Effect of cationic substitution on the pressure-induced phase transitions in calcium carbonate

The high-pressure CaCO3 phase diagram has been the most extensively studied within the carbonates group. However, both the diverse mineralogy of carbonates and the abundance of solid solutions in natural samples require the investigation of multi-component systems at high pressures (P) and temperatures (T). Here we studied a member of the CaCO3-SrCO3 solid-solution series and revealed the effect of cationic substitution on the pressure-induced phase transitions in calcium carbonate. A synthetic solid solution Ca0.82Sr0.18CO3 was studied in situ by Raman spectroscopy in a diamond-anvil cell (DAC) up to 55 GPa and 800 K. The results of this work show significant differences in the high-pressure structural and vibrational behavior of the (Ca,Sr)CO3 solid solution compared to that of pure CaCO3. The monoclinic CaCO3-II-type structure (Sr-calcite-II) was observed already at ambient conditions instead of the expected rhombohedral calcite. The stress-induced phase transition to a new high-pressure modification, termed here as Sr-calcite-IIIc, was detected at 7 GPa. Sr-calcite-VII formed already at 16 GPa and room T, which is 14 GPa lower compared to CaCO3-VII. Finally, crystallization of Sr-aragonite was detected at 540 K and 9 GPa, at 200 K lower T than pure aragonite. Our results indicate that substitution of Ca2+ by bigger cations, such as Sr2+, in CaCO3 structures can stabilize phases with larger cation coordination sites (e.g., aragonite, CaCO3-VII, and post-aragonite) at lower P-T conditions compared to pure CaCO3. The present study shows that the role of cationic composition in the phase behavior of carbonates at high pressures should be carefully considered when modeling the deep carbon cycle and mantle processes involving carbonates, such as metasomatism, deep mantle melting, and diamond formation

Aragonite-II and CaCO₃‑VII: New High-Pressure, High-Temperature Polymorphs of CaCO₃

The importance for the global carbon cycle, the <i>P</i>–<i>T</i> phase diagram of CaCO₃ has been under extensive investigation since the invention of the high-pressure techniques. However, this study is far from being completed. In the present work, we show the existence of two new high-pressure polymorphs of CaCO₃. The crystal structure prediction performed here reveals a new polymorph corresponding to distorted aragonite structure and named aragonite-II. In situ diamond anvil cell experiments confirm the presence of aragonite-II at 35 GPa and allow identification of another high-pressure polymorph at 50 GPa, named CaCO₃-VII. CaCO₃-VII is a structural analogue of CaCO₃-<i>P</i>2₁/<i>c</i>-l, predicted theoretically earlier. The <i>P</i>–<i>T</i> phase diagram obtained based on a quasi-harmonic approximation shows the stability field of CaCO₃-VII and aragonite-II at 30–50 GPa and 0–1200 K. Synthesized earlier in experiments on cold compression of calcite, CaCO₃-VI was found to be metastable in the whole pressure–temperature range