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Silicate Chemistry

 

The chemistry of the solid Earth is predominated by the chemistry of silicon.  Silicon is in the same group or family as carbon and, like carbon, has four “valence” electrons.  These electrons can participate in covalent bond formation.  The geometry of silicon compounds is that of a tetrahedron, as can be understood by the Valence Shell Electron Pair Repulsion theory (VSEPR).  The shared electron pairs form four covalent bonds and are separated by the greatest distance when they lie at the vertices of a tetrahedron. 

 

                                  

 

When two tetrahedrons share an oxygen atom, the result is the ion Si2O7-6.  Although the net charge for this ion is higher than it was for the individual tetrahedrons, the net charge per silicon atom is smaller.  This is because two electrons are involved in covalent bonding in the larger ion. 

 

The charge-to-size ratio for ions is extremely important.  Recall that in the case of ionic bonding, the interaction between a pair of positively and negatively charged ions follows the law for Coulomb force, and this force changes as a function of the square of the separation between charges.

For covalent bonding, the electrons do not interact with free charges in their environment as strongly.  Their interactions are primarily through charge-dipole interactions that fall off as distance cubed.  Therefore, these bonds are not as strong as the ionic bonds.

 

In silicate minerals, the tetrahedra are packed together, so that the entire mineral crystal can be thought of as a pile of tightly packed oxygen atoms with silicon atoms between some of the oxygens, and other metallic atoms occupying other spaces between the oxygens. Because silicon atoms can share oxygen atoms, there are a variety of ways to build silicate structures. This gives rise to a set of basic groups of silicate minerals:

 

Minerals, like everything else, are electrically neutral on a large scale.  How does the -4 charge of a silicon-oxygen tetrahedron become reduced?  They can be reduced through ionic bonding with positively charged metal ions, through sharing of their negatively charged oxygen atoms in covalent bonding, or a combination of the two.

 

In the case of ionic bonding, the cations that typically balance the negative charges of silicate ions are Mg+2, Fe+2, Fe+3, Al+3, Na+, K+, and Ca+2.  In different minerals the relative abundance of these cations is different.  In oceanic crustal rocks, magnesium and iron cations are more common.  Rocks formed by minerals enriched in these cations are called mafic, where “ma” stands for magnesium and “fic” is derived from the latin word for iron.  Another term for mafic is ferromagnesian.  The minerals forming the continental crust are enriched with sodium, potassium, and calcium cations.  The rocks containing these minerals are called felsic, where “fel” refers to feldspars, a common crustal mineral, and “sic” refers to the high percentage of silica.

 

Isosilicates

 

Olivine is a good example of the variability in the chemical composition of minerals. Olivine is a ferromagnesian silicate. In olivine, the ratio of iron to magnesium can vary from 0 to 1. In other words, some olivines might contain no iron, only magnesium. Other olivines might contain no magnesium, only iron, while most olivines contain a mixture of iron and magnesium.

 

Olivine represents a simple case of ionic bonding between a negatively charged silicate ion and positively charged (+2) metal ions.  The chemical formula for olivine is (Mg,Fe)2SiO4, meaning that both magnesium and iron serve as cations.  The formula tells us that there are two metal ions per silicate ion, but the formula doesn’t tell us how the metal ions are distributed.

 

The diagram shows how a magnesium (or iron) ion is shared between oxygen atoms from adjacent silicate tetrahedrons.  This forms an ionic bond between the tetrahedrons, reducing the net charge for both.  Consider the central tetrahedron in this diagram.  Each of its four oxygen atoms share a magnesium ion with an oxygen from an adjacent tetrahedron.  This results in the tetrahedron having

half of each magnesium’s 2+ charge.  Therefore each oxygen forms an ionic bond and becomes electrically neutral, so the silicate ion becomes neutral as well.

 

The arrangement shown in the diagram extends in all directions, with each tetrahedron having either an iron or magnesium ion attached to each of its oxygen atoms.  This creates a 3-dimensional matrix or crystal structure.  The formula for this arrangement, (Mg,Fe)2SiO4, reflects the fact that each tetrahedron possesses half of the four metal ions, for a net gain of two metal ions.

 

Chain silicates

 

Neutrality can also be achieved by sharing oxygen atoms between tetrahedrons (covalent bonding).  In chain silicates, each silicon shares either two, or three oxygen atoms with adjacent silicon atoms. Thus each tetrahedron is not distinct, and they are linked together in strong, covalently bonded chains. The chains are then held together by ionic bonds with metal ions.  Pyroxenes are a single-chain silicate. Amphiboles (including hornblende) are double-chain silicates.

 

 

 

Sheet silicates

 

In sheet silicates, each silicon shares three oxygens with adjacent silicon atoms resulting in strong, covalently bonded sheets. The sheets are then held together with ionic bonds. Micas are sheet silicates. Micas split apart along planes of ionic bonding with K atoms. Biotite mica contains Fe and Mg giving it a dark color, while muscovite contains only K.

 

 

 

Framework silicates

 

In framework silicates, each silicon shares all four of its oxygen with adjacent silicon resulting in a three dimensional framework of strong covalent bonds. The simplest framework silicates is quartz which contains only silicon and oxygen. The feldspars are framework aluminum silicates. Since aluminum occupies the middle of some of the tetrahedra, other metals, namely K, Na and Ca are needed to balance the charge of the crystal.

 

 

Consequences

 

Mineral

Melting Point (°C)

Olivine

1880

pyroxenes

1390

amphiboles

1200

micas

1100

albite feldspar

1100

orthclase feldspar

1000

quartz

  700

 

A consequence of this is that as the strength of electrostatic interactions increases the melting point of the compounds increases.  Minerals in which are primarily ionic silicates melt at a higher temperature than minerals in which the bonds are all or predominately covalent. 

 

The table above displays the melting points of several important silicate minerals at a pressure of 1 atmosphere (101 kPa).