Trying to elucidate the petrological nature of the deep mantle is not easy because it is difficult to re-create such high-pressure - high-temperature conditions in the laboratory. At least for any length of time: Laser heating and momentary shock treatment can do it for a short time, but as silicate reactions usually take a long time to reach equilibrium condition, this leads to huge uncertainties in P-T parameters. However, in the early years, mineral chemistry principles were used to predict high pressure behaviour.
Use of germanates to model high pressure silicates was first suggested by Goldschmidt in 1931. Si and Ge are tetravalent and in same group in the Periodic Table.
|RADII:||Si4+ 0.26A||Ge4+ 0.40A|
Silicates and germanates usually isostructural and there is a wide range of germanate structures. So, if it is possible to synthesize a germanate structure at moderate pressures it is likely that an equivalent silicate structure will exist at higher pressures. If a germanate displays a phase transformation at a given pressure, the corresponding silicate often displays the same transformation but at a much higher pressure. This is because the critical radius ratio RGe/ROxygen for transformation to a new phase is attained at much lower pressures with Ge. Some germanates (e.g. GeO2) can crystallise at zero pressure while the equivalent silicate needs 100 kb pressure.
Many transformations in germanates involve change from 4- to 6-fold co-ordination with oxygen. Compare:
|NaAlSi2O8||>||NaAlSi2O6 + SiO2||28kb (silicate)|
|>||Jadeiite + Rutile str|
|NaAlGe2O8||>||NaAlGe2O6 + GeO2||5kb (germanate)|
|2CoSiO3||>||2Co2SiO4 + SiO2||100kb (silicate)|
|>||Spinel + Rutile str|
|2CoGeO3||>||2Co2GeO4 + GeO2||10kb (germanate)|
These lines of reasoning allowed predictions to be made as to what types of structure might exist at depth in the Earth.