An in situ X ray diffraction study of the kinetics of the Ni₂SiO₄ olivine‐spinel transformation

Abstract

Using the “MAX‐80” cubic‐anvil high‐pressure apparatus with synchrotron radiation, X ray diffraction experiments have been carried out to study the kinetics of the Ni2SiO4 olivine‐spinel transformation at 3.6–4.8 GPa and 740°–980°C. The Ni2SiO4 olivine‐spinel equilibrium boundary has also been redetermined. The starting material for the kinetic experiments was Ni2SiO4 olivine which was hot‐pressed in situ at 980°C and 2.5 GPa for 1.5 hours. During the transformation, X ray diffraction patterns were collected at intervals of either 30 s or 120 s, and estimates of the volume fraction transformed as a function of time have been made from the relative intensities of a total of six diffraction peaks by a least squares procedure. Although the Avrami rate equation can be fitted to the data, it is shown that this equation cannot be used as a basis for extrapolating the results to other temperatures and pressures. Instead, by fitting a more fundamental rate equation for grain boundary nucleation and interface‐controlled growth to the transformation‐time data, rates of nucleation and growth have been estimated. The activation energy for growth at 3.6–3.7 GPa has been estimated as 438 (±199) kJ mol−1. A rate equation has also been fitted to explain the dependence of the estimated nucleation rates on temperature and pressure. The results suggest that because of the kinetics of nucleation, the olivine‐spinel equilibrium boundary needs to be overstepped by ∼0.4 GPa at relatively high temperature (e.g., 1000°C) for the transformation to start, even on a geological time scale. Although the kinetics cannot be evaluated quantitatively at the low temperatures (500°–600°C) which are predicted to occur within subducting slabs at a depth of 300–400 km, the results are consistent with olivine surviving metastably to very significant depths below the equilibrium phase boundary. The results emphasize the importance of understanding the dependence of the rates of nucleation and growth on pressure and temperature when extrapolating experimental kinetic data for this type of transformation to geological conditions.

The influence of OH in coesite on the kinetics of the coesite-quartz phase transition

“…The rate functions k and parameters n determined from kinetic experiments using "wet" coesite varies from 1.2-1248 and 0.12-0.40, respectively. Values of n smaller than one are theoretically not possible and may indicate to the weakness of the simple model of Cahn [3,4]. But, nevertheless the direct comparison of the results of the "wet" and "dry" experiments performed under the same experimental conditions shows that the transformation kinetics of coesite to quartz is several orders of magnitude faster when coesite contains considerable amounts of intrinsic OH.…”

Intracrystalline Transformation of Olivine to Wadsleyite and Ringwoodite Under Subduction Zone Conditions

“…Investigations o n natural ringwoodite and wadsleyite in meteorites ( 8 , 9 ) as well as experiluental studies at high pressure have been carried out to elucidate the nature of these transitions. Most ex~erilnents have been performed o n analog compositions s~l c h as MgLGeO,, NiLSiO,, Fe2Si0,, MgLSiO,, and Co2Si0, (10)(11)(12)(13)(14)(15)(16). These studies indicate that the a-to-y transformation can occur either by incoherent grain bo~rndary n~~c l e a t i o n and interfacecontrolled growth (17), when differential stresses are low and the pressure is fairly close to equilibrium (1 0 , 1 1 , 13, 16), or by an intracrystalline mechanism that is presumed to involve shear (10-12, 14, 15, 18).…”

Evaluation of the Nucleation Rate of Olivine

“…Experimental data have shown that grain boundary nucleation and interface-controlled growth is the major mechanism of the phase transformation from olivine to wayaslite/ringwood [1,5] . However there is no nucleation rate of olivine phase transformation as yet, people have to employ the data from analogous minerals [6] .…”