Transformation Behaviour of Cummingtonite: a Neutron Diffraction Study

Neutron powder diffraction patterns at the HRPD line of neutron spallation facility, ISIS, at the Rutherford Appleton Laboratory can be collected as a function of temperature with exceptionally high quality. This gives the opportunity for full Rietveld refinement of crystal structures and, hence, permits detailed insights to be obtained into structural evolution as a function of temperature. In the present study, this approach has been used to characterise geometrical aspects of and spontaneous strain associated with the P21/m to C2/m phase transition which occurs with increasing temperature in the amphibole mineral, cummingtonite. The cummingtonite structure consists of double chains of SiO₄ tetrahedra parallel to the c-axis and linked laterally by Mg and Fe cations which occupy four crystallographically distinct sites M1, M2, M3 and M4. The phase transition which occurs in Mg-rich cummingtonites involves, primarily, distortion of the tetrahedral chains. Two series of experiments were carried out in the temperature interval 4-570 K, using a natural sample with composition close to Mg_4.6Fe_2.4Si₈O₂₂(OH)₂. This temperature range includes the phase transition itself and order parameter saturation effects at low temperatures. Eleven diffraction patterns were collected at about 40 K steps for 10 hours in order to obtain structural parameters and isotropic temperature factors from full Rietveld refinements. The largest atomic displacements obtained are, as expected, due to rotations of the SiO₄ tetrahedra in the amphibole double chains. Chains that are related by symmetry in the C2/m space group become distinct in the P21/m structure with a consequent change in the coordination of the cations at the M4 sites. This gives rise to a significant decrease of its average bond length due to the phase transition. The evolution of the unit-cell parameters as a function of temperature was obtained by refinements of diffraction patterns collected every 5 K for twenty minutes. The spontaneous strains obtained from the lattice parameters are found to vary in a manner consistent with a classical second order transition, including order parameter saturation.