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The coordination sphere of trace elements in fluids affects the partitioning of these elements between fluids and minerals and/or silicate melts. It has previously been shown by Lüdemann and Franck that transition metals such as Co(II) change their coordination in a fluid phase from octahedral to tetrahedral, if the chloride concentration and/or the temperature is increased. The type of cell used in most previous studies limits the P - T range to 500°C and 6 kbar. We used a modified Bassett type diamond anvil cell (DAC) for extending the P - T range to 1000°C and 100 kbar, in order to cover geologically important systems such as subduction zones or magma bodies in the deep continental crust.

In a first step ligand field spectra of an aqueous solution (0.1 mol/liter) of CoCl₂ were measured at temperatures to 700°C and pressures to 8.6 kbar (Fig. 3.5-10). Spectra were acquired using a Bruker IFS 120 spectrometer with A590 microscope, Xe-arc lamp, Quartz beam splitter and a Si-diode.
 

Fig. 3.5-10: Ligand field spectra of a 0.1 m CoCl2 - solution from 25°C and 1 bar to 700°C and 8.6 kbar. Th = temperature of homogenization.

At room temperature the aqueous CoCl₂ solution has a broad absorption band system at about 19,250 cm^-1 resulting in a pink colour. This is attributed to the octahedral Co(H₂O)₆²⁺ complex. Heating the solution to 700°C results in a color change from pink to blue. The spectrum at high temperature consists of a split band with maximum at about 15,500 cm^-1 and a shoulder at about 18,000 cm^-1 arising from a tetrahedral Co²⁺ complex (Fig. 3.5-10).

The change in coordination depends on temperature and pressure. Increasing temperature stabilizes the tetrahedral complex, increasing pressure stabilizes the octahedral complex. Increasing pressure increases the temperature of the coordination change from octahedral to tetrahedral.