Dense Potassium-Bearing Carbonates – Structure and Stability in Relation to the Earth´s Deep Carbon Cycle

Carbonates are probably the most important carriers of carbon in the planetary interior of our Earth. The occurrence of carbon in the interior of the planet, its distribution between the metallic core and the oxidic mantle, but also the exchange with the atmosphere and the biosphere are currently important research questions in the course of exploring the global carbon cycle. In the oxidic mantle of our planet, CO 2 and carbonates are the presumably essential components that form a large reservoir in the Earth`s mantle in the form of fluids, melts, but also as solid carbonate mineral phases. Small inclusions in natural diamonds, some of which are believed to have formed as far as the lower mantle just before the core-mantle boundary at a depth of 2750 km, prove the existence of carbonate minerals with significant sodium and potassium contents. These minerals formed under high pressures provide rare natural evidence of the existence of CO2 in the depths of the Earth`s mantle. Regarding their formation, however, it is not clear whether these were already enclosed as crystalline components in the depths during the growth of the diamonds, or whether the growing diamond included carbonate melt droplets that crystallized later in the course of cooling. This question is crucial in order to be able to determine the conditions of the formation beyond doubt. The existence of such potassium-containing carbonates is undoubtedly proven by the investigations on diamond inclusions. Potassium itself has the effect of lowering the melting point of carbonate phases, so that potassium-rich carbonates melt at lower temperatures, or their existence as solids would suggest higher pressures. The aim of the investigations in this project is to investigate the stability of such crystalline phases, possible transformations into other yet unknown forms. In addition, the so-called thermomechanical properties are to be determined, which are necessary in order to be able to make appropriate modeling of the conditions of formation. With the help of established high-pressure techniques, the so-called diamond anvil cell, and with the application of combinable heating techniques, synthetic carbonates are to be examined in terms of their properties and their behavior by means of X-ray diffraction methods and vibration spectroscopy. The synthesis of the carbonates is carried out by the Russian research partners using the technology of a multi-anvil press in Novosibirsk. The investigations in the diamond anvil cells are carried out in the laboratories in Vienna as well as in Novosibirsk, supplemented by measurements with synchrotron radiation at respective European large-scale facilities. 1

Carbonates are probably the most important carriers of carbon in the so-called Earth's mantle within the interior of our planet. The occurrence of carbonates and their distribution in the depth addresses the important research questions related to the understanding of deep carbon reservoirs within the global carbon cycle. The existence of compact, high-pressure carbonate minerals is a particular topic of current interest, as they can be detected even in nanosized inclusions in diamonds. Potassium-rich carbonates are of particular interest because they lower melting temperatures and thus enable the formation of carbonatitic melts even at high pressures. The most common system in nature is the binary subsystem K2CO3-CaCO3, for which a total of four stoichiometric subsolidus compounds of intermediate composition have been described. The aim of this work was to explore the stability ranges of these phases through in-situ measurements and, above all, to identify possible structural polymorphism and the character of any polymorphic transformations. In this context, synthetic analogue phases of the four phases, i.e. K2Ca(CO3)2, K2Ca2(CO3)3, K2Ca3(CO3)4 and K8Ca3(CO3)7, were investigated by an Austrian-Russian research consortium using diamond-anvil high-pressure cells under georelevant pressure and temperature conditions applying in-situ vibrational spectroscopy and X-ray diffraction techniques. The results of the investigations show that three of the four compounds underwent polymorphic high-pressure transitions. Second-order phase transformations were determined for K2Ca(CO3)2 and K2Ca3(CO3)4, with a relatively high critical transition pressure of 6.2 and 7.0 GPa, respectively. In both cases, due to the displacive nature of the structural transformation, the stability ranges of the high-pressure forms are not expected to have a significant influence on the formation pathway of the crystallization products. In contrast, the phase transition in K2Ca2(CO3)3 is clearly first-order in nature and isosymmetric, occurring at significantly lower pressures (2.8-4.4 GPa) with a corresponding hysteresis. Structural investigations revealed that the low-pressure phase is structurally unstable, with structural stabilization only occurring under pressure, and the stability range getting expanded under pressure. This also has consequences for the formation pathway, whether K2Ca2(CO3)3can be maintained under pressure stabilization or whether the well-known disproportionation into K K2Ca(CO3)2 + CaCO3 occurs. This finding also allows for genetic statements regarding the formation of carbonate inclusions, which theoretically enables the direct formation and maintenance of the stable high-pressure form.