Paper Detail

Lithology and definition of Shales

To define shales, you must first know:

Shales are classified into four categories according to their content of silt minerals. Fine-grained shales, that lack silt grains, can be classified according to their chemical decomposition or their stone sand layers. To classify them more accurately, an X-ray is used to determine their minerals. In sedimentary (non-calcareous) rocks, the amount of potassium is usually higher than that of sodium, the amount of CO in them is low, and the amount of Al2O3 is usually high. Shales contain varying amounts of chlorite, detrital silica or colloids, small amounts of calcite in the form of cement, and sometimes fossils. Detrital accessory minerals in the form of rutile needles, zircon, and others are also seen in shales. Sodium is present in the  form of detrital albite. In water, it is seen as sediments, as well as the ions substituted other minerals.

The first obvious change in ordinary shale minerals is the re-crystallization of illite and chlorite clay minerals and the emergence of Muscovite and chlorite. At the same time, Calcite combines with clay minerals to give epidote and Zoisite (The type depends on the amount of iron available). Quartz, albite, and re-crystallized accessory minerals destroy the sedimentary fabric effect. In the next step, muscovite and chlorite combine to form biotite. In the more advanced stage, the residual chlorite is converted to Almandine, which results in the release of MgO in the early stages. It should be noted that in any changes, all rock minerals start to react, and no reaction resulting from the combination of two minerals or the decomposition of one mineral is self-sufficient. Product diversity is the result of different raw materials used by factories. In Iran, bricks have long been produced with clay. But suitable clay is not found everywhere, and the clay soil used usually contain large amounts of lime. That’s why the resulting bricks are not in accordance with the national and international standards and do not have the appropriate quality in the facade. Gradually, in the brick industry, other minerals, such as shale, have replaced clay. The characteristics of shales and shale bricks are as follow:

Definition of shale and its constituent compounds:

Shale is a type of sedimentary rock that consists mainly of fine grains. The shale has parallel plates and layers that are easily laminated. The thickness of each layer is less than one centimeter. Shale is mainly a combination of clay minerals and quartz grains, and clay is their most important component. It is usually gray, but the amount of shale constituents determines its color. Black shale, for example, contains more carbon compounds. Being red is due to the presence of iron oxide (hematite). Iron hydroxides, including goethite and limonite in shales, produce brown and yellow shade, respectively. Chlorite, biotite, and illite minerals in shales create green shade.

Location of shale and its sources in Iran:

The shales are formed in places where the speed of water is slow and sedimentation is easy. Therefore, shales can be found mainly in lakes, river deltas, and sedimentary plains.

Shales are one of the main rocks in Iranian rock factories. The proliferation of shale units is seen in almost all structural units in the country. The largest one is in Sanandaj-Sirjan region. The other one is located in Alborz-Azerbaijan and Central Iran.

Shale’s classification (according to the definition of shale):

Shales are classified into four groups based on the silt minerals they contain. Fine-grained shales, that lack silt grains, can be classified according to their chemical decomposition or their stone sand layers. To classify them more accurately, an X-ray is used to determine their minerals. The most important types of shale are:

Quartzose Shales:

These rocks contain a large amount of fine-grains quartz with the dimensions of silt. Feldspathic grains are usually rare, but the cement of these shales may be calcareous, gluconite, iron-bearing, or carbon-bearing. They are usually green to gray and are sometimes seen in brown, red, and black. These shales, like siliceous orthocoartites, usually represent the progress of the sea.

Feldspathic Shales:

Feldspathic shales, sometimes called kaolinite shales, always contain more than 10% feldspar, and their matrix is made up of kaolinite or clay minerals. These shales are about the size of sandy silt, and clay silt, and sometimes the grains are larger. With regard to the size, these shales are usually seen with Arkose. They are usually gray, green, red, and chocolate.

Chloritic shales:

These rocks are similar to phillarnite and are often seen with them. Feldspar is always present in these shales, and its amount may be higher than quartz, and chlorite may be seen in the rock matrix. These rocks represent severe erosion in mountainous areas.

Micaceous Shales:

These rocks are similar to sub-phillarnite and are often seen with them. A large number of mica sheets are found in these rocks, which are often associated with sericite. They are usually gray or brownish-gray, but sometimes they are also seen in red and green.

Shale in the brick industry

In the article “What is brick”, it was mentioned that brick is baked in the kiln, and the initial soil for the brick production is provided by sedimentary particles such as clay, shale, and marl.

Previously, producers produced facade brick with clay, which had low strength and was very fragile. It also gets dirty and damaged very quickly and destroys the facade of the building after a short time. Today, with the advancement of technology and many experiments, producers can produce high-quality bricks using sedimentary rock such as shales. The brick produced by shale is very similar to a rock in terms of strength. It is also highly resistant to corrosion.

Shale bricks are considered postmodern because they are a combination of modern and traditional design.

Advantages of using shale stone in brick production:

1- High resistance to pressure and frost

Due to the raw materials of this type of brick, which resembles stone, they are almost twice as resistant as clay bricks. Therefore, this type of brick is used in interior and exterior designs.

2. Anti-erosion

Being high resistant, this brick is used in paving stones and sidewalks.

3. Very low water absorption

The water absorption in shale products is very low. It absorbs very little water when it rains. Therefore, it does not cause cracks in cold and heat. Also, its color remains constant.

4- Environmentally-friendly

The raw material of shale is very clean. It causes a low level of contamination and does not pollute the air. On the other hand, previously bricks were produced by clay. Clay is used in agriculture and is very fertile. By producing bricks from shale stones, this environmental problem has also been solved.

5. Beauty

No artificial color is used in shales. Therefore, the color of the brick will not change in case of heavy rain and prolonged sun exposure. On the other hand, this stone brick also has anti-efflorescence properties.