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 (approximately
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6 cm x 4 cm (2+1/2 in x 1+1/2 in)) Slate Macro 1.JPG
A piece of slate (approximately 6 cm × 4 cm (2+12 in × 1+12 in))

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.

Description

Slate with pyrite Slate with pyrite.JPG
Slate with pyrite

Slate is a fine-grained metamorphic rock that shows no obvious compositional layering, but can easily be split into thin slabs and plates. [2] [3] It is usually formed by low-grade regional metamorphism of mudrock. [4] [5] This mild degree of metamorphism produces a rock in which the individual mineral crystals remain microscopic in size, [5] producing a characteristic slaty cleavage in which fresh cleavage surfaces appear dull. This is in contrast to the silky cleaved surfaces of phyllite, which is the next higher grade of metamorphic rock derived from mudstone. [6] The direction of cleavage is independent of any sedimentary structures in the original mudrock, reflecting instead the direction of regional compression. [7]

Slaty cleavage is continuous, meaning that the individual cleavage planes are too closely spaced to be discernible in hand samples. The texture of the slate is totally dominated by these pervasive cleavage planes. Under the microscope, the slate is found to consist of very thin lenses of quartz and feldspar (QF-domains) separated by layers of mica (M-domains). [8] These are typically less than 100 microns thick. [4]

The process of conversion of mudrock to slate involves a loss of up to 50% of the volume of the mudrock as it is compacted. Grains of platy minerals, such as clay minerals, are rotated to form parallel layers perpendicular to the direction of compaction, which begins to impart cleavage to the rock. Slaty cleavage is fully developed as the clay minerals begin to be converted to chlorite and mica. Organic carbon in the rock is converted to graphite. [9]

Slate is mainly composed of the minerals quartz, illite, and chlorite, which account for up to 95% of the composition of the slate. The most important accessory minerals are iron oxides (such as hematite and magnetite), iron sulfides (such as pyrite), and carbonate minerals. Feldspar may be present as albite or, less commonly, orthoclase. [10] Occasionally, as in the purple slates of North Wales, ferrous (iron(II)) 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. However, there is evidence that reduced spots may also form after deformation and acquire an elliptical shape from preferential infiltration along the cleavage direction, so caution is required in using reduction ellipsoids to estimate deformation. [11]

Before the mid-19th century, the terms slate, shale and schist were not sharply distinguished. [12] 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. [13] 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. [14]

The British Geological Survey recommends that the term slate be used in scientific writings only when very little else is known about the rock that would allow a more definite classification. For example, if the characteristics of the rock show definitely that it was formed by metamorphosis of shale, it will be described in scientific writings as a metashale. If its origin is uncertain, but the rock is known to be rich in mica, it will be described as a pelite. [2]

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. [15]

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 resistant to frost damage. [16] Natural slate, which requires only minimal processing, has an embodied energy that compares favorably with other roofing materials. [17] 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, [18] often with little or no maintenance. [16] Natural slate is also fire resistant and energy efficient. [19]

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. [20] In the UK, fixing is typically with double nails onto timber battens (England and Wales) [21] or nailed directly onto timber sarking boards (Scotland and Northern Ireland).[ citation needed ] Nails were traditionally of copper, although there are modern alloy and stainless steel alternatives. [22] Both these methods, if used properly, provide a long-lasting weathertight roof with a lifespan of around 60–125 years. [16]

Some mainland European slate suppliers suggest that using hook fixing means that: [23]

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

Slate tiles are often used for interior and exterior flooring, [25] stairs, [26] walkways [27] and wall cladding. [28] Tiles are installed and set on mortar and grouted along the edges. Chemical sealants are often used on tiles to improve durability and appearance, [29] increase stain resistance, [25] reduce efflorescence, and increase or reduce surface smoothness. Tiles are often sold gauged, meaning that the back surface is ground for ease of installation. [29] 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. [30] Due to its thermal stability and chemical inertness, slate has been used for laboratory bench tops and for billiard table tops.

Slate was used by earlier cultures as whetstone to hone knives, [31] [32] but whetstones are nowadays more typically made of quartz. [33]

In 18th- and 19th-century schools, slate was extensively used for blackboards and individual writing slates, for which slate or chalk pencils were used. [30]

In areas where it is available, high-quality slate is used for tombstones and commemorative tablets. [34] In some cases slate was used by the ancient Maya civilization to fashion stelae. [35]

Slate was the traditional material of choice for black Go stones in Japan. It is now considered to be a luxury. [36]

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. [37]

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. [38]

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. [39] [40]

Related Research Articles

Gneiss Common high-grade metamorphic rock

Gneiss is a common and widely distributed type of metamorphic rock. Gneiss is formed by high-temperature and high-pressure metamorphic processes acting on formations composed of igneous or sedimentary rocks. Gneiss forms at higher temperatures and pressures than schist. Gneiss nearly always shows a banded texture characterized by alternating darker and lighter colored bands and without a distinct cleavage.

