2.1 Chemical substitutions

The principles of coordination and charge balance limit the possible structures of the silicates, and therefore constrain the total range of possible compositions found in natural minerals. The fact that many cations have similar sizes allows for a large range of chemical substitutions in the silicates. For instance Fe2+and Mg have a similar size and identical valency. Their similar size implies that they have the same coordination number (6) and thus they are able to fit in the same crystallographic sites, while their identical valency implies that the subPstitution does not violate the electronic stability of the structure. We find therefore that many naturally occurring silicates show a complete range in substitution of Fe and Mg (for example, in the pyroxene, amphibole and mica groups). The substitution of Fe for Mg on a octahedral site can be written conveniently :

Fevi Mgvi-1

where vi refers to the cations located on a octahedrally (6-fold) coordinated site.

Aluminium plays an important role in silicates since apart from Si it is the only common cation which can exist in tetrahedral coordination. It is therefore able to substitute for Si. However, since Al is trivalent while Si is quadrivalent this substitution violates the charge balance requirement and thus cannot therefore occur by itself. However it can occur if it is coupled with another substitution on a nearby crystallographgic site which preserves charge balance. For instance the substitution of a tetrahedral Al (written Aliv) for Siiv:

AlivSiiv-1

could be coupled with the substitution of octahedral Al for Fevi or Mgvi :

AlviMgvi-1

The complete coupled substitution can then be written :

AlivAlviMgvi-1Siiv-1

Note that this is a valid substitution since it is charge balanced. All valid substitutions must be charge balanced.

The role of Aliv substitution is particularly important in the tektosilicate feldspar group. Feldspars consist of 3-dimensional frameworks of linked Si-O and Al-O tetrahedra. The ratio of Al-O to Si-O tetrahedra in feldspars is never greater than 1 : 1 (and may be as little as 1:3) because Al-O tetrahedra tend to avoid linking to each other (the so-called Al-avoidance principle). Oxygens linking two adjacent Si-O tetrahedra are electronically neutral, while oxygens linking adjacent Si-O and Al-O tetrahedra have an excess of 1/4 of a negative charge. This excess charge cannot be balanced by Mgvi or Fevi (which would contribute 1/3 of a positive charge to each linked oxygen) but can be balanced by univalent or bivalent cations with larger coordination numbers (8-fold or 12-fold coordination). Cations which fit the bill include Ca, Ba, Na and K, all of which are found in naturally occurring feldspars.