Abstract

The common occurrence of rhodochrosite with hausmannitc (Mn3O4) and its rare equilibrium association with manganosite (Mn1–XO), bixbyite (Mn2O3,) and pyrolusite (MnO2) are natural expressions of the fo2, fco2, and temperature dependence of the rhodochrosile stability field. The relationships of those variables of the bounding reactions which produce manganese oxides and graphite were determined using rhodochrosite charges surrounded by oxygen buffers in a CO2+CO atmosphere. The rhodochrosite-manganositegas equilibrium surface, determined on the graphite, Ni-NiO, and manganosite-hausmannitc buffers at 1001), 1500, and 2000 atmospheres is given by the equation (°K, atm)
logfCO2=8.6625556T0.0944pT.
This equilibrium is independent of f02 within the limits of experimental error (±5°C, ±40 atm pCO2). The rhodochrosite-hausmannite-gas equilibrium constant, determined on the manganosite-hausmannite and hausmannite-bixbyite buffers, is given by the equation
logK=6logfCO2logfO2=40.399429T+0.486pT

Experimental data for reactions between rhodochrosite and the manganese oxides, supplemented by thermodynamic calculations, indicate that at l000 atmospheres the rhodochrosite field boundary decreases from 715o±5°C,– 12.5 log fo2 (atm.) units on the Mn1-x O-Mn3O1 buffer, to 330°±23oC,–8.4 log fo2 units on the MniO4–MnsO3 buffer, and to 260 ±35o C,–5.6 log fo2, units on the Mn2O3-MnO2 buffer. Rhodochrosite is stable at high temperatures only at low fo2 Comparison of the siderite, calcite, and magnesite stability fields with that of rhodochrosite suggests that the addition of manganese expands the siderite, reduces the calcite, and causes little change in the magnesite fields. Oxygen fugacities encountered in the MnO-C-O system are higher than those in corresponding iron-bearing systems. Metamorphic manganese oxide bodies have high inherited fo2 values due to internal buffering, which causes gradients between such bodies and country rocks containing iron oxides or graphite.

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