Thermodynamics of manganese oxides: Effects of particle size and hydration on oxidation-reduction equilibria among hausmannite, bixbyite, and pyrolusite

Nancy Birkner, Alexandra Navrotsky

Research output: Contribution to journalArticlepeer-review

59 Scopus citations

Abstract

The surface enthalpies of manganese oxide phases, hausmannite (Mn 3O 4), bixbyite (Mn 2O 3), and pyrolusite (MnO 2), were determined using high-temperature oxide melt solution calorimetry in conjunction with water adsorption calorimetry. The energy for the hydrous surface of Mn 3O 4 is 0.96 ± 0.08 J/m 2, of Mn 2O 3 is 1.29 ± 0.10 J/m 2, and of MnO 2 is 1.64 ± 0.10 J/m 2. The energy for the anhydrous surface of Mn 3O 4 is 1.62 ± 0.08 J/m 2, of Mn 2O 3 is 1.77 ± 0.10 J/m 2, and of MnO 2 is 2.05 ± 0.10 J/m 2. Supporting preliminary findings (Navrotsky et al. 2010), the spinel phase (hausmannite) has a lower surface energy than bixbyite, whereas the latter has a smaller surface energy than pyrolusite. Oxidation-reduction phase equilibria at the nanoscale are shifted to favor the phases of lower surface energy-Mn 3O 4 relative to Mn 2O 3 and Mn 2O 3 relative to MnO2. We also report rapidly reversible structural and phase changes associated with water adsorption/desorption for the nanophase manganese oxide assemblages.

Original languageEnglish (US)
Pages (from-to)1291-1298
Number of pages8
JournalAmerican Mineralogist
Volume97
Issue number8-9
DOIs
StatePublished - Aug 2012
Externally publishedYes

Keywords

  • Calorimetry
  • Manganese oxides
  • Nanomaterials
  • Phase equilibria
  • Surface energy
  • Surface hydration

ASJC Scopus subject areas

  • Geophysics
  • Geochemistry and Petrology

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