Solubility of the hydrated Mg-carbonates nesquehonite and dypingite from 5 to 35 °C: Implications for CO 2 storage and the relative stability of Mg-carbonates

Anna L. Harrison, Vasileios Mavromatis, Eric H. Oelkers, Pascale Bénézeth

Research output: Contribution to journalArticleResearchpeer-review

Abstract

Hydrated Mg-carbonate minerals form during the weathering of ultramafic rocks, and can be used to store atmospheric CO 2 to help combat greenhouse gas-fueled climate change. Optimization of engineered CO 2 storage and prediction of the composition and stability of Mg-carbonate phase assemblages in natural and engineered ultramafic environments requires knowledge of the solubility of hydrated Mg-carbonate phases, and the transformation pathways between these metastable phases. In this study, we evaluate the solubility of nesquehonite [MgCO 3 ·3H 2 O] and dypingite [Mg 5 (CO 3 ) 4 (OH) 2 ·(5 or 8)H 2 O] and the transformation from nesquehonite to dypingite between 5 °C and 35 °C, using constant-temperature, batch-reactor experiments. The logarithm of the solubility product of nesquehonite was determined to be: −5.03 ± 0.13, −5.27 ± 0.15, and −5.34 ± 0.04 at 5 °C, 25 °C, and 35 °C, respectively. The logarithm of the solubility product of dypingite was determined to be: −34.95 ± 0.58 and −36.04 ± 0.31 at 25 °C and 35 °C, respectively, with eight waters of hydration. This is the first reported dypingite solubility product. The transformation from nesquehonite to dypingite was temperature-dependent, and was complete within 57 days at 25 °C, and 20 days at 35 °C, but did not occur during experiments of 59 days at 5 °C. This phase transformation appeared to occur via a dissolution-reprecipitation mechanism; external nesquehonite crystal morphology was partially maintained during the phase transformation at 25 °C, but was eradicated at 35 °C. Together, our results facilitate the improved evaluation of Mg-carbonate mineral precipitation in natural and engineered ultramafic mineral weathering systems that sequester CO 2 , and for the first time allow assessment of the saturation state of dypingite in aqueous solutions.

Original languageEnglish
Pages (from-to)123-135
Number of pages13
JournalChemical Geology
Volume504
DOIs
Publication statusPublished - 20 Jan 2019

Keywords

  • CO sequestration
  • Dissolution-reprecipitation
  • Dypingite
  • Mg-carbonates
  • Mineral phase transformation
  • Mineral solubility
  • Nesquehonite
  • Ultramafic mineral weathering

ASJC Scopus subject areas

  • Geology
  • Geochemistry and Petrology

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