The cubic-tetragonal phase transition in the system majorite (Mg 4 Si 4 O 12 ) - pyrope (Mg 3 Al 2 Si 3 O 12 ), and garnet symmetry in the Earth's transition zone

Abstract

Garnets along the join Mg4Si4O12 (majorite end member) – Mg3Al2Si3O12 (pyrope) synthesized at 2000 °C, 19 GPa are, after quench, tetragonal in the compositional range up to 20 mol% pyrope, but cubic at higher Al contents. Lattice constants atet and atet in the tetragonal compositional range converge with increasing pyrope contents towards the lattice constant of the cubic garnets. The elastic strain and the intensity of the (222) reflection as a function of composition indicate a second-order phase transition near 20 mol% pyrope. From the wedge-like shape of pseudomerohedral twins and their interaction near 90° twin-boundary corners, as well as from the absence of growth-induced dislocations, it is concluded that the Al-poor garnets are also cubic at synthesis conditions but invert by (Mg,Si) ordering on the octahedral sites into tetragonal phases of space group I41/a upon quench. This implies that the cubic-to-tetragonal phase transition in Mg4Si4O12 garnet occurs below 2000 °C at 19 GPa and at even lower temperatures in more aluminous compositions. A composition-dependent Landau model is consistent with a direct transformation from Ia3d to I41/a. Comparison of the T-X stability field of majorite-pyrope garnets with the chemistry of majorite-rich garnets expected to occur in the Earth's transition zone shows that the latter will be cubic under all conditions. Softening of elastic constants, which commonly accompanies ferroelastic phase transitions, may affect the seismic velocities of garnets in the deeper transition zone where majorite contents are highest.