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 —  Metamorphic Rock  —
Slate Image
Primary quartz, muscovite/illite
Secondary biotite, chlorite, hematite, pyrite
Slate Macro (~ 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. The result is a foliated rock in which the foliation may not correspond to the original sedimentary layering. 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.

Ninety percent of Europe's natural slate used for roofing originates from the Slate Industry in Spain.[1]


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, the term slate was commonly used to refer to shale well into the 20th century.[3] For example, roof slate refers to shale above a coal seam, and draw slate refers to roof slate that falls from the mine roof as the coal is removed.[4]

Mineral composition

Slate is mainly composed of quartz and muscovite or illite, often along with biotite, chlorite, hematite, and pyrite and, less frequently, apatite, graphite, kaolin, 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.


Slate roof.

Slate in buildings

Slate can be made into roofing slates, also called roofing shingles, installed by a slater.[5] 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 easily stackable.

Slate is particularly suitable as a roofing material as it has an extremely low water absorption index of less than 0.4%. Its low tendency to absorb water also makes it very resistant to frost damage and breakage due to freezing.

Slate roof tiles are usually fixed using either nail fixing, or the hook fixing method as is common with Spanish slate. Using hook fixing means that [6]:-

  • Areas of weakness on the tile are fewer since no hole has had to be drilled
  • Roofing features such as valleys and domes are easier to create since narrow tiles can be used
  • Hook fixing is particularly suitable in regions subject to severe weather conditions, such as Scotland and parts of Wales, since there is a greater resistance to wind uplift as the lower edge of the slate is secured.

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 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.

Other uses

A grave with inscription on slate.

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 gravestones, commemorative tablets and by artists in various genres.

Slate extraction

Historical Pit Vogelsberg 1 at Fell

See main article at Slate industry

In Eurasia

Slate-producing regions in Europe include Wales (see slate industry in Wales), Cornwall (famously the village of Delabole), Cumbria (see Burlington Slate Quarries, Honister Slate Mine and Skiddaw Slate) in the United Kingdom; parts of France (Anjou, Ardennes, Bretagne, Savoie); Belgium (Ardenne); 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, Eifel, Westerwald, Thuringia and north Bavaria); Alta, Norway (actually schist not a true slate) and Galicia. Some of the slate from Wales and Cumbria is colored slate (non-blue): (purple and formerly green in Wales) and (green in Cumbria). China has vast slate deposits; in recent years its export of finished and unfinished slate has increased, it has slate in various colors.

In the Americas

Slate is abundant in Brazil (the second-biggest producer of slate) around Papagaio in Minas Gerais (responsible for 95% of the extraction of slate in Brazil). The independent report by Consultant Geologist J A Walsh describes how certain products originating from Brazil on sale in the UK, are not entitled to bear the CE mark. Such products are sedimentary rocks that have split along their original bedding plane. True slate has been subjected to metamorphism and does not split along bedding, but rather along planes associated with the realignment of minerals during metamorphism. This realignment, known as ‘schistosity’, bears no relationship to the original horizontal bedding planes. Because such Brazilian products display higher water absorption indexes than those from other areas such as Galicia, this makes them less suitable for use as roofing tiles, since the study showed a significant loss of strength when subject to thawing and freezing[7].

Other areas known for slate production are the east coast of Newfoundland, the Slate Belt of Eastern Pennsylvania, and the Slate Valley of Vermont and New York, where colored slate is mined in the Granville, New York area.

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

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


Shale can metamorphose into slate; sometimes the fossils may remain intact.

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.[8]

See also


  1. ^ European Association of Mining Industries website retrieved on 26/01/2010
  2. ^ R. W. Raymond, Slate, A Glossary of Mining and Metallurigical 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. ^ Hart, Diane (1991) The building slates of the British Isles. Watford: Building Research Establishment ISBN 0851254837
  6. ^ Galician and Spanish Slate website “Hook Fixing” . Retrieved on 26/01/2010
  7. ^ Fundación Centro Tecnológico de la Pizarra’s 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, retrieved on 27/01/2010
  8. ^ BBC Video : David Attenborough : Lost Worlds, Vanished Lives
  • Page, William (ed.) (1906) The Victoria History of the County of Cornwall; vol. I. (Chapter on quarries.) Westminster: Constable

Further reading

  • Hudson, Kenneth (1972) Building Materials; chap. 2: Stone and slate. London: Longman ISBN 0582127912; pp. 14-27

