A phyllosilicate, which was previously known as a disilicate, is a compound with a structure in which the silicate tetrahedrons are arranged in sheets. Hence, they are also known as sheet silicates. In these sheet silicates, the central atom is silicon and is surrounded by four atoms of oxygen at the corners of a tetrahedron. The sheet silicates form the parallel sheets of the silicate tetrahedra as given in the formula Si2O5 in a 2:5 ratio. These sheet silicates may or may not be present in hydrated form.
Common examples of phyllosilicate or sheet silicate include mica and talc.
A rock-forming mineral that is found to be made up of silicate groups is known as silicate minerals. It is one of the largest and most important classes of minerals and makes up approximately 90% of the Earth’s crust. Silicon dioxide (SiO2) or silica is one of the common silicate minerals. This silica is found in nature in the form of the mineral quartz and its polymorphs. There is a wide variety of silicate minerals that are found in the crust of the Earth, with a wider range of combinations because of the processes going on for thousands of years during the formation of the Earth’s crust and the processes that are currently going on in the reworking of the crust of the Earth. Some of these processes are melting, crystallization, fractionation, metamorphism, weathering and diagenesis.
General Structure and Main Groups of Silicate Minerals
It is clear from above that a silicate mineral is the one in which the anions are predominantly of silicon and oxygen atoms. In most forms of silicate found in the crust of the Earth, the silicon atom takes the central atom position in an ideal tetrahedron and the oxygen atoms take the corners while being covalently bonded to the central atom. Two of the adjacent tetrahedra may share a single vertex which results in a bridge-like formation in-between the two central silicon atoms of the two tetrahedra. In case there is an unpaired oxygen atom on the vertex that is bound only to a single silicon atom of a single tetrahedron, it acquires a negative charge and that negative charge is imparted to the silicate anion.
In the above mentioned general structure, the silicon atoms present at the central positions may be replaced by other atoms of other elements but still would be bound to the four oxygen atoms present at the corners of the tetrahedron. In case the substituted atom does not form a tetravalent bond it then contributes an extra charge to the anion which then requires an extra cation.
In mineralogy, the silicate minerals are classified into seven groups mainly depending on the structure of their silicate anion. They are given below in the following table:
The Phyllosilicate Group of Silicate Minerals
It is already mentioned that the phyllosilicate or sheet silicates are a group of silicate minerals in which the silicate anion or silicate tetrahedra are arranged in the form of a sheet with the chemical formula Si2O5 in the ratio of 2:5. Few images are shown below explaining the structures of phyllosilicates.
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These are few examples of the sheet structure of the phyllosilicates. In these sheets, the silicon atoms are arranged at the corners of the hexagons, while the unshared oxygen atoms are commonly oriented on the same side of the sheet. As these atoms have the capability of formation of chemical bonds with different metal atoms and thus, the silicate sheets are interleaved with the layers of other elements. The different layers are stacked together thus leading to the formation of grouping with the unshared oxygen atoms toward the centre and these groups are weakly held together and thus it results in giving the phyllosilicates their distinct cleavage that is present parallel to the layers.