Introduction to Siliceous Rock
Any rock belonging to the group of sedimentary rocks that largely or entirely consists of silicon dioxide (SiO2) in either of the two forms of quartz or amorphous silica and cristobalite is widely known as siliceous rock. In the class of the sedimentary rocks, the rocks that are formed as a result of the chemical precipitation are included in the category and examples of siliceous rock or silica rock, as it contains silicon dioxide as the main constituent. This siliceous sedimentary rock does not include any rock that is formed due to detrition or fragmentation.
Classification of Rocks
Rocks are an integral part of the Earth’s crust. They are found in various shapes and sizes and are formed because of extreme geological processes going on in the crust of the Earth. They are majorly classified into three distinct types based on their formation processes and because of which they have their own unique characteristics. The classification of the rocks is as given below:
Igneous Rocks: These are the types of rocks that are formed because of the solidification of the lava and the magma. This solidification can occur either below the surface or above the surface of the Earth. When it occurs below the surface, they are known as intrusive rocks and they are highly crystalline in feature whereas, when the solidification takes place above the surface, they are known as extrusive rocks and they are highly glassy in feature. Granite is one of the most common examples of igneous rocks in the world and it is an intrusive igneous rock.
Sedimentary Rocks: These are the rocks that are formed by the deposition of the fragments of rocks that weathered away due to exogenous processes. Mostly these rocks are formed at the river beds and the fragments are converted into rocks by compaction through a process called lithification. Limestone and silica rock is the most common form of sedimentary rocks found across the world.
Metamorphic Rocks: These are the kind of rocks that are formed because of the recrystallisation of the already existing rocks or fragments of rocks. There are various types of metamorphic rocks depending on the layer patterns. Gness is the best example that is found for metamorphic rocks.
Sedimentary Rocks
As mentioned before the sedimentary rocks are the ones that are formed by the settling of the fragments of rocks that are subjected to different exogenous processes. These fragments are transported because of different processes such as the river flow and then over time are settled whenever suitable conditions are present. These deposits are converted into rocks through compaction in a process called lithification. The sedimentary rocks are classified into the following three major categories depending on their mode of formation:
I. Mechanically Formed: Examples include sandstone, limestone, shale, loess, etc.
II. Organically Formed: Examples include geyserite, chalk, limestone, coal, etc.
III. Chemically Formed: Chert, limestone, halite, potash, etc. are examples of such types.
As can be seen, most of the types of rock examples that are formed due to chemical precipitation are the ones that are mostly included in the category of siliceous sedimentary rocks. The siliceous chert is one of the most common forms of such rocks followed by the siliceous limestone which is also one of the pretty common examples of siliceous rocks. Other examples of siliceous sedimentary rocks, also known as silica rocks as these are rocks high in silica, include siliceous dolomite and calc gneiss. Some of the less common forms also include siliceous sandstones in places where they are formed because of certain chemical processes. Of the different types of available silicate mineral rocks, pyroxene rock is also one of them which is nothing but a type of inosilicate i.e. the chain silicates of silicate tetrahedra.
These siliceous rocks are also not devoid of life. They are known to commonly form the silica-secreting organisms such as the radiolarians, diatoms or other types of sponges. These are one of the salient features of siliceous rocks.
FAQs on Siliceous Rock
1. What are siliceous rocks?
Siliceous rocks are a type of sedimentary rock characterised by a high concentration of silica (SiO₂). This silica can be in the form of microcrystalline quartz, such as chert or flint, or from the accumulation of silica-secreting organisms. They are defined by their chemical composition rather than their grain size.
2. What are the most common examples of siliceous rocks?
Common examples of siliceous rocks, based on their origin and composition, include:
- Chert: A hard, dense microcrystalline rock composed of silica that forms from chemical or biochemical precipitation.
- Flint: A dark-coloured variety of chert, known for its hardness and historical use in making tools.
- Diatomite: A light, porous rock formed from the fossilised remains of diatoms, a type of algae with silica shells.
- Radiolarite: A rock formed from the compacted silica skeletons of microscopic marine organisms called radiolarians.
- Siliceous Sandstone: A type of sandstone where the sand grains are cemented together by silica, making it exceptionally durable.
3. How are siliceous rocks formed?
Siliceous rocks are primarily formed through two main processes:
- Biogenic Origin: This involves the accumulation of silica-rich skeletons from microscopic organisms like diatoms and radiolarians on the ocean floor. Over millions of years, these deposits are compacted and lithified (turned into rock).
- Chemical Origin: This occurs when silica dissolved in water precipitates directly to form layers of chert. This can happen in deep marine environments or when silica-rich fluids replace other minerals in existing rocks, like limestone.
4. What is the difference between siliceous, argillaceous, and calcareous rocks?
The main difference lies in their primary chemical composition. Siliceous rocks are rich in silica (SiO₂). Argillaceous rocks, like shale and mudstone, are dominated by clay minerals (hydrous aluminium silicates). Calcareous rocks, such as limestone and chalk, are composed mainly of calcium carbonate (CaCO₃).
5. Is granite considered a siliceous rock?
This is a common point of confusion. While granite is very rich in silica, it is an igneous rock, formed from cooled magma. The term "siliceous rock" is specifically used in geology to describe sedimentary rocks with a high silica content. Therefore, while granite is a silica-rich rock, it is not classified as a siliceous sedimentary rock.
6. What are the main industrial uses of siliceous rocks?
Due to their unique properties, siliceous rocks have several important applications. Diatomite is highly porous and is widely used in filtration systems, as a mild abrasive in products like toothpaste, and as an industrial absorbent. Harder siliceous rocks like chert and durable siliceous sandstone (quartzite) are used as high-quality construction aggregates and building stones.
7. Why is sandstone sometimes classified as a siliceous rock?
Sandstone is classified as a siliceous rock when its sand grains are predominantly made of quartz (a silica mineral) and, more importantly, when the grains are cemented together by silica. This siliceous cement makes the sandstone exceptionally hard and resistant to weathering compared to sandstones held together by other materials like calcite or clay.
