Thermal behavior of complex, tunnel-structure manganese oxides

The manganese oxide minerals coronadite, romanechite, and todorokite occur in a wide range of geologic settings and are common minor phases in massive deposits and as fracture linings. We have studied the thermal behavior of these minerals using a combination of thermogravimetric analysis, differential scanning calorimetry, water analysis, and X-ray powder diffraction. All of these minerals are relatively insensitive to heating up to 400 °C, and the most significant result of heating is reduction of Mn⁴⁺ with concomitant evolution of oxygen and structural transformation. The total water content of the tunnel-structure manganese oxides is related primarily to the tunnel size and secondarily to the nature of the tunnel cation. Both coronadite and romanechite retain much of their tunnel water to at least 500°C. Todorokite gradually loses most of its tunnel water below 400°C. The lower-temperature evolution of water from todorokite as compared with romanechite and coronadite is probably a reflection of the larger, less-filled tunnels in todorokite. Romanechite and todorokite (and perhaps coronadite) appear to contain water in crystallographically well-defined sites, but water evolution does not appear to have a significant structural impact on these minerals. The structural effects of heating romanechite and todorokite up to 300°C are limited to a minor change in the β angles. Coronadite is not significantly affected until over 600°C. At higher temperatures, all of these minerals transform to Mn₂0₃ or Mn₃0₄, admixed with other complex oxides in some cases. The relative insensitivity of the todorokite structure to heating strongly supports the now-accepted tunnel structure of todorokite.