A ferroelastic phase transition in vesuvianite

Abstract

Vesuvianite is a common rock-forming silicate mineral, with the general chemical formula X₁₉Y₁₃ Z₁₈ T_0–5O₆₈ W₁₀, where ′ = Ca, Na, Ln, Pb, Sb; Y = Al, Mg, Fe, Ti, Mn, Cu, Zn; Z = Si; T = B; W = (O, OH, F). Vesuvianite is ideally tetragonal with space group P4/nnc. However, a combination of optical and X-ray diffraction evidence indicates that the true symmetry of vesuvianite is P2/n or its subgroups (Pn, or less likely P2, P or P1) at room temperature. Birefringence measurements show that there is a phase transition at approximately 780 K. If the correct space group is P2/n this may be classified as a proper ferroelastic transition. The empirical order parameter exponent β is close to 1/2 which indicates typical Landau type behaviour. X-ray powder diffraction spectra show only slight changes associated with the phase transition. Any systematic variation in lattice parameters with increasing T is obscured by the associated errors; this indicates that the strain is extremely small, which allows relatively large fluctuaticns in the order parameter. Calculations show that there are 155 infrared-active and 232 Raman-active phonons in the paraelastic phase. Peak positions in room temperature Raman spectra indicate that they are primarily a result of Si tetrahedra in the vesuvianite structure. Comparison of peak intensities of the xx, yy, and zz spectra indicate that the deviation from tetragonal symmetry at room temperature is very slight. All of the evidence suggests that vesuvianite belongs to a system with a non-conserved order parameter.

Key Words:

• Vesuvianite
• phase transition
• optical birefringence
• Raman spectroscopy
• kinetics