The class of silicate minerals is of greater importance than any other, for about 25% of the known minerals and nearly 40% of the common ones are ‘silicates’. The silicates make up 90% of the earth’s crust.
Of every 100 atoms in the crust of the earth, more than 46 arc oxygen, over 27 are silicon and 7 to 8 are aluminium. The crust has been pictured as a box-work of oxygen-ions bound together by the small highly charged silicon and aluminium ions.
The interstices of this more or less continuous silicon-oxygen- aluminium net-work are occupied by ions of magnesium, iron, calcium, sodium and potassium. The predominance of alumino-silicates and silicates reflects the abundance of oxygen, silicon and aluminium.
All the silicate structures, so far investigated show that the silicon atoms are in four-fold co-ordination with oxygen. This arrangement appears to be universal in these compounds and the bonds between silicon and oxygen are so strong that the four oxygen are always found at the corner of a tetrahedron of nearly constant dimensions, and regular shape, whatever the rest of the structure may be like.
The radius ratio of the four valent silicon ion to that of oxygen ion is equal to 0.318 [since Si (radius)=0–42A and oxygen (radius) -1.32A, It indicates that four-fold co-ordination will be the stable state for silicon-oxygen grouping.
Although electron-sharing is present in the silicon-oxygen bond, the total bonding energy of the silicon ion is still disturbed equally among its four closest oxygen neighbours. Hence, the strength of any single silicon-oxygen bond is equal to just one-half the total bonding energy available in the oxygen ion.
Each oxygen ion has, therefore, the potentiality of bonding to another silicon ion and entering into another tetrahedral grouping, thus uniting the tetrahedral groups through the shared oxygen. This sharing may involve one, two, three or all four of the oxygen ions in the tetrahedron, giving rise to a diversity of structural configurations.
Sharing of one oxygen between any two adjacent tetrahedra. May, if all oxygen’s are so shared give rise to structures with a very high degree of connectivity, such as quartz structure. This linking of tetrahedron by sharing of oxygen is known as polymerisation. It has been observed that the higher the temperature of formation the lower the degree of polymerisation and vice-versa.
It has long been noted that the silicate minerals in igneous rocks display a fairly regular and predictable sequence of crystallization, beginning with olivine and progressing through pyroxene to amphibole and thence to mica. This sequence is in the order of increasing polymerization of the silicate tetrahedra.
Depending on the degree of polymerisation and the extent of oxygen-sharing between tetrahedra, the silicate frame-work may consist of separate tetrahedra, separate multiple tetrahedral groups, chains, double chains, sheets or three-dimensional box-works.
Up to the 1930s, the analyses of silicates were interpreted and their formulas generally written in terms of a number of hypothetical oxy-acids of silicon. Thus, olivine Mg2SiO4, was termed an orthosilicate and considered to be a salt of orthosilicic acid H4Si04; Enstatite, MgSiO3 was called a metasilicate and considered to be a salt of metasilicic acid H2Si03. The silicate structures, so far recognized are of the following types:
These are independent or isolated Si04– tetrahedra which are bound to each other only by ionic bonds -through interstitial cations. Their structures depend chiefly on the size and charge of the interstitial cations.
Considering the valencies of the elements composing the Si04 group, it is found that silicon has four positive and each oxygen has two negative valencies. Thus there are eight negative valencies in all and ‘he group as a whole therefore has four negative valencies in excess. In olivine structure, cations (mainly Mg) lie between the tetrahedral groups and contribute the necessary ‘+ve’ charges to make the structure electrically neutral.
Olivine, Garnet, Zircon, Sillimanite, Kyanite,Andalusite, Staurolite, Phenacite, Topaz, Willemite, Sphene etc.
The soro-silicates are characterised by isolated double-tetrahedral groups formed by two Si04 tetrahedra sharing single apical oxygen. The resulting ratio of silicon to oxygen is 2 : 7. They have a net charge of ‘-6’. As the charge is -6| three divalent ions are needed to balance it.
