Garnet-perovskite transformation under conditions of the Earth's lower mantle: an analytical transmission electron microscopy study

Natural pyrope garnets with three different chemical compositions have been transformed at 30-60 GPa in the laser-heated diamond-anvil cell (DAC). Recovered samples were examined by analytical transmission electron microscopy (ATEM). For all three pyropes, the dominant post-garnet phase was Mg-rich orthorhombic silicate perovskite, The Al content of Mg-perovskite increased significantly with increasing pressure and temperature, and its chemical composition became close to that of the starting material. Mg-perovskite with Al2O3 content less than 25 mol% was quenched as a single phase of orthorhombic perovskite at ambient conditions, whereas Mg-perovskite with Al2O3 content of 25-28 mol% transformed into the alternating lamellae of orthorhombic perovskite and lithium niobate phases. With further increasing the Al2O3 content, Mg-perovskite converted into a single phase of the lithium niobate structure with polysynthetic twinning on {1012} at ambient conditions. The high Al content may also induce the stabilization of Fe3+ in the perovskite structure accompanied by metallic iron. Two Al-rich phases, corundum and a new Al-rich phase (the NAL phase) were recognized with Mg-perovskite mostly at lower pressures. The NAL phase is close to M3Al4Si1.5O12 (M = Mg, Fe, Ca, Na, K), i.e., the middle of A(2)O(3) and AB(2)O(4) type compounds, and is accompanied by stishovite. The electron diffraction patterns are consistent with the space group P6(3)/m or P6(3) with a = 8.85 (2) Angstrom, c = 2.78 (2) Angstrom, V = 188 (1) Angstrom(3). The existence of large cations such as Ca, K, and Na seems to stabilize the NAL phase relative to the corundum phase, but excess Ca and Na contents are likely to induce the formations of Ca-rich cubic perovskite and the calcium ferrite phase, respectively. (C) 1999 Elsevier Science B.V. All rights reserved.