Geochemistry

Gaining Quadruple Points

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Science  18 Feb 2005:
Vol. 307, Issue 5712, pp. 1017
DOI: 10.1126/science.307.5712.1017a

The most important binary mixture on Earth, as well as a significant one in industrial applications, is that of H2O and CO2. Their binary chemistry affects the atmosphere and ocean; determines volatiles in Earth's mantle and crust, including volcanoes; and plays a role in numerous industrial reactions. Although there have been many studies of the relation between CO2 and H2O at temperatures above the freezing point of water—involving gases, liquids, or supercritical fluids—the phase relations at lower temperatures, those at or below the freezing point of water, are less well documented. At low temperatures, the mixture is important in the upper atmosphere; in ice cores (where CO2 is trapped as a gas); in clathrates in the deep ocean; and on other planets, notably Mars with its polar caps of water and dry ice.

Longhi explores this parameter space using thermodynamic data on the various pure phases and the H2O + CO2 clathrate and shows that the phase diagram is richer than previously thought. The analysis suggests that CO2 clathrate will be stable in only some regions of the deep ocean, a critical issue with respect to carbon sequestration, and that conditions may be appropriate in some polar ice sheets for the accumulation of liquid CO2. — BH

Geochim. Cosmochim. Acta 69, 529 (2005).

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