Introduction

Silicates are the most abundant class of minerals representing 75% of all known mineral species. With few exceptions nearly all the igneous rock-forming minerals are silicates and as such account for around 90% of the Earth's crust. Recognition of the silicates as a group of minerals is relatively easy since most are hard, transluscent, give a colourless or white streak and are insoluble in all common acids. However, because of their relative inertness the separate members are usually much harder to identify individually.

The common structural feature of silicates are Si-O tetrahedra. Each unit is composed of a central Si4+ ion surrounded by four O2- ions at each of the points of the tetrahedron. The individual bonds between Si and O are approximately 50% ionic and 50% covalent. This means that the bond partially arises from the attraction of oppositely charged ions and partly from sharing of electrons. The strength of any single Si-O bond is roughly equal to one half of the total bonding energy available in the oxygen ion. Consequently each O2- has the potential of bonding to another silicon ion, thus taking part in another tetrahedral grouping. Such linking is usually referred to as polymerisation and the numerous possibilities of linking the tetrahedra together gives rise to the great variety of silicate structures and is the basis of the subdivision of silicates into subclasses.

The general chemical formula for silicates is XmYn(ZpOq)Wr.

X = cations with large ionic radii and small valence numbers (1 or 2), forming a coordination number of 6, 8 or12 with O

Y = cations with medium size ionic radii and 2-4 valence numbers, forming a coordination number of 6 with O

Z = cations with small ionic radii and large valence numbers (3 or 4), forming a coordination number of 4 with O

W = usually is OH-1, F-1 or Cl-1 or equivalent

p, q, m, n, r = subscript numbers used to maintain electroneutrality. The p:q ratio defines the subclass of silicates.

Nesosilicates

Nesosilicates (nesos, Greek for "island"), formerly known as Tetrahedral Silicates or Orthosilicates, are minerals in which the (SiO4) tetrahedra are isolated and share no oxygen bonds. Instead, the charge on each oxygen ion is satisfied by metal ions between the tetrahedra. These (SiO4) 4- groups, thus act as complex negative ions. The metal ions can bond to six or eight such tetrahedra and hold the complex structure together. More rarely five coordinated metal ion structures are also found. The p:q ratio is 1:4 or (3:12) and the basic unit are (SiO4)-4 or (Si3O12)-12. The general formulae for these minerals therefore end with SiO4 or a multiple of SiO4, for example, Mn3Al2Si3O12. Because the SiO4 tetrahedra are independent the crystal habit of these silicates is equidimensional and cleavage is absent. The structure of nesosilicates produces stronger bonds and a closer packing of ions. This gives them more hardness along with higher densities and refractive indexes, than some of the other chemically similar silicates in other subclasses. Consequently, there are more gemstones in the nesosilicates than in any other silicate subclass.

The main mineral groups of the nesosilicates are:

• Olivine Group - a solid solution series between Forsterite and Fayalite varying in the amount of magnesium and iron. Important rock forming minerals.
• Phenakite Group - two closely related minerals containing beryllium and zinc. Both form hexagonal crystals.
• Garnet Group - a large group of minerals with closely related structures. They make up two solid solution series, pyrope-almandine-spessarite and uvarovite-grossular-andradite. They crystallise in the hexoctahedral class and are similar in crystal habit.
• Zircon Group - minerals which have zirconium or thorium ions packed amongst the silica tetrahedra in a way that each metal ion has eight oxygens around it. Zircon is common and widely distributed accessory mineral in all types of igneous rocks.
• Humite Group - several species containing OH and F. Most have 2/m symmetry and a structure similar to olivine.
• Al2O5 Group - contains the polymorphic species Kyanite, Andalusite, Sillimanite. All three are found in metamorphosed aluminous rocks. Closely related minerals include Topaz and Staurolite.
• Silicate apatites - several nesosilicates containg PO4 in the structure. All have the 6/m symmetry.

Sorosilicates

Sorosilicates (soros, Greek for "group"), formerly known as Double Tetrahedral Silicates, are characterised by isolated, double tetrahedral groups formed from two SiO4 tetrahedra sharing a single oxygen atom. This structure forms an unusual hour-glass like shape. Some of the more complex members of this this subclass may also contain normal silicate tetrahedrons as well as the double tetrahedrons. The p:q ratio is 2:7 giving the basic unit as (Si2O7)6-. The shared O ion is fully saturated. The remaining six O ions which each carry a single negative charge, are balanced by the positively charged metal ions that hold the structure together. Because Si2O7 unit has an unusual shape the structures tend to be very complex. Sorosilicates are thus relatively rare and the sorosilicates subclass is the smallest of the silicate familes.

