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Beta MnOOH is precipitated preferentially (with respect to Mn304) at temperatures near 0øC when Mn 2+ is oxidized in aerated aqueous solutions. Upon aging in solutions open to the atmosphere a slurry of/3MnOOH tends to disproportionate to form MnO2 and Mn 2+. In such aged solutions, Mn 2+ and H + activities can be constant, and both the oxidation reaction Mn 2+ + x402 (aq) + «H20 •/3MnOOH (c) + 2H + and the disproportionation reaction 2/3MnOOH (c) + 2H + • MnO2 (c) + Mn 2+ + 2H20 can have positive reaction afiSnities. It is not possible for both reactions to be in thermodynamic equilibrium in the same system unless oxygen is almost completely absent. A value for AGf of -129.8 _+ 0.6 kcal/mol was obtained for/3MnOOH from experimental data by assuming that the reaction afiSnity for the oxidation reaction is equal to that for the disproportionation. A value for AGf for/3MnOOH of -129.8 -+ 0.5 kcal/mol was determined by measuring the redox potentials for the postulated half-reaction MnO2 (c) + H + + e-•/3MnOOH (c) at 0 ø, 5 ø, and 15øC and extrapolating to 25øC. Both these values are consistent with laboratory observations that/3MnOOH is less stable than 7MnOOH or Mn304 at 25øC. Analytical data for manganese-depositing springwater samples are consistent with a nonequilibrium model involving disproportionation of either/3MnOOH or Mn304.

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