Silicate Classification

Silica Polymerization

The silicate minerals form a group of inorganic compounds of great chemical, as well as structural, complexity.

The one common component of all silicates, however, is the tetrahedral SiO₄^-4 structural subunit. The manner in which silica tetrahedral units associate provides a basis for classification of the silicates into 6 distinct groups.

Before we examine these 6 silicate structural groups individually, it would be well to highlight a few fundamental features of the silica tetrahedra.

The basic tetrahedral grouping of 4 oxygens about each Si provides the strongest, most stable bonds within the silicate species. This very stable configuration is compatible with both the radius ratio rule and the strong covalent 3sp³ bonding.

Since the size of the Al³⁺ ion is similar to that of the Si⁴⁺ ion, we may expect some isomorphous substitution, or proxying, of Al³⁺ for Si⁴⁺ within tetrahedral units. Such substitution seldom exceeds 50%.

Silica tetrahedral units (SiO₄^-4) may occur within silicate structures as independent units or they may polymerize in varying degree through the sharing of common oxygens.

As we examine the silica tetrahedron, we see a tendency toward polymerization. In the Si-O union, bond valency is unity, or exactly 1/2 the valency, or charge, of the oxygen. Each oxygen within the unit, therefore, is free to bond with another cation with charge up to equal bond valency. It is apparent that a bond with a second Si, with equal bond valency, and in such case with equal bond strength, would form a very stable polymer.

When SiO₄^-4 tetrahedra polymerize, it is always through common sharing of oxygens, never by sharing of edges or faces of the tetrahedra (Pauling's third rule).

There are three possible types of tetrahedral polymerization:

In the face-sharing tetrahedra shown above, note that the Si cations (yellow) are not screened from each other by intervening anions and are also very close to each other. There is a strong repulsion between these two highly charged cations that leads to instability of the structure.

In the edge-sharing tetrahedra example above, the Si cations are still not screened from each other by intervening oxygen (red) anions. Although they are positiioned further apart than in the face-sharing example, there is still a strong repulsion between them and an inherent stability to the structure.

Finally, in the corner-sharing example, the highly charged Si cations are screened from each other by an intervening oxygen and they are as far apart as can possibly be arranged. To add even more stability to the structure, the oxygens in the basal (unpolymerized) parts of each tetrahedron should be rotated 1/6th of a turn so that they are as far apart as possible. However, other intervening cations may alter their arrangement somewhat.

Corner sharing by Si tetrahedra is preferred, as stated in Pauling's third rule: "the sharing of edges and particularly of faces by two anion polyhedra decreases the stability of an ionic crystal structure". In other words, highly charged cations prefer:

1. to maintain as large a separation as possible within a structure, and
2. to have anions intervening between them so as to screen the charges of one from the other.

The preference for corner sharing is also enhanced in this case because it causes the least distortion of the bond angles imposed by Si-O covalency.

Pauling's rules were formulated with reference to minerals in which bonding is appreciably ionic. The rules, therefore, may be incompatible with strongly covalent structures. In tetrahedral structures such as SiO₄^-4, Pauling's rules are at least partially applicable because the disposition of anions about the central Si, which is due in a large part to sp³ orbital sharing, is very similar to what would prevail if the bonding were strictly ionic.

Silicate Structural Group Classification

The extent of polymerization of silica tetrahedra varies widely within the silicate mineral group, from no polymerization to complete polymerization of all tetrahedral oxygens.

Nearly all silicate mineral species can be placed in one of 6 distinctive structural groups based on the manner of polymerization or association of silica tetrahedral units.

This is the most widely accepted classification system, but other systems of silicate classification do exist, some of which delineate as many as 8 or even 10 silicate structural groups.

As we consider the 6 silicate structural groups, we will find that the complexity of silica tetrahedral polymerization follows in the same order as the Bowen reaction series-the order of crystallization of primary silicate minerals from solid solution igneous magma during cooling from high to low temperatures.

For additional information, see the Athena Mineralogy site for a description of the entire classification system plus compositioinal and structural information regarding thousands of minerals.

Silicate Structural Classes

Nesosilicates (unitetrahedral, island)

In the nesosilicates, silica tetrahedra do not polymerize. SiO₄^-4 tetrahedra exist as discrete anionic structural subunits.

