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Questions and Answers
What is the fundamental unit in all silicate structures?
How many oxygen atoms are bonded to a central silicon cation in the SiO4 tetrahedron?
What type of bonds develop in silicate structures due to the attraction of oppositely charged ions and sharing of electrons?
What is the term used to describe the linking of SiO4 tetrahedrons by shared oxygen atoms?
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Which type of silicate structure contains isolated groups of SiO4 tetrahedrons?
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What composition do ring silicates, also known as cyclosilicates, have when they are of the sixfold variety?
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What is the unit composition of single chains in inosilicates?
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When three oxygen atoms are shared with adjoining tetrahedrons, what type of silicate is formed?
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Which type of silicate is characterized by an SiO2 unit composition?
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What is the role of aluminum in silicate structures?
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Which elements can be found in octahedral coordination with oxygen in silicate structures?
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How is solid solution involving ions of different charge maintained in silicate structures?
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Study Notes
Silicate Structures
- The fundamental unit in silicate structures is the silicon-oxygen (SiO4)4– tetrahedron, composed of a central silicon cation (Si4+) bonded to four oxygen atoms.
- The silicon-oxygen bonds are approximately 50% ionic and 50% covalent, developing from the attraction of oppositely charged ions and the sharing of electrons.
- The positive charge (+4) of each silicon cation is satisfied by its four bonds to oxygen atoms.
Polymerization
- Each oxygen ion (O2–) contributes only half of its total bonding energy to a silicon-oxygen bond, allowing it to also bond to the silicon cation of another tetrahedron.
- This shared bonding leads to polymerization, where SiO4 tetrahedrons become linked by shared oxygen atoms.
Types of Silicates
- Nesosilicates have isolated groups of SiO4.
- Sorosilicates contain pairs of SiO4 tetrahedrons linked into Si2O7 groups.
- Ring silicates (cyclosilicates) are closed, ringlike silicates with a composition of Si6O18.
- Inosilicates are composed of infinite chains of tetrahedrons with a unit composition of SiO3 or Si2O6 (single chain) or Si4O11 (double chain).
- Phyllosilicates (sheet silicates) are formed when three oxygen atoms are shared with adjoining tetrahedrons, resulting in infinite flat sheets with a unit composition of Si2O5.
- Framework silicates (tectosilicates) have an infinite three-dimensional network with an SiO2 unit composition, where tetrahedrons share all their oxygen ions.
Aluminum in Silicates
- Aluminum, with a radius slightly larger than silicon, can be surrounded by four oxygen atoms arranged tetrahedrally or six oxygen atoms in sixfold coordination.
- Aluminum can occupy tetrahedral sites as a replacement for silicon and octahedral sites in solid solution with elements such as magnesium and ferrous iron.
- Aluminum can form ionic bonds with SiO4 tetrahedrons in sixfold coordination.
Octahedral Coordination
- Several ions can be present in silicate structures in octahedral coordination with oxygen, including Mg2+, Fe2+, Fe3+, Mn2+, Al3+, and Ti4+.
- These ions have approximately the same dimensions and can occupy equivalent atomic sites despite having charges ranging from positive two to positive four.
- Solid solution involving ions of different charge is accomplished through coupled substitutions, maintaining neutrality of the structures.
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Description
Explore the fundamental unit of all silicate structures, which is the silicon-oxygen (SiO4)4– tetrahedron. Learn about the bonding between a central silicon cation and four oxygen atoms forming a regular tetrahedron. Understand how these tetrahedrons play a crucial role in holding together the terrestrial crust.
