Carbonic acid formation via a solid-solid reaction under high P-T conditions

We report the formation of anhydrate H₂CO₃ via a solid-solid reaction pathway under high P-T conditions using Raman spectroscopy, X-ray diffraction, and visual observations. The x-ray results showed that H₂O was thermally diffused into a cubic (CO₂-I) and a distorted cubic lattice (CO₂-IV). At low P-T region, monohydrate H₂CO₃ (H₂CO₃·H₂O) formed between CO₂-I and ice-VII, whereas anhydrate H₂CO₃ was formed between CO₂-IV and ice-VII at higher P-T region. The crystal structure of H₂CO₃ is orthorhombic and the calculated density lies between that of CO₂-IV and ideal CO₂(IV)/H₂O(VII) mixture. Stability region of H₂CO₃ was determined between the lower bound of CO₂-IV and the CO₂-saturated ice melting line. The present results show that H₂CO₃ can be produced under higher P-T conditions in the Earth’s subduction zone, and suggesting an important role for H₂CO₂ in the water-rich planets