Independent SiO₄^-4 tetrahedra are joined, and the excess negative charges neutralized, by linkages with "charge-balancing" cations which form mostly ionic bonds with the electrically active, unsatisfied oxygens of adjacent SiO₄^-4 tetrahedra.

Silica tetrahedra in the nesosilicates are arranged so that alternate SiO₄^-4 subunits are inverted, and are linked by Mg²⁺ or Fe²⁺ in octahedral coordination. The bond valency of the Mg,Fe-O bonds = 1/3, which makes for a relatively weak bond, and one that cleaves easily.

A number of primary silicates crystallize in the nesosilicate mode.

Examples:

Group	Species	Composition
Olivine	Forsterite	Mg2SiO4
Olivine	Fayalite	Fe2SiO4
Zircon	Zircon	ZrSiO4
Garnet	Almandine	Fe3Al2(SiO4)3

An example of the structure of forsterite, one of the nesosilicates, is shown below. Click here for instructions on how to view the structure.

 

The nesosilicates form from high temperature fluid magma. Providing the magma has a sufficient content of neutralizing cations, the nesosilicates follow the Pauling rule that highly charged cations (Si⁴⁺) maintain as large a separation as possible. Hence, tetrahedra do not need to polymerize and share common oxygens. Plenty of other cations are present to maintain electrical neutrality.

A second factor contributes to the nesosilicate stability at high temperature. The cations and oxygen expand to a differential degree with increasing temperature. Hence, the octahedral space becomes larger, and can more easily accommodate the octahedral cations.

Stability of the 6-fold Mg- or Fe-O octahedral linkages between adjacent tetrahedral oxygens is thus enhanced by the better fit of Mg or Fe in the expanded octahedral space.

Sorosilicates (duotetrahedral, double-island)

As nesosilicates crystallize from magma, the fraction that remains fluid often becomes enriched in Si with respect to oxygen and to other cations.

Consequently, silicates with higher Si to O ratios begin to form.

This relative increase in the Si to O ratio promotes the sharing of oxygens, and tetrahedral polymerization occurs.

The simplest polymerization involves the sharing of one corner oxygen by two Si tetrahedra to form a double tetrahedral unit.

This tetrahedral dimer forms a discrete composite group in a few, rather rare sorosilicate species.

The composition of this characteristic unit is: Si₂O₆^-7

These double tetrahedral units are linked to other similar units through bonding with charge-balancing cations to the active oxygens of adjacent composite units.

Examples of sorosilicate mineral species.

Species	Composition
Melilite (Åkermanite)	Ca2(Mg,Fe2+)Si2O7
Thortveitite	(Sc,Y)2Si2O7
Epidote	Ca2Al2.4Fe0.6Si3O13H

The structure of epidote, a sorosilicate mineral, is displayed below.

 

Discrete three and four member linear silica polymers also form, but are rare. They are also classified in the sorosilicates.

Cyclosilicates (ring silicates)

The cyclosilicates represent a further stage in the complexity of silica tetrahedra polymerization. Two oxygens of each tetrahedron are shared to form discrete structural units of limited dimension.

The cyclosilicates corner-share two oxygens by forming closed rings. Several types of ring structures may be formed. Rings containing 3, 4, or 6 silica units are relatively common. Rings containing 8, 9, or even 12 silica units are fairly rare, but do exist. The individual rings are linked together by cations that interact with the unshared oxygen of adjacent rings. Doubled rings containing 2 linked layers of 3, 4, and 6 member rings are rare. The two rings are connected by sharing oxygens between them, which means that 3 of the 4 oxygens of each Si unit are shared.

Some relatively common examples are:

Species	Ring Type	Composition
Wadeite	3-member	K2ZrSi3O9
Joaquinite	4-member	NaFeBa2Ce2Ti2Si8O26OH·H2O
Tourmaline	6-member	NaMg3Al6(BO3)3(OH)4Si6O10
Beryl	6-member	Be3Al2Si6O18
Milarite	double 6-member	(K,Na)Ca2Be2AlSi12O30·0.75H2O

An example of the structure of beryl, a 6-member ring cyclosilicate mineral is displayed below.

 

It may be noted that with increasing polymerization of silica tetrahedra, the ratio of oxygen to silicon decreases and the number of "active" oxygens decreases.

Inosilicates (chain silicates)

Sharing of corner oxygens of tetrahedra can also be manifested in linear chains of tetrahedra. These chains have no finite length, or number of members, as do the sorosilicates. The chains may take 2 forms:

A. Single chain

As with the cyclic cyclosilicates, two oxygens of each tetrahedron are shared.

