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

Abstract

The importance for the global carbon cycle, the P–T 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-P21/c-l, predicted theoretically earlier. The P–T 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.

Postaragonite phases of CaCO3 at lower mantle pressures

“…The observations in Ref. 47 of mixed-phase CaCO 3 at 50 GPa reinforces our discussion in later sections regarding the kinetics of P mmn post-aragonite.…”

“…The recent publication by Ref. 47 reportedly features P mmn post-aragonite at 50 GPa alongside a proposed new structure with the P 2 1 /c-h unit cell, supplementing the argument that transformations into P mmn are severely kinetically hindered. With this in mind and with support from mechanistic calculations reported here showing the arrival of a structure above 50 GPa which is identical to P 2 1 /c-h, we postulate that − in spite of its enthalpic favorability (Fig.…”

New High‐Pressure Phase of CaCO3 at the Topmost Lower Mantle: Implication for the Deep‐Mantle Carbon Transportation

“…Analysis of the inclusions in deep diamonds has revealed evidence of the transportation of CaCO 3 to depths of at least the topmost lower mantle via sinking slabs (Brenker et al, ; Kaminsky et al, , ; Wirth et al, ). Previous studies have further shown that CaCO 3 undergoes a series of phase transitions at high pressures (Catalli & Williams, ; Gavryushkin et al, ; Lobanov et al, ; Oganov et al, , ; Ono et al, ; Pickard & Needs, ; Smyth & Ahrens, ; Suito et al, ). At 300 K, CaCO 3 in calcite structure will transform into CaCO 3 ‐III at 2.5 GPa, whereas CaCO 3 ‐III transforms into CaCO 3 ‐VI at 15 GPa (Merlini et al, ).…”

“…Although previous experimental and theoretical studies have shown that CaCO 3 crystallizes in the form of aragonite at pressure‐temperature conditions of the topmost lower mantle, a new phase between 32 and 48 GPa with a space group of P 2 1 / c has been predicted by a recent theoretical study (Oganov et al, , ; Ono et al, ; Pickard & Needs, ; Santillan & Williams, ; Suito et al, ). A recent experimental study reported two new phases of CaCO 3 , named aragonite‐II and CaCO 3 ‐VII, between 30 and 50 GPa at high temperatures (Gavryushkin et al, ). Yet neither aragonite‐II nor CaCO 3 ‐VII was observed to be a pure phase at high pressures and temperatures (Gavryushkin et al, ).…”

“…A recent experimental study reported two new phases of CaCO 3 , named aragonite‐II and CaCO 3 ‐VII, between 30 and 50 GPa at high temperatures (Gavryushkin et al, ). Yet neither aragonite‐II nor CaCO 3 ‐VII was observed to be a pure phase at high pressures and temperatures (Gavryushkin et al, ). Aragonite‐II coexists with aragonite and CaCO 3 ‐VII at ~26 GPa between 1,400 K and 2,400 K, while CaCO 3 ‐VII coexists with postaragonite between 40 and 50 GPa at 1,200–2,200 K (Gavryushkin et al, ).…”