Kinetics of Phase Transitions Determined by Synchrotron X-Ray Diffraction Using MAX80 High-pressure Cell. J. Lauterjung¹, P. Zinn¹, T. Peun¹ and E. Hinze², ¹GeoForschungsZentrum Potsdam, Potsdam, Germany, ²Institut für Geowissenschaften, Universität Giessen, Germany

The kinetics of the phase transition (-Quartz to Coesite and Olivine-typ to Spinel-type structure in Mg₂GeO₄ have been studied in the pressure range up to 6 GPa and temperatures up to 1500deg.C by energy-dispersive in-situ X-ray diffraction using a MAX80 cubic-anvil high pressure apparatus at the Hamburger Synchrotronstrahlungslabor (HASYLAB), Germany. Diffraction patterns were collected in time intervals of 10 to 60 seconds. Transformed volume fractions could be derived from diffracted line intensities of the respective phase as a function of time. Nucleation- and growthrates were calculated from the transformed volume fractions using rate equations for grain boundary nucleation and interface-controlled growth. The pressure and temperature dependance of growth rates allow to determine thermodynamic parameters like activation energy, activation volume and heat of transition (see table).

Mg₂GeO₄ Quartz-Coesite

Activation energy, kJ/mol 200(3) 172(1)

Activation volume, cm³ 10(2) 13(2)

Heat of transition, kJ/mol 10(1) 8(1)

The structural behaviour of Mg₂GeO₄ during the transition was monitored by structure-factor calculations. The results indicate a fast (martensitic) rearrangement of the oxygen lattice and a slow ordering of the cations in the spinel lattice. The high pressure equipment and the energy dispersive diffraction techniques are discussed. Recent and future developments of the experiment are discussed. Implications of the results to the interpretation of geodynamic processes are given.