Simplest repeat formula: (SiO₃)^2-n

Single chains link to others in 3 dimensions through intermediate cations (typically in 6 or 8-fold coordination) bonding to active oxygens. Some common single-chain inosilicates are:

Species	Composition
Enstatite	MgSiO3
Diopside	CaMg(SiO3)2
Jadeite	NaAl(SiO3)2

An example of the structure of a single-chain inosilicate mineral is displayed below.

B. Double chain

Double chain silicates represent a further stage in tetrahedra polymerization.

In the double chain structure, we find infinite chains in which three corner oxygens of 1/2 the tetrahedra are shared by adjacent Si and in which two corner oxygens of 1/2 the tetrahedra are shared by adjacent Si.

Repeat formula: (Si₄O₁₁)^6-_n

Again, the double chains are linked by interchain cations bonding to "active" oxygens. Because of the complexity of the silicate framework, we have more than one type of coordination site present. Some examples are:

Species	Composition
Tremolite	Ca2Mg2(Si4O11)2(OH)2
Hornblende	Ca2(Mg,Fe2+)4(Al,Fe3+)(Si7Al)O22(OH)2

An example of the structure of a double-chain inosilicate mineral is displayed below.

Pyroxenoids and Other Inosilicates

Pyroxenoids are single chain inosilicates that have repeat distances along the chain direction > 2 silica tetrahedral units. The longer repeat distances are due to rotation or bending of the chain structures.

Examples include:

Species	Composition	Chain Repeat Distance
Wollastonite	CaSiO3	3 silica units
Rhodonite	MnSiO3	5 silica units
Pyroxmangite	(Mn,Fe)SiO3	7 silica units

Other inosilicates have larger numbers of chains polymerized together. The hornblende group has paired silica chains, but minerals exist with 3, 4, 5, 6, 7, and 12 chains joined together. Even more complex inosilicates exist, having chains and pairs in combination, chains that have been wrapped into cylinders, and so forth.

Some examples include:

Species	Composition	Chain Width
Jimthompsonite	(Mg,Fe)5Si6O16(OH)2	3 chains
Gageite	(Mn,Mg,Zn)42Si16O54(OH)40	4 chains
Santaclaraite	CaMn4Si5O14(OH)2·H2O	5 chains
Zektzerite	NaLiZrSi6O15	6 chains

Polymerization of Si tetrahedra in chain structures is usually reflected in crystal form. Silicates with chain structures usually form elongate crystals. Some forms of asbestos (e.g., Tremolite) crystallize with the double chain structure.

Similarly, polymerization in chain structures results in directional orientation of the relatively strong Si - O bonds as well as of the weaker interchain cation - O bonds. The result of this bond orientation is expressed in mineral cleavage.

Phyllosilicates (layer silicates)

The phyllosilicates represent a further degree of tetrahedra polymerization. In the phyllosilicates, three oxygens (the "basal" oxygens) of all tetrahedra bond to the Si of adjacent tetrahedra. The result is the creation of an infinite, 2-dimensional sheet of tetrahedra.

Repeat formula: (Si₂O₅)^2-_n

The infinite 2-dimensional sheet structure is responsible for the structural similarities of the mica, chlorite, smectite, vermiculite, serpentine and kaolin groups. All these minerals have platy habit with perfect cleavage parallel to the tetrahedral sheets. The ease of cleavage, or bond breakage, is made possible by the weaker intersheet (parallel) bonds. These are, in some cases, ionic in nature - in others, they are hydrogen bonds or van der Waals bonds. They occur in three groups: 1:1, 2:1, and 2:2 (also known as 2:1:1). These numbers refer to the relative numbers of tetrahedral and octahedral sheets in a unit cell.

Some examples include:

Group	Species	Composition
1:1, dioctahedral	Kaolinite	Al2Si2O5(OH)4
1:1, trioctahedral	Lizardite	MgsSi2O5(OH)4
2:1, dioctahedral	Pyrophyllite	Al2Si4O10(OH)2
2:1, dioctahedral	Talc	(Mg,Fe)3Si4O10(OH)2
2:1, dioctahedral	Muscovite	KAl2(AlSi3)O10(OH)2
2:1, dioctahedral	Beidellite	Ca0.25(Si3.5Al0.5)Al2O10(OH)2
2:2	Chlorite	(Mg,Fe,Al)6(Si,Al)4O10(OH)8

The structure of kaolinite, a 1:1 phyllosilicate, is depicted below.

