Lecture 16: Ionic Solids and Silicates

Before coming to class:

Carefully read the information on this page. If it is not clear or if you want additional information, you may also want to review material from your textbook in sections 13.9 (partial).
Watch the 15 minute video lecture Silicates and Other Ionic Materials on the MediaSite page.
Take a short quiz on WebBoard.

Bonding in Ionic Solids

A simple example of an ionic solid is sodium chloride. This solid consists of a close-packed array of chloride anions with sodium cations in octahedral holes. Each sodium cation is surrounded by 6 chloride anions and each chloride anion is surrounded by 6 sodium cations. The cations and anions form an interpenetrating face-centered cubic structure.

The difference between a molecular formula and an empirical formula is important! A molecule of any ionic, crystalline material depends on the size of the crystal. Carbon monoxide has a molecular formula of CO and there is always precisely one carbon and one oxygen in every molecule of CO. The formula NaCl describes the ratio of sodium cations and chloride anions in sodium chloride but it is NOT a molecular formula. You could also say that sodium chloride is Na₂Cl₂, or Na₁₅₄₇₈Cl₁₅₄₇₈.

The interactions within the solid NaCl crystal are mainly (but not entirely) electrostatic. There is an attraction between the anions and the cations that reduces the total energy of the structure. The LATTICE ENERGY is a measure of the electrostatic attractions and repulsions in an ionic solid. For NaCl, 788 kJ M^-1 is released in formation of the crystal. There are also some weaker covent interactions. The empty sodium orbitals overlap with filled chlorine orbitals.

In any close packed array of atoms or ions, there are both octahedral and tetrahedral holes where smaller atoms or ions could reside.

In this figure, there are three layers of atoms or ions. Look at the first layer. There are holes surrounded by 3 of these anions.

We add a second layer (red) so that each atom or ion fits into a depression in the layer below it. Some of the holes in the first layer are capped by another atom or ion in the second layer. These are tetrahedral holes. Other holes are not capped in this way. A bigger atom or ion could fit into these octahedral holes which are surrounded by 3 anions from one layer and 3 from another layer.

A third layer covers these octahedral holes in the ABC layering of the cubic close packed structure (ccp). If the third layer were a position identical to the first layer, the structure would have hexagonal close packing (hcp).

We can think about NaCl as a ccp array of chloride anions with sodium cations in the octahedral holes or as a ccp array of sodium cations with chloride ions in the octahedral holes.

Examples of Close-Packed Solids

formula	ion, type of array	ion, holes
NaCl	Na⁺, ccp	Cl^- in all O-holes
CaF₂	Ca⁺², ccp	F^- in all T-holes
ZnS, zinc blend	Zn⁺², ccp	S^-2 in 1/2 of T-holes
ZnS, wurtzite	Zn⁺², hcp	S^-2 in 1/2 of T-holes
NiAs	As^3-, hcp	Ni⁺³ in all O-holes
alpha-Al₂O₃	O^2-, hcp	Al⁺³ in 2/3 of O-holes
alpha-Fe₂O₃	O^2-, hcp	Fe⁺³ in 2/3 of O-holes
TiO₂, rutile	O^2-, hcp	Ti⁺⁴ in 1/2 of O-holes
TiO₂, anatase	O^2-, ccp	Ti⁺⁴ in 1/2 of O-holes
FeTiO₃	O^2-, ccp	Fe⁺³ in all O-holes

Orthosilicates

Silicate minerals are based on the [SiO₄]^4- oxyanion.

Minerals with isolated silicate anions are called orthosilicates.

Examples include olivine (Mg, Fe)₂SiO₄, phenacite Be₂SiO₄, zircon ZrSiO₄, and garnets M₃M'₂(SiO₄)₃ where M= Al³⁺, Cr³⁺, or Fe³⁺ and M'= Ca²⁺, Mg²⁺, or Fe²⁺.

⁺

⁴⁺

Pyrosilicates and Metasilicates

If two silicate tetrahedra share one vertex (Si₂O₇^6-), the anion is called a pyrosilicate.

Pyrosilicate minerals include thorveitite Sc₂SiO₇, and barysilite MnPb₈(SiO₇)₃.

Metasilicates contain cyclic [SiO₃]_n^2n- anions.

Beryl Be₃Al₂Si₆O₁₈ is a metasilicate.

Pyroxenes: Single Chains

Each silicate tetrahedra in pyroxenes share a vertex with two other tetrahedra forming single chains of silicate [SiO₃]_n^2n-.

Examples include enstatite MgSiO₃, and diopside CaMg(SiO₃)₂.

The picture at left shows the silicate tetrahedra (blue) and the oxygen atoms (red).

Amphiboles: Double Chains

When two single chains are connected side-to-side through shared vertices, the double chain structure is called an amphibole.

Amphiboles and pyroxenes are structureally similar.

The picture on the right shows two, connected, zig-zag chains going into the distance. Below the top double chain is a layer of octahedrally coordinated cations. Below the cations is another double chain.

Two examples of amphiboles are blue asbestos Na₂Fe₅(OH)₂(Si₄O₁₁)₂, and gray-brown asbestos (Mg, Fe)₇(OH)₂(Si₄O₁₁)₂.

Phyllosilicates: Sheet Silicates

When three oxygen vertices are shared among silicates, a sheet structure is formed.

The fourth oxygen is coordinated to a metal cation.

Talc, Mg₃(OH)₂[Si₄O₁₀], is a typical phyllosilicate. It has a slippery feel because the layers can slide over one another.

Chrysotile or white asbestos, Mg₃(OH)₄[Si₄O₁₀], is a commonly used insulator. This material always curves in one direction creating tubular fibers.

Aluminum Substitution in Silicates

Aluminum(III) is approximately the same size as silicon(IV) and it can replace silicon in any of the anionic silicates. When that happens, there is a change in the charge of the material. The negative charge increases by one for every aluminum in the structure.

Asbestos

Asbestos is a fibrous mineral which does not burn or rot, is resistant to most chemicals, is flexible and possess high tensile strength. This unique combination of properties makes it an extremely useful material for lightweight reinforced cement products, friction materials, high temperature seals and gaskets and a host of other products.

Since the turn of the century, asbestos has been recognised as an occupational health hazard. It has been linked with asbestosis, lung cancer and mesothelioma in humans.

Lung disease almost exclusively affects miners and those involved in manufacuring of asbestos products. Asbestos is classified by regulatory authorities as carcinogenic to humans.

Combined exposure to asbestos and cigarette smoke very greatly increases the risk of lung cancer. They act synergistically and the combined risk is much greater than the individual risks for exposure to asbestos or for smoking. Lung cancers caused by asbestos are clinically indistinguishable from those caused by cigarette smoking. Exposure to asbestos is comparable to exposure to second-hand smoke in increasing a non-smoker's risk of lung cancer.

The degree of hazard (pathogenicity) is intrinsically related to fibre type and fibre size distribution. Chrysotile is much less dangerous than the amphibole types of asbestos because the small, curly fibers have a shorter residence time in the lung. Amphibole asbestos is composed of long, straight fibers that easily break and become embedded in lung tissue.

amphibole fiber	chrysotile fiber	chrysotile fiber diagram

Today, only one type of asbestos is used: chrysotile. In addition, the industry now only markets dense and non-friable materials in which the chrysotile fibre is encapsulated in a matrix of either cement or resin. These modern products include chrysotile-cement building materials, friction materials, gaskets and certain plastics.

Silicon Dioxide

When all four oxygen of a silicate are shared with other silicon atoms, the three-dimensional structure of SiO₂ forms.

Beach sand and quartz crystals are both examples of silicon dioxide.