SINGLE CRYSTAL DIFFRACTION AT EXTREME CONDITIONS: SPIN PAIRING TRANSITION AND STABILITY OF SIDERITE, FeCO3

Experimental mineralogy and petrology at extremely high pressure and temperature presents severe challenges that make synchrotron radiation a fundamental and exciting tool for the exploration of the planets’ interiors. X-Ray diffraction of single crystals loaded in the diamond anvil cell (DAC) has been recently developed with the aim of improving the accuracy of the structural analysis of minerals at extreme conditions. Several studies has been conducted reaching the pressure of the core/mantle boundary, we obtained accurate equation of state parameters, characterization of pressure induced structural rearrangements and bond compressibility. Crystals of new phases were synthesized in the diamond anvil cell at high pressure and temperature, their analysis provides accurate constraints for the modeling of the Earth’s interior. Carbonates are involved in several high pressure processes in nature. It is well known that the fate of carbonates at the subduction zone plays a significant role in the dynamics and chemistry of our planet, moreover in the history of Earth and planets carbonates experienced shock impact events that might have significantly altered the atmosphere composition. The study of the high pressure-high temperature behavior of the iron member of the calcite series is significant not only to define the stability of carbonates but also to advance our understanding of the properties of mantle minerals that are affected by the spin pairing transition of Fe. We characterized in detail the effect of the spin transition, occurring at ~ 44 GPa at ambient temperature, leading to a large collapse of the volume and the gain of intense coloration. Structural refinements were conducted up to 55 GPa, providing a robust estimate of the C-O bond compressibility, of the shrinkage of the ionic radius of Fe²⁺ caused by the spin pairing transition, and of the subtle structural rearrangement accompanying such a sharp volume collapse. High pressure and temperature experiments were conducted by laser heating through the diamond anvils. These experiments impose new constraints on the stability field of siderite at mantle conditions.