External links

1911 encyclopedia

Up to date as of January 14, 2010

From LoveToKnow 1911

SLATE (properly Clay Slate; in M. Eng. slat or sclat, from O. Fr. esclat, a small piece of wood used as a tile; esclater, to break into pieces, whence modern Fr. éclat, the root being seen also in Ger. schleissen, to split), in geology, a fissile, fine-grained argillaceous rock which cleaves or splits readily into thin slabs having great tensile strength and durability. Many other rocks are improperly called slate, if they are thin bedded and can be used for roofing and similar purposes. One of the best known of these is the Stonesfield slate, which is a Jurassic limestone occurring near Oxford and famous for its fossils. Slates properly so-called do not, except on rare occasions, split along the bedding, but along planes of cleavage, which intersect the bedding usually at high angles. The original material was a fine clay, sometimes with more or less of sand or ashy ingredients, occasionally with some lime; and the bedding may be indicated by alternating bands of different lithological character, crossing the cleavage faces of the slates, and often interrupting the cleavage, or rendering it imperfect. Cleavage is thus a superinduced structure, and its explanation is to be found in the rearrangement of the minerals, and the development of a certain degree of crystallization by pressure acting on the rock. Slates belong mostly to the older geological systems, being commonest in Pre-Cambrian, Cambrian and Silurian districts, though they may be found of Carboniferous or even of Tertiary age, where mountain-building processes have folded and compressed these more recent formations. The action of pressure is shown also by the fossils which sometimes occur in slates; they have been drawn out and distorted in such a way as to prove that the rock has undergone deformation and has behaved like a plastic mass. Evidence of the same kind is afforded by the shape of the knots and concretions sometimes present in the slate. If the bedding be traced, either in the slates or in the other rocks which accompany them, flexures will be frequently observed (the folding often being of an isoclinal type), while reversed faulting, or thrusting, is usually also conspicuous.

Missing image

The origin of slaty cleavage is in some measure obscure. This structure is by no means confined to slates, though always best exemplified in them, owing probably to the finegrained, argillaceous materials of which they consist. Grits, igneous rocks, ashbeds and limestones may and often do show cleavage. Coarse rocks and rocks consisting of hard minerals are always imperfectly cleaved. The cleavage of slates must be distinguished from cleavage of minerals, the latter being due to different degrees of cohesion along definite crystallographic planes. The connexion of cleavage with pressure, however, is unmistakable. It is never exhibited except by rocks which have been sub jected to the tangential stresses set up in the earth's crust by folding. These stresses may operate in several ways. They will alter the shape of mineral particles by broadening them in a direction at right angles to the principal pressures, while they are thinned in the direction in which the pressure acted. Probably the size of the particle will be slightly reduced. This method of reasoning, however, does not carry us far, as the minerals of slates vary considerably in form. Pressure will also tend to produce an expansion of the rock mass in a direction (usually nearly vertical) at right angles to the compression, for such rocks as slates are distinctly plastic in great masses. This flowage will help to orientate the particles in the direction of movement, and, operating conjointly with the flattening above explained,will accentuate the liability to cleave in a definite set of planes. The recrystallization induced by pressure is probably of still greater importance. Slates consist largely of thin plates of mica arranged parallel to the cleavage faces. This mica has developed in the rock as it was folded and compressed. In the moist and plastic slate the mineral particles slowly enlarged by the addition of new crystalline molecules. Those faces which were perpendicular to the pressure would grow slowly, as the great pressure would promote solution, and inhibit deposition; the edges or sides, on the other hand, being less exposed to the pressure would receive fresh deposits. In this way thin laminae would form, lying at right angles to the direction of greatest stress. Micas and other platy minerals (such as chlorite), which naturally grow most rapidly on their edges, would show this tendency best, and such minerals usually form a large part of the best slates; but even Sketch (by Du Noyer) of a block of variegated slate from Devil's Glen, Co. Wicklow. The crumpled bands mark the bedding, and the fine perpendicular striae in front are the cleavage planes; the fine lines on the darkened side merely represent shadow, and must not be taken for planes of division in the rock. It will be observed that the cleavage planes do not pass through the white bands.

quartz and felspar, which under ordinary conditions form more equidimensional crystals, would assume lenticular forms. In the necessary co-operation of these three causes, viz. flattening of particles by compression, orientation of particles by flow and formation of laminar crystals, the fundamental explanation of slaty cleavage is found. The planes of cleavage will be approximately perpendicular to the earth pressures which acted in the district; hence the strike of the cleavage (i.e. its trend when followed across the country) will be persistent over considerable areas.

Where the rock masses are not homogeneous (e.g. slates alternating with gritty bands), the cleavage is most perfect in the finest grained rocks. In passing from a slate to a grit the direction of the cleavage changes so that it tends to be more nearly perpendicular to the bedding planes. A structure akin to cleavage, often exemplified by slates especially when they have been somewhat contorted or gnarled, is the Ausweichungsclivage of Albert Heim. It is produced by minute crumplings on the cleavage faces all arranged so that they lie along definite planes crossing the cleavage. These slight inflections of the cleavage may be sharp-sided, and may pass into small faults or steps along which dislocation has taken place. A secondary or false cleavage, less perfect than the true cleavage, may thus be produced (see Petrology, Pl. IV. fig. 7). The faces of slates have usually a slightly silky lustre due to the abundance of minute scales of mica all lying parallel and reflecting light simultaneously from their pearly basal planes. In microscopic section the best slates show much colourless mica in small, thin, irregular scales. Green chlorite is usually also abundant in flakes like those of the mica. The principal additional ingredient is quartz in minute lens-shaped grains. The size of the individual particles may be approximately one-five-hundredth of an inch. Minute rods or needles of rutile are also common in slates, and well-formed cubes of pyrites are often visible on the splitting faces. The brownish colour of some slates is due to limonite and haematite, but magnetite occurs in the darker coloured varieties. Other minerals which occur in the rocks of this group are calcite, garnet, biotite, chloritoid, epidote, tourmaline and graphite or dark carbonaceous materials.

