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What is Chert Rock?

Chert is a fine-grained and hard sedimentary rock that is composed of micro-crystals of quartz (SiO2). It commonly occurs in the form of nodules, layered deposits and concretionary masses. Chert rock is composed of remains of siliceous ooze, the sediment which covers the major portion of the deep ocean floor. Chert often breaks up into pieces with sharp edges and this was used by people to make weapons and tools out of chert.


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Properties and Characteristics of Chert

Chert is just as hard as crystalline quartz and a tough rock also which makes it quite hard to penetrate. It stands above the landscape and prevents soil erosion. It has a waxy lustre and is never transparent.


Formation of Chert

Do you know how the chert stone forms? Chert usually occurs in the form of carbonate rocks to irregular nodules in limestone, chalk and dolomite formations. In the sediments, microcrystals of silicon dioxide grow up to form irregular nodules when the silica is moved over to the site of formation by the groundwater. If these nodules are large in number, they grow to form layers of chert rock within the sediment mass. In some parts of oceans, many diatoms and radios live which have a silica skeleton. When they die, they fall to the floor, dissolve and recrystallize. Thus, chert is also considered a biological sedimentary rock.


Composition of Chert

Chert rock is usually a biogenic rock which is made up of extracts of diatoms, radios and sponge spicules. It is a microcrystalline silicon dioxide. It may also move like a liquid rich in silica and forms nodules in rocks. This is done by replacing the original chert mineral usually carbonates. For this reason, it is also termed to be of chemogenic origin.


Colours of Chert Rock

Chert stone can be seen in many colours. The range exists between white and black or cream and brown. Other chert stones such as green chert or red chert are also very common. It gets its darker colour from the addition of higher organic and mineral matters. Higher organic content can lead to grey or black chert rocks and higher amounts of iron oxides lead to red colour.


Types of Chert Rock

There exists a large variety in the types of chert, which depends on the chert minerals, their visibility and physical properties.

Let us know about some of the chert rock types and their properties.

1. Jasper

It is primarily formed as primary deposits found in connection with magmatic formations. This provides its characteristic red colour. It also occurs in yellow, black or green colours.

2. Flint

It is microcrystalline quartz which is formed by the replacement of calcium carbonate with silica. It is usually found as nodules and was historically used to make tools.

3. Opal

It is a form of hydrated silicon dioxide and is often of Neogenic origin.

4. Common Chert

It is the most abundant form of chert which forms in limestone formations by replacing calcium carbonate with silica. It is less used to make gemstones and tools while flint is much more preferable.

5. Radiolarite

It is another form of chert rock that is formed as primary deposits and contains radiolarian microfossils.

6. Magadi

It is a chert rock that is formed from a sodium silicate precursor in alkaline lake environments.

7. Tripolitic Chert

A light-coloured porous siliceous sedimentary rock, Tripolitic Chert is formed by the weathering of chert or siliceous limestone.


Uses of Chert Stone

In today’s life, Chert has minimal uses but in the historic periods, it was often used for construction purposes and makings of tools.

A. Tools

In historic times, chert was frequently used as a material for the preparation of stone tools. Like other stone tools, chert also fractures in a Hertzian cone when it is struck with some force. When a chert stone is beaten against iron or steel, it results in sparks making it ideal for starting fires. Both common chert and flint were frequently used for making fire igniting tools.


B.   Construction Purposes

It is ubiquitous in some regions as stream gravel and fieldstone. Chert is used as a construction material and for road surfacing. The primary reason why chert is used for it is that it tends to get more firm and compact during rain while others get wet and muddy.

FAQs (Frequently Asked Questions)

1. What are Sedimentary Rocks? Name Some Sedimentary Rocks?

Answer: The types of rocks that are formed by the deposition and accumulation of organic or mineral particles at the surface of Earth. The particles of the sedimentary rocks are called sediments. Some common sedimentary rocks are sandstone, siltstone, shale, chert, flint, limestone and rock salt.

2. What are the Concerns With Using Chert?

Answer: Following are the issues concerned with chert:

  1. Chert has several problems when it is used as concrete aggregates. When chert gets weathered while it is used in concrete, it develops pop-outs that undergo freezing and thawing. This is due to the highly porous nature of weathered chert.

  2. Certain varieties of chert undergo a specific reaction with highly alkaline types of cement. This leads to expansion and cracking of concrete which leads to failure of chert.

3. What Does the Term Chert Mean?

Answer: The term chert in petrology usually refers to rocks composed majorly of microcrystalline, cryptocrystalline and micro-fibrous quartz. 

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