DNS DINÁMICO

2.3.1.a Polymerisation.

Molten carbonates are defined as ionic melts and unlike silicate melts are believed to be incapable of polymerisation. The inability of carbonate to polymerise and the fundamental differences between silicates and carbonates are attributable to their electronic configurations and subsequent bonding characteristics. Consideration of the electronic configurations of Si^ and C ^ demonstrates that the outer shells of both atoms have identical electron occupation, that is, Si"^ - 3s^p^ and - 2s^p^, and hence it would be expected that bonding characteristics would be similar. However, the electronegativities of Si"^ and C f* of 1.9 and 2.6 respectively (Pauling 1948), indicate that S i-0 bonds will

be more polarised than C -0 bonds, with a 50% ionic character and localised charge distribution on oxygen atoms. Since the small ionic radius of Si^ of 0.34 predicts tetrahedral coordination with oxygen, Si"*^ readily adopts sp^ hybrid covalent bonds, where as, C^, due to its lower ionic character, is not restricted by the requirements of close packing and adopts a sp^ hybridisation in order to reduce the columbic interaction of the oxygen atoms (fig.2.9). Note that in both C ^ and Si'*^ the adoption of hybrid bonding

4+ B^o bond n^-oztdtal «p?o bond kxM pair p-orfaital BÇ» o r t t t e l Atm bonding iMxrUtal

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00* CO*

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Figure 2.9. Schematic diagram of bonding in silicates (B) and carbonates (C) and the shell energy levels for the carbonate group

orbitals is facilitated by the excitation of an electron from an s orbital to occupy an empty p orbital, which is energetically favourable due to the lower energies of the bonded orbital (fig.2.9.D).

A consequence of sp^ hybridisation is the formation of sp^a bonds between and nuclei and the formation of two p7t bonds, above and below the plane of the molecule by interaction of and p orbitals. Note, not only does the presence of a p7C bond result in a double bond, which is in reality shared over the three C-O bonds, but also results in incorporation of all bonding p orbitals in C -0 bonds, leaving only lone pair p orbital per oxygen orientated in the plane of the molecule. Hence, unlike the SiO^^ tetrahedra, which demonstrates no pTC bonding and free oxygen unpaired p orbitals, the trigonal group has no unpaired orbitals available for covalent bonding, and is hence, unable to polymerise.

The inability of to polymerise prohibits the formation of three dimensional network structures and as a result carbonates form structures based on ionic and coordination bonding.

2.3.l.b Coordination.

The stable coordination of carbon to oxygen as described above is trigonal and unlike silicates, in which both V fold SiOg^ and VI fold SiOg®", coordinated Si^ have been observed at high pressures in both crystalline and melt phases (Xianyu et al 1991, Stebbins et al 1989), coordination increases have not yet been reported from carbonates, although this may be a reflection of the lack of structural experimental data on carbonates, due to technical difficulties, above 15kb.

Within this Section I will discuss the mechanisms by which coordination changes could occur, using Si'*^ coordination changes as comparative examples, in order

to evaluate the possibility of coordination changes in carbonates at very high pressures. Increases in Si'*^ coordination from SiO^^ to SiOg®" requires relatively excited electronic configurations in order to produce octahedral d^sp^ hybridisation, involving excitation of electrons into the 4d orbitals. Energetically such an increase in coordination may be facilitated by its mixed ionic-covalent character, which is likely to be relatively sensitive to the requirements of close packing at higher pressure.

An increase in coordination of from C O ^ to CO^^, however, would not require

increased electronic excitation as the electronic configuration of in satisfies the requirements of both sp^ trigonal and sp^ tetrahedral hybridisation. However, such an increase in coordination would involve the breaking of the pTC bond and subsequent increases in energy due to resulting antibonding orbitals.

It is apparent therefore that the occurrence of at high pressure is theoretically possible. However, physical mechanisms suggested for Si^ coordination increases by the response of network structures to viscous flow under close packing (Xianyu et al 1991), could not apply to carbonate melts, as viscous flow would occur due to the breaking and forming of ionic bonds.

2.3.I.e. Complexation.

As discussed in Section 2.3.1.a the carbonate group is unable to form covalent bonds with metal cations due to the lack of non bonding unpaired oxygen orbitals and, hence, it has been suggested that there is no definite association between metal cations and ionic groups in carbonate melts (Trieman and Schedl 1983). If, however, such complexation does occur then it must be due to ionic bonding between carbonate groups and metal cations, and hence, may be expected to be a function of electronegativity. Also the presence of three lone pair oxygen p-orbitals in the plane of the could result in

coordinate bonding to certain transition metals. Coordinate bonding is similar to covalent bonding except that both electrons are supplied by the same atom.