The structure of pyrophyllite, a 2:1 phyllosilicate, is depicted below.

The structure of biotite, a 2:1 phyllosilicate, is depicted below.

Tectosilicates (framework silicates)

In the tectosilicates, all 4 oxygens of all tetrahedra are shared by adjacent tetrahedra. In some of the groups, Al may substitute for Si; however, since it is in the same position and is in the same coordination, it is considered to be similar to Si in these structures. The repeat formula is: SiO₂ (modified by substitution). SiO₄^-4 ---> SiO⁰₂. Some examples of tectosilicates from a number of classes are:

Group	Species	Composition
Quartz	Alpha-Quartz	SiO2
Feldspar	Albite	NaAlSi3O8
Feldspar	Anorthite	CaAl2Si2O8
Feldspathoid	Sodalite	Na8Si6Al6O24Cl2
Zeolite	Analcime	NaAlSi2O6·H2O

The structure of quartz is depicted below.

 

The structure of albite, a feldspar, is depicted below.

 

The structure of clinoptilolite, a zeolite, is depicted below.

Other Silicates (not readily classifiable)

As in nearly all classification systems, there are a few individuals that aren't readily classifiable. The same is true for the silicates.

The majority of silicates that are not readily classifiable display polymerization characteristics typical of two or more silicate classes. Most examples are quite rare, however the two most common ones are palygorskite and sepiolite, both clay-sized minerals.

Species	Composition	Silicate Structure
Palygorskite	Si8Mg5O20(OH)2(OH2)4·4H2O	Tetrahedral sheets inverted every 2 Si units
Sepiolite	Si12Mg8O30(OH)4(OH2)4·8H2O	Tetrahedral sheets inverted every 3 Si units
Hyalotekite	Pb2Ba2Ca2{B2[Si1.5Be0.5]Si8O28}F	4-member rings joined by 2 Si tetrahedra, producing 8-member rings
Nordite	Na3(La,Ce,Nd,Pr)(Sr,Ca)(Zn,Mg,Fe,Mn)Si6O17	A bent chain structure with 4-member rings in the chains
Pentagonite	Ca(VO)[Si4O10]·4[H2O]	Sheets with alternating tetrahedral units
Aenigmatite	Na2Fe5TiSi6O20	Branched chains

The structure of palygorskite is depicted below. By structure is similar to a sheet structure as in the phyllosilicated, but every two rows of tetrahedra are inverted, making for a much more complex structure.

General Stability Trends

Mineral stability is related to silicate structural configuration. In general, silicate mineral stability is related to the degree of polymerization of Si tetrahedra.

The strongest bonds within the silicate minerals are the Si-O bonds.

With increasing polymerization, the O / Si ratio decreases, and conversely, the number of Si-O bonds increases.

With increasing polymerization, the proportion of relatively weaker bonds linking tetrahedral units through intermediate cations decreases.

In situations where we find less O neutralization through sharing with adjacent Si (lower degree of tetrahedral polymerization), we find higher proportions of the more weakly bonded, more chemically reactive cations.

1. Mg - readily hydrolized
2. Fe - readily oxidized

As a general rule, therefore, we can say that resistance to weathering (under most earth surface conditions) tends to increase with degree of polymerization within the silicates.

Configurations of Various Linkage Arrangements

Although the tetrahedrally coordinated Si-O units, and the manner in which they join or are joined together, forms the basis for silicate classification, it is obvious that the tetrahedral Si-O structural unit is not the only structural unit in any of the silicate minerals, with the exception of quartz and its polymorphic variants.

The Si-O tetrahedral linkage represents the strongest bonds within the silicate structures and, therefore, tends to dominate other bonding linkages with respect to stability and general properties of the silicate minerals.

Other structural groupings of ions are also very important in distinguishing specific silicate mineral structures and in influencing silicate mineral chemical and physical properties.

Author: Ed Nater
Department of Soil, Water, and Climate

Copyright: Ed Nater and the Regents of the University of Minnesota
Copyright for mineral models held by the Minerals & Molecules Project

The opinions expressed herein are those of the authors and do not necessarily represent those of their respective universities or their Regents.