The main mineral groups of the sorosilicates are:

• Epidote Group - includes complex minerals containing both single and double tetrahedra. These are held together by ions of aluminium, iron, manganese and calcium. Water is present as OH-. Individual members are difficult to distinguish. Important rock-forming minerals, especially in low and medium grade metamorphic rocks.
• Melilite Group - a complete solid solution series between gehlenite and akermanite. Melilite is intermediate. The double tetrahedra are held together by Mg2+ or Al3+ ions which substitute for each other in the series.

Cyclosilicates

Cyclosilicates (cyclo, Greek for "ring"), formerly known as Ring Silicates, are minerals in which the tetrahedra of silica are joined in rings, each tetrahedron sharing O2- ions with two other adjacent tetrahedra. Several cyclic configurations are possible. The simplest is the Si3O9 ring represnted by the rare Benitoite. The Si4O12 ring occurs together with BO3 triangles in the triclinic mineral Axinite. The Si6O18 structure is the most common and stable form giving rise to the largest family inlcuding Beryl and Tourmaline. There are a handful of species with eight-membered rings, including Muirite.

The chemical formula of cyclosilicates can be summarised as SinO3n where n is the number of tetrahedra in the ring. The ratio p:q is 1:3 or (6:18), the basic unit being (Si6O18)-12. Charges on the rings are balanced by metal ions which hold the rings together in the crystal structure. Because the bonds between these positive ions and oxygen are relatively weak, cyclosilicates do not exhibit well-developed cleavage. The symmetry of the rings is usually reflected in the crystal form, as it the case of Beryl, which has six tetrahedra arranged in a hexagonal ring, Si6O18, giving rise to hexagonal crystals.

The main mineral groups of the cyclosilicates are:

• Beryl Group - several minerals containing six membered rings. Most contain aluminium and nearly all occur in pegmatities. Beryl is an important gemstone. The cloured varieties are emerald (green), aquamarine (blue) and heliodor (yellow).
• Tourmaline Group - a complex group of closely related cyclosilicates containing B and Al with substituion by Ca-Na, Mg-Al and Fe-Mn. Occur in granite pegmatites and good transparency form makes them popular gemstones.
• Cordierite Group - a solid solution series replacing Fe by Mg or Mn. Minerals often show strong dichriosm.

Inosilicates

Inosilicates (inos, Greek for "chain"), formely known as Chain Silicates, resemble sorosilicates in that they are formed by each silica tetrahedron sharing two O2- ions. However in this case the tetrahedra form infinite chains rather than pairs. The p:q ratio is 1 to 3 making the basis subunits (SiO3)2- or (Si2O6)4-. The chains can be simple such as in the Pyroxene family or more complex whereby different O 2- sharing positions give rise to linear chains that lack symmetry. In both cases the long chains are aligned and held together by metal ions. In a simple chain structure the tetrahedrons alternate to the left and then to the right along the line formed by the linked oxygen ions. More complex chains may appear to spiral. In a cross section view the chain forms a trapezium and this shape produces the angles between the crystal faces and cleavage directions. Therefore inosilicates usually have a prismatic, radiating or fibrous habit along the direction of the chains. The Inosilicates are a major group of rock-forming minerals and are therefore greatly studied.

Double Chain Silicates, are a special subclass class of the inosilicates. They differ from the single chain family by the fact that exactly half the tetrahedra in one chain are cross-linked with half of those those in the other by sharing O atoms. The two single chains lie side by side so that all the right sided tetrahedrons of the left chain are linked by an O to the left sided tetrahedrons of the right chain. The double chain looks like a chain of six sided rings. The cross section is similar in the double chains to that of the single chains except the trapezium is longer in the double chains. This difference produces different cleavage which makes this feature a useful way of distinguhing between single and double chain silicates.In single chain silicates the cleavage directions lie at nearly 90 degress whereas in double chain silicates the cleavage angles are close to 120 and 60 degrees. The group is frequently referred to as the Amphiboles and includes a series of important minerals that occur in igneous and metamorphic rocks. Because of the additional O2- sharing the p:q ratio is 4 to 11 and the general chemical formula for the basic unit is (Si4O11)6- or (Si8O22)12-. As with single chain minerals the structural alignment crystal tends to favour prismatic, radiating or fibrous habits along the direction of the chains.

