The present cumulative thesis describes an experimental investigation of the elastic properties and crystal chemistry of rhombohedral carbonates at pressures and temperatures relevant to the Earth's mantle. The goal of the thesis is to investigate the high-pressure behavior and determine the stability fields of several endmember minerals that represent the calcite group (FeCO3, MnCO3, CoCO3, ZnCO3, NiCO3), as well as fer-romagnesian [(Fe,Mg)CO3] compositions, in order to determine

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... ical regularities and describe high pressure polymorphs and dis-sociation products at extreme conditions. In addition, the seismic de-tectability of Fe-bearing carbonates in the Earth's mantle is explored by deriving sound velocities and comparing them to the velocity profiles of the bulk mantle. The main tools used in these studies are: laser-heated diamond anvil cells for generating the high pressures and temperatures that exist in the Earth's mantle; powder and single-crystal X-ray diffrac-tion for phase identification, structure solution and refinements, and in situ characterization of the chemical compositions of novel compounds; Raman spectroscopy for the investigation of the molecular vibrational modes; and nuclear inelastic scattering for the determination of acoustic velocities. The equations of state of synthetic single crystals of siderite (FeCO3), ferromagnesite [(Mg0.74Fe0.26)CO3], rhodochrosite (MnCO3), spherocobaltite (CoCO3), gaspeite (NiCO3), and smithsonite (ZnCO3) were obtained during cold compression. All of the above carbonates remain stable in the calcite-type structure (R3⁻c)