Methods for investigating deep mantle mineralogy


Direct sampling of the deeper mantle is obviously impossible. Observed seismic velocity-depth functions however constrain the densities of likely mantle phases. Moreover, possible phase transformations in the transition zone of the mantle are very difficult to verify experimentally. For instance, in the 400-900 km depth region pressures are in the range 130-340 kilobars and temperatures 1500-3000C ... beyond the range of most experimental equipment until recently. Now with diamond anvil apparatus, laser heating and on-line X-ray determinations it is possible to reach into this range, at least momentarily.

Indirect methods have also proved reasonably successful. Fortunately high pressure phases tend to crystallise in structures which are already known (isomorphs). For instance we can compare silicates with germanates because germanates form a series of crystal structures closely parallel to those of silicates, but the transformations occur at lower pressures.

Thermodynamics requires that the high pressure polymorph be denser, which limits possible structures. Once structure is known the bond lengths between cations and anions enable densities to be calculated.

The Radius Ratio (Rcation/Ranion) determines type of crystal structure. At high pressures effective radii contract differentially, thus altering the radius ratio. Thus a new high pressure phase may appear when radius ratios exceed certain critical values. Large ions (e.g. Oxygen 1.40Å) contract more under pressure than small ions. Oxygen is more polarizable than smaller cations:

Element Polarizability Radius (Å)

O--

3.1 10-24cm3 1.4

Si4+

0.04 10-24cm3 0.26

Mg2+

0.12 10-24cm3 0.72

 

Pressure thus increases covalent component of chemical bond.