| Summary: | Carbonic acid (H 2 CO 3 ) forms in small amounts when CO 2 dissolves in H 2 O, yet decomposes rapidly under ambient conditions of temperature and pressure. Despite its fleeting existence, H 2 CO 3 plays an important role in the global carbon cycle and in biological carbonate-containing systems. The short lifetime in water and presumed low concentration under all terrestrial conditions has stifled study of this fundamental species. Here, we have examined CO 2 / H 2 O mixtures under conditions of high pressure and high temperature to explore the potential for reaction to H 2 CO 3 inside celestial bodies. We present a novel method to prepare solid H 2 CO 3 by heating CO 2 /H 2 O mixtures at high pressure with a CO 2 laser. Furthermore, we found that, contrary to present understanding, neutral H 2 CO 3 is a significant component in aqueous CO 2 solutions above 2.4 GPa and 110 degrees C as identified by IR-absorption and Raman spectroscopy. This is highly significant for speciation of deep C-O-H fluids with potential consequences for fluid-carbonate-bearing rock interactions. As conditions inside subduction zones on Earth appear to be most favorable for production of aqueous H 2 CO 3 , a role in subduction related phenomena is inferred.
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