The main mineral groups of the inosilicates are:

• Pyroxene Group - single chain minerals lacking (OH)x. Contains Enstatite-Ferrosilite and Diopside-Hedenbergite series. All are very important rock-forming minerals. Examples include Augite and Spodumene.
• Amphibole Group - double chain minerals with (OH)x present. Contains Tremolite-Ferroactinolite, Cummingtonite-Grunerite and Glaucophane-Riebeckite series. Most are important rock-forming minerals. Examples include Horneblende.

      

Phyllosilicates

Phyllosilicates (phyllo, Greek for "leaf"), formerly known as Sheet Silicates, are minerals in which all the SiO4 tetrahedra share corners with three others, thus forming infinitely extending layers or sheets. The fourth corner is unshared and all these unshared O ions point in the same direction. In the middle of the hexagon formed by the unshared corner there is room for an OH group. The p:q ratio in the silicate layer is 2 to 5 giving the basic unit (Si2O5)2- or (AlSi3O10)5- where Al substitutes for Si. The sheets are held together in stacks by metal ions and because these can bridge the sheets in a number of alignments they gives rise to the huge variety of minerals in this group. Some members of this subclass have the sheets rolled into tubes. These tubes form fibrous structures as in Chrysotile (asbestos). On the basis of chemistry and geometry of the octahedral layers the phyllosilicates are divided into two major sub-groups: trioctahedral and dioctahedral. In spite of their diffrences, the layered structures common to all phyllosilicates produce very similar physical characteristics. For example the minerals normally occur as tabular or platy crystals, they have one cleavage direction parallel to the layers, and because of the relatively weak bonding between the layers the are typically very soft. Most are very important rock-forming minerals.

The main mineral groups of the phyllosilicates are:

• Serpentine Group - a group of polymorphs having little substitution of Si by Al. Members include Antigorite, Lizardite and Chrysotile. The latter often forms long fibrous structures referred to as asbestos.
• Clay Group - a group consisting of hydrous aluminium silicates. Examples include Kaolinite and Talc. Mg or Fe sometimes substitutes for Al. Clay is used to refer to fine-grained earthy material that becomes plastic when mixed with water.
• Mica Group - form thin sheets and there is considerable substitution of Si by Al with little or no OH. The crystals are usually tabular with prominent basal planes and have either diamond or hexagonal outlines with angles of 60 or 120 degrees.
• Chlorite Group - A number of minerals which have very similar chemical, crystallographic and physical properties. Without X-ray analysis it is extremely difficult to distinguish between the members. These include Chamosite, Clinochlore and Pennantite.

Tectosilicates

Tectosilicates (tecto, Greek for "framework"), formerly known as Framework Silicates, are minerals in which the Silica tetrahedra share all four O 2- corners with adjacent tetrahedra. The result is an infinite lattice stretching out in three dimensions. As every O atom is bonded to two Si atoms, the p:q ratio becomes 1 to 2 as in SiO2. The diversity of Tectosilicates is created by the partial substitution of Si 4+ by Al 3+. In this way a deficit of positive charge is created which is in turn balanced by the introduction of large metal cations such as Potassium, Sodium or Calcium. These ions are accomodated in cavities in the relatively open three dimensional framework. The tectosilicates include many of the main rock-forming minerals and are a fairly homogenous group of minerals. They are typically colourless, or only pale coloured, have a vitrouos lustre and a relatively low density because of the open structure. The latter also contributes to the relative softness of most member species of between 5 and 7.

The main mineral groups of the tectosilicates are:

• Quartz Group - consits mainly of many varieties of quartz inluding Chalcedony, Opal, Agate, Chert, Onyx and Jasper. Most are very important rock-forming minerals, especially quartz as hydrothermal vein filler. Other SiO2 polymorphs include Tridymite and Cristobalite.
• K-feldspar Group - contains species in which there is significant substitution of Si by Al. Atomic charge neutrality is preserved by other positive ions, in this case by Potassium (K). Important examples include Orthoclase and Microcline.
• Plagioclase Feldspar Group - similar substitution of Si by Al, but the charge balancing ions are Sodium and Calcium. they form a solid solution series between Albite (100-90%) Na to Anorthite (90-100%) Ca.
• Feldspathoid Group - form in silica poor regions and therefore the Al substitution of Si is much higher than in Feldspars. The charge can be balanced by Na, Li or Ca. Examples include Nepheline, Eucryptite, Sodalite and Cancrinite.
• Zeolite Group - minerals with a very open tetrahedral lattice which contains large ions and water. The water is easily removed by heating. The process is reversible. The ions can also be exchanged, a process known as ion echange. Both properties are very useful industrially. More than 120 species are known including Stilbite, Heulandite and Analcime.