Pressure-induced phase transition in synthetic trioctahedral Rb-mica

The crystal structure of a synthetic Rb analog of tetra-ferri-annite (Rb-TFA) 1M with the composition Rb0.99Fe3.032+(Fe1.043+Si2.96)O-10.0(OH)(2.0) was determined by the single-crystal X-ray diffraction method. The structure is homooctahedral ( space group C2/m) with M1 and M2 occupied by divalent iron. Its unit cell is larger than that of the common potassium trioctahedral mica, and similar lateral dimensions of the tetrahedral and octahedral sheets allow a small tetrahedral rotation angle alpha = 2.23(6)degrees. Structure refinements at 0.0001, 1.76, 2.81, 4.75, and 7.2 GPa indicate that in some respects the Rb-TFA behaves like all other micas when pressure increases: the octahedra are more compressible than the tetrahedra and the interlayer is four times more compressible than the 2: 1 layer. However, there is a peculiar behavior of the tetrahedral rotation angle alpha: at lower pressures (0.0001, 1.76, 2.81 GPa), it has positive values that increase with pressure [from 2.23(6)degrees to 6.3(4)degrees] as in other micas, but negative values -7.5(5)degrees and -8.5(9)degrees appear at higher pressures, 4.75 and 7.2 GPa, respectively. This structural evidence, together with electrostatic energy calculations, shows that Rb-TFA has a Franzini A-type 2: 1 layer up to at least 2.81 GPa that at higher pressure yields to a Franzini B-type layer, as shown by the refinements at 4.75 and 7.2 GPa. The inversion of the alpha angle is interpreted as a consequence of an isosymmetric displacive phase transition from A-type to B-type structure between 2.81 and 4.75 GPa. The compressibility of the Rb-TFA was also investigated by single-crystal X-ray diffraction up to a maximum pressure of 10 GPa. The lattice parameters reveal a sharp discontinuity between 3.36 and 3.84 GPa, which was associated with the phase transition from Franzini-A to Franzini-B structure.