抄録
Tetragonal cryptomelane from the Tirodi mine, India transforms to monoclinic structure at 710°C in air. The composition of the tunnel is same before and after transformation (Na:K = 1:2), but the site occupancy increase from 68% to 86%. The increase of the occupancy is due to the movement of tunnel cation which were in the part converted to bixbyite. The structure refinements of the both tetragonal and monoclinic cryptomelanes clearly show the change of the MnO6 framework. In the monoclinic cell, two symmetry-independent octahedra exist. Mn(2) octahedra has greater distortion than the octahedra in tetragonal cell, but Mn(1) octahedra has less distortion. It is possible to consider that some of the Mn cations are reduced to Mn3+ to maintain the charge balance with the tunnel cations and Mn3+ ions exist mainly in Mn(2) octahedra for greater distortion. The transformation was caused by the increase of the tunnel site cation occupancy.
