Phase transitions in the system MgO-CO sub(2)-H sub(2)O during CO sub(2) degassing of Mg-bearing solutions

• Published in: Geochimica et cosmochimica acta 2012-01, Vol.76, p.1-13
• Main Authors: Hopkinson, Laurence, Kristova, Petra, Rutt, Ken, Cressey, Gordon
• Format: Article
• Language: English
• Subjects: Agitated; Carbon dioxide; Degassing; Disorders; Heating; Minerals; Sparging; Transformations
• ISSN: 0016-7037
• DOI: 10.1016/j.gca.2011.10.023

This study documents the paragenesis of magnesium carbonates formed during degassing of CO sub(2) from a 0.15 M Mg super(2+) aqueous solution. The starting solutions were prepared by CO sub(2) sparging of a brucite suspension at 25 degree C for 19 h, followed by rapid heating to 58 degree C. One experiment was performed in an agitated environment, promoted by sonication. In the second, CO sub(2) degassing was exclusively thermally-driven (static environment). Electric conductance, pH, and temperature of the experimental solutions were measured, whereas Mg super(2+) was determined by atomic absorption spectroscopy. Precipitates were analysed by X-ray diffraction, Fourier transform (FT) mid-infrared, FT-Raman, and scanning electron microscopy. Hydromagnesite [Mg sub(5)(CO sub(3)) sub(4)(OH) sub(2) times 4H sub(2)O] precipitated at 25 degree C was followed by nesquehonite [Mg(HCO sub(3),OH) times 2H sub(2)O] upon heating to 58 degree C. The yield of the latter mineral was greater in the agitated solution. After 120 min, accelerated CO sub(2) degassing resulted in the loss of nesquehonite at the expense of an assemblage consisting of an unnamed mineral phase: [Mg sub(5)(CO sub(3)) sub(4)(OH) sub(2) times 8H sub(2)O] and hydromagnesite. After 240 min, dypingite [Mg sub(5)(CO sub(3)) sub(4)(OH) sub(2) times 5H sub(2)O (or less H sub(2)O)] appears with hydromagnesite. The unnamed mineral shows greater disorder than dypingite, which in turn shows greater disorder than hydromagnesite. In the static environment, there is no evidence for nesquehonite loss or the generation of [Mg sub(5)(CO sub(3)) sub(4)(OH) sub(2) times XH sub(2)O] phases over the same timeframe. Hence, results indicate that the transformation of nesquehonite to hydromagnesite displays mixed diffusion and reaction-limited control and proceeds through the production of metastable intermediates, and is interpreted according to the Ostwald step rule. Nevertheless, variations in the chemistry of nesquehonite, combined with the established tendency of the mineral to desiccate, implies that its transformation to hydromagnesite is unlikely to follow a single simple sequential reaction pathway.