High-pressure transformation behaviour of the cummingtonite-grunerite solid solution

The displacive C2/m ⟷ P2₁/m phase transition that occurs with increasing pressure in cummingtonites has been investigated by means of X-ray single-crystal diffraction at different compositions and degrees of Mg/Fe²⁺ order. Unit-cell parameters and a sub-set of intensity data of some “b” and “a” reflections were collected for four natural and two annealed samples as a function of pressure. The phase transition is close to second order in character, either with a significant contribution from the sixth order term of the Landau potential or with the fourth order coefficient depending on pressure via the bare elastic constants. The transition pressure, P_c, obtained at the different compositions and degrees of order, displays a plateau at the grunerite end. The evolution of the spontaneous strain associated with the phase transition does not depend on the Mg/Fe²⁺ substitution or ordering, but it is very sensitive to the impurity (Al, Na, Ca) content.