Shale Fine-grained, clastic sedimentary rock

Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of clay minerals and tiny fragments of other minerals, especially quartz and calcite. Shale is characterized by its tendency to split into thin layers (laminae) less than one centimeter in thickness. This property is called fissility. Shale is the most common sedimentary rock.

Sedimentary rock Rock formed by the deposition and subsequent cementation of material

Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles to settle in place. The particles that form a sedimentary rock are called sediment, and may be composed of geological detritus (minerals) or biological detritus. The geological detritus originated from weathering and erosion of existing rocks, or from the solidification of molten lava blobs erupted by volcanoes. The geological detritus is transported to the place of deposition by water, wind, ice or mass movement, which are called agents of denudation. Biological detritus was formed by bodies and parts of dead aquatic organisms, as well as their fecal mass, suspended in water and slowly piling up on the floor of water bodies. Sedimentation may also occur as dissolved minerals precipitate from water solution.

Metamorphic rock Rock that was subjected to heat and pressure

Metamorphic rocks arise from the transformation of existing rock to new types of rock in a process called metamorphism. The original rock (protolith) is subjected to temperatures greater than 150 to 200 °C and, often, elevated pressure of 100 megapascals (1,000 bar) or more, causing profound physical or chemical changes. During this process, the rock remains mostly in the solid state, but gradually recrystallizes to a new texture or mineral composition. The protolith may be an igneous, sedimentary, or existing metamorphic rock.

Rock (geology) Naturally occurring mineral aggregate

In geology, rock is any naturally occurring solid mass or aggregate of minerals or mineraloid matter. It is categorized by the minerals included, its chemical composition, and the way in which it is formed. Rocks form the Earth's outer solid layer, the crust, and most of its interior, except for the liquid outer core and pockets of magma in the asthenosphere.

Metamorphism Change of minerals in pre-existing rocks without melting into liquid magma

Metamorphism is the transformation of existing rock to rock with a different mineral composition or texture. Metamorphism takes place at temperatures in excess of 150 to 200 °C, and often also at elevated pressure or in the presence of chemically active fluids, but the rock remains mostly solid during the transformation. Metamorphism is distinct from weathering or diagenesis, which are changes that take place at or just beneath Earth's surface.

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.

Siltstone Sedimentary rock which has a grain size in the silt range

Siltstone, also known as aleurolite, is a clastic sedimentary rock that is composed mostly of silt. It is a form of mudrock with a low clay mineral content, which can be distinguished from shale by its lack of fissility.

Phyllite Type of foliated metamorphic rock

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.

Mudrock Class of fine grained siliciclastic sedimentary rocks

Mudrocks are a class of fine-grained siliciclastic sedimentary rocks. The varying types of mudrocks include siltstone, claystone, mudstone, slate, and shale. Most of the particles of which the stone is composed are less than 116 mm and are too small to study readily in the field. At first sight, the rock types appear quite similar; however, there are important differences in composition and nomenclature.

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.

Clastic rock Sedimentary rocks made of mineral or rock fragments

Clastic rocks are composed of fragments, or clasts, of pre-existing minerals and rock. A clast is a fragment of geological detritus, chunks and smaller grains of rock broken off other rocks by physical weathering. Geologists use the term clastic with reference to sedimentary rocks as well as to particles in sediment transport whether in suspension or as bed load, and in sediment deposits.

Slate industry Industry of the extraction and processing of slate

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.

Fissility (geology) Tendency of a rock to split along flat planes of weakness

In geology, fissility is the ability or tendency of a rock to split along flat planes of weakness. These planes of weakness are oriented parallel to stratification in sedimentary rocks. Fissility is differentiated from scaly fabric in hand sample by the parting surfaces’ continuously parallel orientations to each other and to stratification. Fissility is distinguished from scaly fabric in thin section by the well-developed orientation of platy minerals such as mica. Fissility is the result of sedimentary or metamorphic processes.

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.

Slate industry in Spain

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.

Diphwys Casson quarry Former slate mine

Diphwys Casson quarry was a major slate quarry in the town of Blaenau Ffestiniog, Gwynedd, North Wales. It was one of the major users of the Ffestiniog Railway.

References

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Further reading