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 on Chert
1. What exactly is chert and what is its primary composition?
Chert is a hard, dense, microcrystalline or cryptocrystalline sedimentary rock. Its primary chemical composition is silicon dioxide (SiO₂), the same as quartz. Unlike the large, visible crystals in quartz, the silica crystals in chert are so small that they can only be seen with a high-powered microscope.
2. How is chert formed in a geological context?
Chert is typically formed through two main processes. The first is the biochemical accumulation of silica-rich skeletons from microorganisms like diatoms, radiolarians, and sponges on the ocean floor. The second process involves the chemical replacement of other materials, like limestone or wood, where silica-rich fluids permeate the original substance and replace it over millions of years.
3. What are the key physical properties used to identify chert?
Chert can be identified by a combination of its distinct physical properties:
Hardness: It is very hard, with a Mohs hardness of about 7, meaning it can scratch steel and glass.
Fracture: It exhibits a conchoidal fracture, which means it breaks with smooth, curved surfaces like broken glass. This property made it ideal for crafting sharp tools.
Lustre: It typically has a dull to waxy or vitreous (glassy) lustre.
Texture: It feels smooth to the touch due to its microcrystalline structure.
4. What is the main difference between chert and flint?
The primary difference between chert and flint lies in their formation environment and resulting purity. Flint is a specific, high-purity variety of chert that typically forms as nodules within chalk or marl limestone deposits. Chert is the broader geological term. While often used interchangeably, flint is commonly associated with darker colours (grey, black) and a finer texture due to its specific origin.
5. Where are major chert deposits typically found?
Chert is found worldwide in sedimentary rock formations. It most commonly occurs as nodules in limestone and chalk beds or as extensive layered deposits known as bedded chert. Significant formations include the Flint Hills in the USA, the chalk cliffs of Dover, England (which contain flint nodules), and the Rhynie chert in Scotland, famous for its well-preserved fossils.
6. What are some common historical and modern uses of chert?
Historically, chert was invaluable for making stone tools, such as arrowheads and scrapers, due to its hardness and predictable conchoidal fracture. It was also used as a fire-starter when struck against iron pyrite. In modern times, it is mainly used as a road aggregate, in construction fill, and for making gravel.
7. Why does the colour of chert vary so much, from white to black?
The wide range of colours in chert is due to the presence of various impurities and trace elements within its silica structure. White or light grey chert is very pure, while black or dark grey chert (flint) gets its colour from included organic matter. Red, brown, or yellow varieties, known as jasper, owe their colour to iron oxide impurities.
8. Why is chert considered a biochemical sedimentary rock?
Chert is often classified as a biochemical sedimentary rock because its silica frequently originates from the remains of living organisms. Microscopic marine life such as diatoms and radiolarians build their skeletons from silica in seawater. When these organisms die, their skeletons settle on the seabed, accumulate, and eventually compact and solidify into chert over geological time.
9. What are the main issues with using chert as an aggregate in concrete?
Some types of chert can cause significant problems when used as a concrete aggregate due to a chemical reaction known as the alkali-silica reaction (ASR). This reaction with the alkaline cement paste forms a gel that expands, causing internal pressure that leads to cracking and structural failure of the concrete. Additionally, porous chert can cause surface 'pop-outs' during freeze-thaw cycles.
