To start with this content featuring the discussion of Sedimentary Rocks we must know there are basically three kinds of rock, namely - igneous, sedimentary, and metamorphic. The Igneous rocks evolve from the molten rock known as magma or lava, this lava cools and thus solidifies. While, the sedimentary rocks generally originate when the particles get settled down in the water or the air, or by the means of precipitation of the minerals from water. They get accumulated in layers.
So, we see, if the rocks could talk it would definitely be in these three languages – Igneous, Sedimentary and Metamorphic. This indicates the importance of its classification, sedimentary being among it, we will take up its discussion in this section.
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Siliceous rock is one of the groups of sedimentary rocks which consists mainly of silicon dioxide (SiO2), which is either in the form of quartz or amorphous silica and cristobalite. They are included by these rocks which have formed as precipitates of chemical composition and are excluded from those of detrital or fragmental origin.
Among the other siliceous rock, the most common siliceous rock is chert which is a dense, microcrystalline rock that has chalcedony and quartz as its composition. This occurs in beds and in nodules. The Bedded chert consists of siliceous fossils like diatoms and radiolaria. They form the siliceous oozes which are on the seafloor. When these organisms sinks, the portions of their shells get dissolved near the bottom and they reprecipitate in the void spaces between these shell fragments which results in a hard bedded rock. The Bedded cherts are very common in areas that are enriched in silica-bearing organisms, like near volcanoes, they are the volcanic glass that is rapidly altered to produce these high-silica concentrations. Similar processes occur in freshwater lakes that are associated with volcanoes.
These rocks are also known as silica sand or even called ‘quartz sand’. The chemical composition of silica is silicon dioxide (SiO2). The Silicon compounds are the significant component lying on the Earth’s crust. The sand is plentiful, they are easy to mine and are relatively easy to process. Sand is the primary ore source of silicon. Other sources are - metamorphic rock, quartzite.
Silicon (Si) is a semi-metallic or a metalloid structure, this property occurs because it has several of its metallic characteristics. The Silicon is never found in its natural state, while found in combination with the oxygen as the silicate ion which is SiO44- in silica-rich rocks like obsidian, granite, diorite, and even in sandstone. Feldspar and quartz account for the most significant silicate minerals. The Silicon alloys include a variety of other metals, which includes iron, aluminium, copper, manganese, nickel and ferrochromium.
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Pyroxenes are the dark coloured rock-forming type of minerals that are found mainly in igneous and metamorphic rocks worldwide. Pyroxene rocks are formed under acute high temperature or pressure conditions.
Rocks such as Augite, Jadeite, Diopside and Spodumene are the most commonly known members among the pyroxene group. However, the pyroxene group consists of many other members as well.
The minerals of pyroxene are defined by their chemical composition and their crystal structure. The common chemical composition is described by the formula is - XYZ2O6
Here X may be one or more of the following – Ca, Na, Fe++, Mg, Mn, Zn or Li.
While, Y can be – Mg, Fe++, Al, Cr, Co, Sc, Mn or Vn.
Z can be Si, Al or even a combination of them both
There can be a wide range of cations occurring in the X and Y positions.
Siliceous Sedimentary Rock
The siliceous sediments are the composition of silica. This silica has precipitated at or near the site of the deposition or this has replaced the pre-existing sediments. These sediments are different from the clastic or the terrigenous sediments. The later sediments are made of grains which are derived from the rocks elsewhere, while physically transported to the site where the deposition took place.
In the oceans, the siliceous sediments are dominated in the oozes which are composed of microscopic silica particles precipitated biologically by the diatoms.
Non-ferromagnesian are the silicates minerals which are without the substantial Fe and also without Mg which is in their crystalline structure. They are generally lighter-coloured in texture than the ferromagnesian silicates.
This Limestone very often contains variable amounts of silica which is in the form of chert or the form of siliceous skeletal fragments (like sponge spicules, diatoms, or the radiolarians). Fossils are quite common in the components of limestone. However, the majority of the limestones consists of sand-sized grains which are in a carbonate mud matrix.
Siliceous Shale is the hard, fine-grained rock that is shaly structured, they are generally believed to be the shale that is altered by the silicification.
Low Silica Rocks
The Ultramafic rocks (which is also referred to as the ultrabasic rocks, they are although the terms which are not totally equivalent) but they are the igneous and meta-igneous rocks which have a very low silica content (that is less than 45%), they are generally >18% MgO, with high FeO, low in the potassium content, and they are composed of greater than 90%of the mafic minerals (which are dark coloured, high magnesium).
Examples of Siliceous Rocks
Examples of Siliceous rocks are as follows:
Chert and flint.