Ž . Physics of the Earth and Planetary Interiors 116 1999 117–131 www.elsevier.comrlocaterpepi Garnet-perovskite transformation under conditions of the Earth’s lower mantle: an analytical transmission electron microscopy study Nobuyoshi Miyajima a, ) , Kiyoshi Fujino a , Nobumasa Funamori b,1 , Tadashi Kondo b,2 , Takehiko Yagi b a DiÕision of Earth and Planetary Sciences, Hokkaido UniÕersity, Sapporo 060-0810, Japan b Institute for Solid State Physics, UniÕersity of Tokyo, Tokyo 106-8666, Japan Received 2 February 1999; accepted 16 August 1999 Abstract Natural pyrope garnets with three different chemical compositions have been transformed at 30–60 GPa in the Ž . laser-heated diamond-anvil cell DAC . Recovered samples were examined by analytical transmission electron microscopy Ž . 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 Al O content less than 25 mol% was quenched as a single 2 3 phase of orthorhombic perovskite at ambient conditions, whereas Mg-perovskite with Al O content of 25–28 mol% 2 3 transformed into the alternating lamellae of orthorhombic perovskite and lithium niobate phases. With further increasing the Al O content, Mg-perovskite converted into a single phase of the lithium niobate structure with polysynthetic twinning on 2 3 3q  4 1012 at ambient conditions. The high Al content may also induce the stabilization of Fe in the perovskite structure Ž . accompanied by metallic iron. Two Al-rich phases, corundum and a new Al-rich phase the NAL phase were recognized Ž . with Mg-perovskite mostly at lower pressures. The NAL phase is close to M Al Si O M sMg, Fe, Ca, Na, K , i.e., the 3 4 1.5 12 middle of A O and AB O type compounds, and is accompanied by stishovite. The electron diffraction patterns are 2 3 2 4 ˚ ˚ ˚ 3 Ž. Ž. Ž. consistent with the space group P6 rm or P6 with a s8.85 2 A, c s2.78 2 A, V s188 1 A . The existence of large 3 3 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. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Diamond-anvil cell; Garnets; Analytical transmission electron microscopy ) Corresponding author. Institute for Solid State Physics, University of Tokyo, Tokyo 106-8666, Japan 1 Present address: Department of Earth and Planetary Physics, University of Tokyo, Tokyo 113-0033, Japan. 2 Present address: Institute of Mineralogy, Petrology and Economic Geology, Tohoku University, Sendai 980-8578, Japan. 0031-9201r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. Ž . PII: S0031-9201 99 00127-2