Synthesis, structure and properties of carbonates at earth-mantle conditions
Final Report Abstract
Our high pressure investigations of the different carbonates have demonstrated: 1. The phase diagrams of the investigated substances were previously considered as relatively simple. In reality, however, the phase relationships are highly complex. Some polymorphs exist only in a very narrow p-T range. Therefore, in order to get more precise information, future experiments will have to be performed in very small p-T intervals. 2. Our investigations on the CdCO3 cast doubt on the existence of the alleged calcitearagonite phase transition in transition metal carbonates. Our results suggest, that either, if such a transformation takes place, it occurs at a higher pressure than actually achieved in our experiments, or this type of phase transition is really untypical for such carbonates. Further in-situ high pTX-ray or neutron diffraction experiments with improved resolution on carbonates with d transition metal cations are needed to unambiguously elucidate their behavior concerning this type of phase transition. 3. It is remarkable that it is very difficult to extract from the literature consistent information on the high-pressure behavior of carbonates. This is probably due to the fact that the respective experiments were all carried out under different conditions (in-situ / ex-situ; with or without heating), on different materials (synthetic or mineral), and with different pressure transmitting media. However, all these facts play an important role for the kinetics in high pressure experiments. For example, the properties of the pressure transmitting medium used must be considered when interpreting the obtained data. Unawareness or overestimation of the provided hydrostatic pressure can lead to wrong interpretations of the observations. It is well known that non-hydrostatic conditions can strongly influence the behavior of the substances studied. For example, deviations from hydrostatic conditions may either promote, or suppress phase transformation, or may even cause amorphization of the sample. For further investigations on carbonates and other compounds under extreme conditions it seems to be advisable to follow a consistent experiment protocol allowing for a comparative classification and summary of the data obtained. 4. The nomenclature used in the various published reports on the high pressure polymorphs of carbonates with aragonite-type structure at ambient conditions, is highly confusing. For example, it is not clear whether only the first occurring high-pressure phase should be called post-aragonite phase, or all phases appearing in the course of a sequence of phase transitions. An easier nomenclature like phase I, II, III,... seems to be appropriate. Using such a nomenclature the post-aragonite phase would be called phase II.
Publications
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Raman spectroscopic study of PbCO3 at high pressures and temperatures. Phvs. Chem. Minerals, publ. Online 22. May 2009
Minch, R., Dubrovinsky, L., Kurnosov, A., Ehm, L., Knorr, K. & Depmeier, W.
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Evidence for the existence of a PbCO3-ll phase from high pressure X-ray measurements. Z. Kristallogr. 225, - (2010)
Minch, R., Peters, L., Ehm, L., Knorr, K., Siidra, O. I., Prakapenka, V., Dera, P. & Depmeier, W.