Idocrase, minerals of epidote group. Melilite (Ca2MaSij07), Lawsonite [CaAl2(Si207)(0H)2H10)], Hcmimorphite Zn4si2O7(OH)2H1O] etc.
When each Si04-tetrahedron shares two of its oxygen with neighbouring tetrahedra, they may be linked into ring. They have a ratio of Si : 0 = 1 : 3. Three “possible closed cyclic configurations of this kind may exist as
(a) Each of the three tetrahedra shares an oxygen atom.
(b) Each of the four tetrahedra shares an oxygen atom.
(c) Each of the six-tetrahedra shares an oxygen atom.
These are all having the formulas which are multiples of Si03. The simplest is the Si309 ring represented among minerals only by the rare titanosilicate, Benitoite (hexagonal)-BaTiSi309.
The Si4Oi2 ring occurs together with B03-triangles and (OH) groups in complex structure of the triclinic mineral ‘axinite’.
The Si6018 ring however is the basic frame work of the common important minerals like beryl and tourmaline. The hexagonal Si6O18 rings are arranged in planar sheets parallel to (0001) in beryl. These sheets are so firmly bonded by the small beryllium and aluminium ions with their high surface density of charge and high polarizing power that only poor cleavage results.
In tourmaline, however, the rings are polar. This polarity of the fundamental structural unit leads to the well known polar character of the tourmaline crystal.
The cyclosilicates are also known as ‘Ring-structures’.
4. Chain structures:
These are also known as ‘ino-silicates Here Si04-tetrahedra are joined together to form chains of indefinite extent.
There are two-principal modifications of this structure yielding somewhat different composition:
(a) Single chains, in which Si : O is 1 : 3 characterised by the pyroxenes and pyroxenoids.
(b) Double chains, where alternate tetrahedra in two parallel single chains are cross-linked and the Si : O ratio is 4 : 11, characterised by the amphiboles.
(a) Single-chain Structures:
The chains consist of a large number of linked Si04 groups, each sharing two oxygen’s and have the composition n(Si206). Here the excess negative charge on the chain is balanced by the valencies of other cations. The chains fun parallel to the ‘c-axis’ of the mineral and are bonded together y the calcium and magnesium ions which lie between them.
(b) Double-chain structure:
These are also known as the Band-structures’. Here the alternate tetrahedra are arranged in two Parallel ways and these chains are indefinite in extension and elongated usually in ‘c’-crystallocraphic direction’ and are bound by metallic ions.
Sheet structures (Si1O10):
It is also known as ‘phyllosilicates’. It is formed when the Si4O10, tetrahedra are linked by three of their corners and extend indefinitely in a two-dimensional net-work or sheet, which has a silicon and oxygen ratio of 4 : 10. This is fundamental unit-in all mica and clay-structure.
The sheets form a planar net-work responsible for the principal characteristics of minerals of this type-their pronounced pseudo hexagonal habit and perfect basal cleavage parallel to the plane 0f the sheet. Most of the minerals of this class are hydroxyl bearing.
Most of the members have platy or flaky habit and one prominent cleavage. They are generally soft, of relative low specific gravity and may show flexibility or even elasticity of the cleavage lamellae, group there are two types of configuration; one is called the dioctahedral sheet and the other is called the trioctahedral sheet.
It is also known as ‘framework structure When each of the four oxygen atoms of each tetrahedron is shared by another tetrahedron, it results in the formation of tectosilicates. Here every Si04 tetrahedron shares all its corners with other tetrahedra giving a three-dimensional net-work in which Si: 0=1: 2.
Here the bond is stable and strong and the frame-work is electrically neutral and does not contain other structural unit. There are eight different ways in which the linked tetrahedra may share oxygen and at the same time build a continuous electrically neutral three dimensional net-work.
Members of the Felspar, Felspathoid, Zeolite, Quarts group of minerals etc. show this type of silicate structures.