veneers are made from various types of slate, schist, or marble.

References

  1. 1 2 3 4 5 Essentials of Geology, 3rd Ed, Stephen Marshak
  2. R. W. Raymond, Slate, A Glossary of Mining and Metallurgical Terms, American Institute of Mining Engineers, 1881; page 78.
  3. Albert H. Fay, Slate, A Glossary of the Mining and Mineral Industry, United States Bureau of Mines, 1920; page 622.
  4. J. Marvin Weller, ed.,Supplement to the Glossary of Geology and Related Sciences, American Geological Institute, 1960; page 18.
  5. Schunck, Eberhard, and Hans Jochen Oster. Roof Construction Manual Pitched Roofs.. Basel: De Gruyter, 2003. 12. Print.
  6. Natural Slate, the natural option Archived 10 March 2014 at the Wayback Machine
  7. Galician and Spanish Slate website "Hook Fixing". Retrieved on 26 January 2010 archived
  8. Fundación Centro Tecnológico de la Pizarra. Report into the "Technical properties of Bambui Slate from the State of Minas Gerais (Brazil) to ascertain its compliance with the Standard EN12326". Brazilian Slate Report Archived 14 February 2010 at the Wayback Machine , retrieved on 27 January 2010
  9. Granville: Facts Archived 8 January 2011 at the Wayback Machine accessed 23 March 2011
  10. BBC Video: David Attenborough: Lost Worlds, Vanished Lives

Further reading