Minerals of silicate group
Silicate minerals are, so far, considered to be the most common rockforming minerals. They constitute a major part of the earth’s crust, which is about 90%. Silicate minerals include felspar, felspathoids, quartz, pyrozines, amphibole, micas, and olivines. Minerals other than silicate minerals, which will be discussed here, ae garnets, chlorite, serpentine, aluminium, calcium minerals, and some other minerals like ores. There are different groups of minerals, but in this essay we will discuss the Silicate minerals.
Silicate Minerals: The Silicate mineral group is of great importance due to their contribution in forming the large part of the earth’s crust. They constitute about 90% of earth’s crust. They occur in almost in all types of common rocks except limestone. In order to understand the difference between major silicate mineral groups, it is necessary to study their structure. Every silicate mineral contains oxygen and silicon, and all except quarts, contain one or more additional elements to complete their structure. The basic unit in all silicate minerals is silicon-oxygen tetrahedron. This structure is composed of four oxygen atoms with the silicon atom at its center as shown in the following figure;
These tetrahedra can occur in silicate structures either in single units or joined into chains, sheets, or three-dimensional networks by sharing oxygen atoms. Keeping in view the arrangement and the internal link of these tetrahedra, the silicate minerals are classified into the following structures.
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(1). Isolated Tetrahedra (Neosilicates): Olivine is the best example of an isolated tetrahedra silicate structure. It is composed of isolated tetrahedra bonded to iron and/or magnesium ions. There are –4 charges in each silica tetrahedron, which is then balanced by two iron or magnesium cations, each with a charge of +2. Olivine can be pure Mg2SiO4 or Fe2SiO4, or a combination of the two, written as (Mg,Fe)2SiO4. Magnesium and iron can substitute for each other because they both have a charge of +2, and they are similar in size. Magnesium cations have a radius of 0.73 Å, and iron cations have a radius of 0.62.
The crystal habit of these is generally equidimensional and they have poor cleavage. Olivine (Mg, Fe)2SiO4, , Zircon ZrSiO4, and garnets are examples of this class.
Bonding of tetrahedron with Iron or magnesium.
(2). Sorosilicate Silicate: Formerly known as Pyrosilicate; the sorosilicates are characterized by linked pairs of SiO4 tetrahedra. In this structure, only one oxygen is shared giving a ratio of Si:O = 2:7. Hemimorphite H6O9Si2Zn4, is an example of this class.
(3). Chaine Silicate (Inosilicates): The shared oxygen in Silicate mineral tetrahedra is spread into an infinite one-dimensional chain. These chains may be single, double, and even wider. Single chains characterize pyroxenes; double chains characterize amphiboles; and wider chains grade toward sheet structures. In the single chain structure two of the four oxygens in each SiO4 tetrahedron, are shared giving a ratio of Si:O=1:3. In the double chain structure, half of the tetrahedra share three oxygens, while another half share two oxygens yielding a ratio of Si:O = 4:11. Inosilcates split easiy in one crystal direction because bonds within chains are strong but are weaker between them. These minerals commonly form needle-like crystals, such as asbestos. Pyoxemes are examples of single chin minerals and amphiboles are examples of double chain minerals.
(3). Sheet Silicate(Phillosilicates): Mica is an example of the sheet structure of tetrahedra silica. It is a continuous sheet, which is formed by sharing three oxygen ions by two adjacent tetrahedra. Oxygen balances the charged silica, and very few charge-balancing cations are needed for sheet silicate minerals. Bonding between sheets is relatively weak, and this accounts for the tendency of mica minerals to split apart in sheets.
In phyllosilicate three of the four oxygens in each SiO4 tetrahedron are shared with neighbouring tetrahedra giving a trio of Si:0 = 2:5. As the atomic bonding perpendicular to sheet structure is generally weak, these minerals can be spilt easily into thin sheets. Flaky minerals, such as micas, chlorites, and kaolinite are examples of this class.
(4). Framework Silicate (Tectosilicate): In this form of silicate, groups of tetrahedra are connected with one another in a three-dimensional structure. Feldspars are a group of very abundant framework silicates in Earth’s crust. They include alumina tetrahedra as well as silicate tetrahedra. In alumina tetrahedra, there is an aluminum cation at the centre instead of a silicon cation. As all oxygens are in each SiO4 tetrahedron are shared with neighboring tetrahedra. This results in a strongly bonded structure in which the ratio of Si:O = 1:2. The minerals belonging to the tectosilicate group possess uniform properties throughout. Quartz and feldspar are common examples of this group.
(5). Ring Silicate (Cyclosilicate): The ring silicates or cyclosilicates contain rings of linked SiO4 tetrahedra having a ratio of Si:O = 1:3. These rings may consist of groups of three, four, or six linked tetrahedra. Cyclosilicates form extremely strong minerals, such as beryl and tourmaline.