Partitioning of Chromium in the System MgO-Al₂O₃-SiO₂-Cr₂O₃ at 1 atm

Cristiano Brigida (cri@expe.terra.unimi.it)¹, Massimiliano Valle (maxvalle@libero.it)² & Stefano Poli (stefano@biko.terra.unimi.it)¹

¹ Università di Milano - Dip. Scienze della Terra, via Botticelli, 23, 20133, Milano, Italia

² ICRA S.p.A., via Lioni, 8, 24060, S. Paolo d'Argon (BG), Italia

Storage and inertization of toxic wastes containing heavy metals (Cr, Cd, Pb) should be approached by appropriate technological recycling processes. Chromium can be reinserted in a number of crystalline phases forming in the system MgO-Al₂O₃-SiO₂, profiting of petrological knowledge at supersolidus conditions. By this way complete bonding of chromium in a refractory phase assemblage and thus inertization can be obtained.

Phase equilibria were investigated in the spinel, corundum and mullite primary phase fields of the system MgO-Al₂O₃-SiO₂-Cr₂O₃, aimed to study chromium partitioning between solid and liquid phases, at temperatures up to 1560°C. Six different bulk compositions were prepared: three of them with SiO₂:Al₂O₃=1:1 and three with SiO₂:Al₂O₃=2:3. All compositions have Cr₂O₃:Al₂O₃=1:9 in molar proportions. A first set of runs was performed in platinum capsules, heated in a vertical furnace with controlled atmosphere, and quenched in water.

Preliminary chemical characterization of the phases by Electron Microprobe analysis shows strong partitioning of chromium - relative to coexisting liquids - in spinel, corundum-eskolaite series phases and Cr-mullite, for appropriate bulk compositions. Cr₂O₃ content in mullite coexisting with Cr₂O₃-Al₂O₃ solid solutions is, in the most Al-Si-rich sample, up to 11 wt%; such amount is very close to the upper limit known of chromium solid solubility in the mullite structure, as reported by Rager et al. (1990). Cr₂O₃ content in glass ranges from 0.3 to 0.9 wt.% and Cr₂O₃ increases with Al₂O₃ content of the liquid phase, in agreement with previous work by Schwessinger and Muan (1992). Al₂O₃ partitioning between spinel and coexisting liquids shows a major dependence on the composition of the bulk system, i.e. on the composition of the coexisting phases, as found by Schwessinger and Muan (1992) in Al-poor systems. Relationships between chromium and aluminum partitioning in spinels, Cr-mullite and glass are discussed.

Rager H, Schneider H & Graetsch H, Am. Min., 75, 392-397, (1990).

Schwessinger WT & Muan A, J. Am. Ceram. Soc, 75, 1390-1398, (1992).

Aluminium in Mantle Perovskites ­ A Change in Compression Mechanism in the Earth's Lower Mantle

John Brodholt (j.brodholt@ucl.ac.uk)

Dept. of Geological Sciences, University College London, Gower St. London WC1E 6BT, U.K.

Although aluminium is the fifth most abundant element in the mantle (about 5% Al₂O₃) its effect on the physical properties of perovskite has, until recently, largely been ignored or thought to be insignificant. It is becoming clear, however, that many properties of MgSiO3 perovskites are remarkably sensitive to small amounts of aluminium. In particular, a surprising result is that perovskite with a relatively small amount of alumina (about 5%) has a bulk modulus 10% lower than the pure Mg end-member (Zhang and Weidner, 1999). It has been argued that this increased compressibility is due to a high concentration of oxygen vacancies required to charge balance the aluminium (Navrotsky,1999). This would have important consequences for the mantle since aluminous perovskites would be weaker, have lower seismic velocities and greater attenuation, and, in addition, it could be a natural host for water. To test whether significant concentrations of oxygen vacancies exist in aluminous perovskites, ab initio DFT methods have been used to calculate the compressibility of end-member defect perovskites. The results show that only perovskites with oxygen vacancies have significantly enhanced compressibilities. However, oxygen vacancies are only energetically favoured at pressures into the shallower part of the lower mantle; at higher pressures (>30 GPa) the oxygen vacancy mechanism becomes unfavourable and aluminium is incorporated into both cation sites. This high pressure mechanism does not require oxygen vacancies to charge balance the aluminium and the resulting aluminous perovskite has a bulk modulus similar to the Al-free perovskite. This suggests that aluminium can strongly effect the physical properties of only the shallow part of the lower mantle; the deeper lower mantle will have properties more in accord with aluminium-free perovskite.

Zhang J & Weidner D, Science, 284, 782-784, (1999).

Navrotsky A, Science, 286, 1788-1789, (1999).

High Pressure Stability of Antigorite in the Systems MgO-SiO₂-H₂O and MgO-Al₂O₃-SiO₂-H₂O

Geoffrey David Bromiley (gbromiley@fs1.ge.man.ac.uk) & Alison R. Pawley

Department of Earth Sciences, Williamson Building, University of Manchester, Oxford Road, Manchester M13 9PL, UK

Current models of subduction-related volcanism invoke complex sequences of hydration and dehydration reactions in the mantle wedge. An understanding of the phase relationships expected to occur between hydrous phases in ultramafic systems at high pressures and low temperatures is therefore critical. The stability of the serpentine mineral antigorite has been examined in this context using a combination of non-end-loaded and end-loaded piston-cylinder apparatus for pressures up to 4.0 GPa and Walker-type multi-anvil device for pressures between 4.0 and 6.0 GPa. All run products were identified optically and by powder x-ray diffraction analysis. Some runs were further analysed by electron microprobe. Within the MgO-SiO₂-H₂O (MSH) system, antigorite dehydrates at high pressure to enstatite plus forsterite. This reaction was bracketed at 1.6 GPa between 630°C and 650°C, at 2.5 GPa between 620°C and 660°C, at 2.9 GPa between 620°C and 660°C, and at 520°C between 4.5 and 5.0 GPa. These new brackets partially resolve discrepancies noted between previous studies. Within the MgO-Al₂O₃-SiO₂-H₂O (MASH) system, antigorite breaks down to forsterite plus enstatite plus chlorite. This reaction was bracketed at 2.0 GPa between 660°C and 700°C, at 2.9 GPa between 660°C and 680°C, and at 600°C between 5.0 GPa and 5.5 GPa. These brackets suggest that the presence of even small amounts of Al (3.2% by weight) can increase the stability of antigorite considerably. This has important implications for the applicability of studies based on the stability of chemical end-members. The stability field for Al-bearing antigorite also possibly overlaps the stability fields of a number of other hydrous phases suggested to occur in the MASH system.