By advancing crystallization and increased size of their components, slates pass gradually into phyllites, which consist also of quartz, muscovite and chlorite. In the neighbourhood of intrusive granites and similar plutonic igneous rocks, slates undergo "contact alteration," and great changes ensue in their appearance, structure and mineral composition. They lose their facile cleavage and become hard, dark-coloured, slightly lustrous rocks, which have a splintery character or break into small cuboidal fragments. These are known as "hornfelses" (q.v.). Farther away from the granite the slates are not so much altered, but generally show small rounded or ovoid spots, which may be darker or lighter in colour than the matrix. The spots contain variety of minerals, sometimes mainly white mica or chlorite. In these spotted slates andalusite, chiastolite, garnet and cordierite often occur; chiastolite is especially characteristic; cordierite occurs only where the alteration is intense. The chiastolite-slates show elongated, straight-sided crystals with black cores (see Petrology, Pl. IV. fig. 9), which, on transverse section, have the form of a cross constituting the two diagonals of the rhombic or squarish pattern of the mineral. These crystals may be half an inch to several inches in length; they are usually more or less completely weathered to white mica and kaolin. In other cases, especially near mineral veins, slates are filled with black needles of tourmaline or are bleached to pale grey and white colours, or are silicified and impregnated with mineral ores. Frequently in districts where slates are much crumpled they are traversed by numerous quartz veins, which have a thickness varying from several inches up to many feet, and may occasionally be auriferous. (J. S. F.) Slates are widely used for roofing houses and buildings of every description, and for such purposes they are unequalled, the better sorts possessing all the qualities necessary for protection against wind, rain and storm. The finer varieties are made into writing slates, and in districts where cross cleavage exists slate pencils are made. Slabs are also manufactured, and, being readily cut, planed, dressed and enamelled, are used for chimney pieces, billiard tables, wall linings, cisterns, paving, tomb-stones, ridge rolls, electrical switch-boards and various other architectural and industrial purposes.

Slate rocks are quarried both above ground and below ground, according as they lie near to or distant from the surface. When they are near the surface, and their dip corresponds with the slope of the ground, they are in the most favourable position, and are worked in terraces or galleries formed along the strike of the beds and having a height of about 50 ft. The galleries are generally carried on in sections of to yds., worked across the beds, and may rise to any height or be sunk below the surrounding level by excavations. When the rock is much removed from the surface, or inconveniently situated for open workings, it is quarried in underground chambers reached by levels driven through the intervening mass and across or along the beds. Or it may be necessary to sink shafts as in coal-pits before the rock is arrived at, but the cost of doing so forms a serious drawback. The material is sometimes won by the aid of channelling machines which make a series of cuts at right angles to each other in the face of the rock; a block is then broken off at its base by wedges forced into the cuts, and its removal permits access to other blocks. When blasting is resorted to, advantage is taken of the natural cuts or joints, as the rock is readily thrown or worked off these. The explosive used should be of such a character as to throw out or detach masses of rock without much splintering, which would destroy the blocks for slate-making. From the mass thrown out by the blast, or loosened so as readily to come away by the use of crowbars, the men select and sort all good blocks and send them in waggons to the slate huts to be split and dressed into slates. Two men are employed at this operation - one splitting and the other dressing, performing their work in a sitting posture. The splitter places a block on end between his knees, and with chisel and mallet splits it into as many plates as possible of the usual thickness for roofing purposes - namely, a quarter of an inch more or less according to the size and strength required. These plates are then placed horizontally by the dresser on a vertical iron "stand," and cut with a sharp knife into slates of various sizes suitable for the market. For an enumeration of these sizes, see Roofs, where also will be found an account of the different varieties of slates and of the ways in which they are fixed.

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Simple English

[[File:|thumb|right|Blue slate]] Slate is a metamorphic rock. It is formed from shale by being under pressure and moderate heat. Shale itself is formed from muddy clay, and splits into thin plates. This characteristic splitting is carried over into slate.

Schist is a rather similar type of metamorphic rock, and students need to learn the difference.


File:Slate Macro
Grey-green slate (~ 6 cm long and ~ 4 cm high)

Roofing slates are used in many countries, though ceramic tiles have gradually taken over. Heavier slates can be used for flooring (inside and outside). Slates are used for the cladding (surface skin) of buildings, and sometimes in Wales there are whole buildings of slate. Gravestones and memorial plaques are another use. Thick slate is used for billiard tables and laboratory bench tops, where its resistance to temperature changes and chemicals is useful. Slate was used for blackboards and writing tablets in schools.


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