KINETIC STUDIES ON THE CRYSTALLISATION OF COESITE USING THE HP/HT-DEVICE MAX80 AND SR. Zinn, P.¹, Hinze, E.² & Lauterjung, J.¹; ¹GeoForschungsZentrum Potsdam, D-14473 Potsdam; ²University of Giessen, D-35390 Giessen

The kinetic behaviour of the quartz - coesite transition has been studied in the P/T range of 3.2-5.2 GPa and 680-900C by in-situ X-ray diffraction using synchrotron radiation and a MAX80 cubic anvil high-pressure apparatus at HASYLAB. The quartz - coesite equilibrium phase boundary has been determined and discussed using the experimental results. During the transition, X-ray patterns were collected, and the transformed volume fraction has been estimated from the diffracted intensities of the respective phases as a function of time. By fitting a fundamental rate equation for grain boundary nucleation and interface-controlled growth to the transformation-time data, rates of nucleation and growth have been estimated [1].

During the in situ observed phase transition over a small P-T stability region an intermediate non quenchable stage could be detected [2]. By constructing a Bragg-Bragg Mo-double-crystal-monochromator the resolution of the MAX80-device could be raised up to . With this configuration the intermediate phase shall be investigated in detail.

Furthermore, by quenching the samples, the reaction products could be investigated by TEM images. A strongly twinning with (100) reflection twins could be observed [3]. Its influence on the thermodynamic behaviour of the a-quartz - coesite phase transition is discussed. Coesite-nuclei in quartz crystals, and in a latter stage penetrating the quartz-crystals could be observed. The growth mechanism and the existing structural relationship between host-quartz and the coesite-nucleus were then analysed.

[1] Zinn,P., Lauterjung, J., Hinze,E.: Kinetic studies of the crystallisation of coesite using SR. Nucl. Inst. Meth B 97 (1995) 89-91

[2] Heaney,P.J. & Veblen,D.R.: Observations of the a-b phase transition in quartz. Am. Mineral., (1991), Vol. 76, pp. 1018-1032,

[3] Bourret,A., Hinze,E, & Hochheimer,H.D., (1986): Twin Structure in Coesite. Studied by HREM. Phys Chem Min, Vol. 13, pp 206-212