The Mineral Slate



The mineral slate is a metamorphic rock of lower class. It is normally the result of the metamorphic activity taking place as a result of low temperature and pressure situation on shale/mudstone or perhaps on basalt. The minerals of clay are the basic rock, which on account of metamorphosis becomes mica with the configuration of Muscovite, Biotote, and Chlorite. The characteristic of slate is finely foliated through its split, leaving surfaces that are flat and smooth (regularly regarded as “slaty cleavage” not to mistake it for mineral cleavage). At times, we see the source of the bed on the planes of foliation. Contrasting to the blurred ‘thud’ sound with shale and mudstone, a ‘ring’ is heard when we strike a slate.

We express the word “slate” when anything is made from the rock slate. It is either a writing slate or a single tile used for roofs. Conventionally, this rock, that is small and smooth is mostly used either as a notice board or a notepad used for writing on with chalk and frequently framed, in particular to display the rates in inns or pubs. The phrases that we use such as “blank Slate” or “clean slate” is the outcome from here. Slate is a metamorphic rock where the grains are very finely foliated. Here the planes of the rock slate are perpendicular to the path where the compression of metamorphosis takes place, where foliation will not be similar to the layering basic sedimentary rocks.

Historical mining terminology

The expressions such as shale, slate and schist were not clearly differentiated in the early 19th century. In the United States coal that was mined beneath the ground was termed generally as shale through the 20th century. For instance, slate used for drawing was called shale, whereas slate used for roofing was termed as shale obtained over the seam of coal.

Physical Properties of Slate

The physical property of slate is Metamorphic-Argillaceous rock. This mineral has properly formed cleavages that split either having or not having gone through the re-crystallization process and with or without knots/spots of Andulasite, Pyrite, Garnet types of minerals. The grains of these minerals are from fine to medium, they are moderately compact and hard, the shades and textures of this mineral vary in their uniformity. It possesses the property of splitting into slender sheets down the cleavage plane; it is on account of the extended and compressed minerals that exist along the plane of cleavage.

Specific Gravity 2.65 to 2.80

Modulus of Rupture 22 to 34 N/mm2

Porosity Low to very low

Hardness 3.0 to 4.0 on Mohs scale

Weather Impact Resistant

Density 2.6 to 2.8 Kg/cm3

Water Absorption: 1.5 to 2%

Corrodibility: 0.4 to 0.7%

Compressive Strength:24 to 36 N/mm2

Chemical Properties of Slate

In the chemical form, slates are metamorphic, Multi-Mineral, argillaceous rocks composed of collection of colloidal and mineral matter. Fundamentally the mineral is composed of mica, quartz, oxides of iron and Sericite. They have at times knots/spots of minerals such as Pyrite, Andulasite, etc.

TiO2 0.4-0.5%

SiO2 55-65%

Lime (CaO) 0.5-1%

Loss On Ignition (LOI) 3-4%

Alumina (Al2O3) 15 to 20%

Iron (Fe2O3) 4%-8%

It is fairly resistant to acids and staining.

Magnesia (MgO) 0.5-3%

Soda (Na2O) & Potash (K2O) 2-4%


Mineral composition

The main composition of slate is Muscovite, Elite or Quartz, frequently amid Chlorite, Hematite, Pyrite and Biotite. Irregularly we find slate along with Tourmaline, Kaolinite, Graphite, Apatite, Magnetite or Feldspar and Zircon. Infrequently, the slates obtained in North Wales, which is purple in color, contain reduced spheres of ferrous which are formed round the nuclei of iron, letting behind a texture of pale green spots. When we observe the example of the cleavage plane they emerge in the shape of an ellipses because of the deformation that takes place in the spheres when stress is subsequently applied to the ovoid.

Slate extraction

A good amount of slate obtained from Europe comes from Spain. In fact, Spain is the major exporter and producer of naturally obtained slate. In addition, 90% of the naturally obtained slate of Europe is made use for roofs which was initially obtained from the slate industry. The world’s second largest producer of slate is Brazil, where it is abundantly available. 95% of Brazil’s slate is obtained in the region of Papagaios in Minas Gerais. But, Brazil’s every slate is not permitted to abide the CE mark.

Colored slated is obtained from the mines of Granville, in the regions of New York. It is in the east coastal regions of Newfoundland, Virginia, Buckingham County, the Slate Valley of Vermont, New York and eastern regions of Pennsylvania where slate is produced.. The slate from Pennsylvania is abundantly used in making turkey calls to hunt turkeys in the United States. The sound created from the slate (when we scratch with variations of strikers on the wood) replicates nearly every call of the four varieties of the turkey of North America: Osceola, Merriam’s, Eastern Wild Turkey, and Rio Grande.

The deposits of slate are extensively in China; recently the exporting of refined and unrefined slate has gone up. Slate having variety of colors are obtained in China.

How Does Slate Form?

The production of slate that occurs in the tectonic surroundings is normally an earlier sedimentary basin, which gets entailed in the boundary of the convergent plate. In this basin mudstones and shale undergo compression by slight heat and the forces that act horizontally. This modification of clay minerals takes place in mudstone and shale by this heat and forces. Development of foliation takes place in the shape of right angles where the forces of compression of the boundary of the convergent plate give way forming vertical foliation which generally traverse the bedding planes that was already in shale.

Uses of the Word "Slate"

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The term “slate” is of inconsistent use in the later period and in certain industries. At present, the majority of the geologists take care in not using the term “slate” when they speak about “shale”. But, in the earlier days the term slate was regularly used in free reference to shale.

The perplexity of the expressions happens partly because of the reality that shale in due course gets transformed to slate. Just think that you are driving towards the east of Pennsylvania, along the region where metamorphism is on the rise, beginning where the rock is actually “shale” and then halting to scrutinize every projection. You are sure to be in a dilemma as to where on the path “shale” was transformed into “slate”. It is hard to name exactly the rock you picked which has undergone slight metamorphism.

In the Appalachian Basin of the industry in the coal mines the term “slate” is made use of by most of the miners, to refer shale that makes up the floor and the roof of the mine, and also the fragmented shale which separates the coal in the manufacturing plants. The fresh miners are trained by the experienced miners and thus the antiquated language is transferred.

During the 1800’s the elementary students of schools were using a little piece of slate framed with wood in order study and write problems in arithmetic. To write on a little pencil slate, clay or soapstone was used. Cleaning the slate was done by wiping with a soft cloth. Tiny slates were made use of in business in order to chalk out the schedules, menus, daily events, prices and various notices and events in schools. Nowadays, these slates used for writing is vanished from the schools, the term “slate” is continuing in usage in the phrases like “wipe the slate clean,” ”Put it on the slate”, “Slated for today” and even more. It might be of interest to know that slates are still used as a writing medium in rural Indian schools.

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