The non-convergent ordering of Mg and Ni over the M1 and M2 sites of synthetic olivines has been studied using “time of flight” neutron powder diffraction and X-ray absorption spectroscopy (EXAFS). The compositional dependence of order/disorder at room temperature was established for solid solutions of general formula (Mgb>1−Xb>Nib>Xb>)b>2b>SiOb>4b>, where X = 0.15, 0.2, 0.25, 0.3, 0.5, and 0.8 atoms Ni (Xb>Nib>; i.e., mole fraction of Ni-olivine end-member). Ni orders into M1 with Kb>Db> = (Ni/Mg in M1)/(Ni/Mg in M2) reaching a maximum of 9.5 at a composition of Mgb>1.6b>Nib>0.4b>SiOb>4b>. The temperature dependence of order/disorder at up to 1100 °C was determined for two samples (Xb>Nib> = 0.2 and 0.5). Between about 600 and 750 °C the samples show an increase in order due to kinetic effects, while above 750 °C the samples show a progressive decrease in order and describe an equilibrium disordering path. Equilibrium data define a Ni-Mg, M1-M2 intersite exchange energy of 21.5 ± 1.9 kJ/mol. On cooling, the blocking temperature for cation exchange is about 800 °C.
The kinetics of disordering behavior were analyzed using a Ginzburg-Landau model giving activation energies for Mg-Ni exchange between M1 and M2 for samples of composition Mgb>1.6b>Nib>0.4b>SiOb>4b> and Mgb>1.0b>Nib>1.0b>SiOb>4b> of 145 ± 5 and 160 ± 5 kJ/mol, respectively. The model also shows that the characteristic time scale for re-equilibration of M1-M2 order decreases from around 2.5 s at 1000° to 0.03 s at 1300 °C. This points to the inapplicability of intracrystalline Ni-Mg partitioning for obtaining geothermometry and geospeedometry information for magmatic conditions. Ni K-edge EXAFS data show that samples with Xb>Nib> = 0.15, 0.2, 0.25 and 0.3 all show Ni clustering on adjacent M1 sites, indicating the presence of domains of Ni-rich and Mg-rich regions on a nanolength scale of < 10 Å. These “precipitates” are at least an order of magnitude too small to be detectable by neutron powder diffraction. We suggest that the elastic strain at the interfaces between the Ni-rich precipitates and the Mg-rich matrix is responsible for the plateau or possible maximum in the b unit-cell parameter as a function of composition across the solid solution, which is observed at a composition of Mgb>1.6b>Nib>0.4b>SiOb>4b> at room temperature. Comparison of our data with earlier studies at high P and T on Mgb>1.0b>Nib>1.0b>SiOb>4b> olivine suggests that the effect of P is to increase the degree of order of Ni into M1 and to slow down the kinetics of intersite exchange with a ΔVb>disorderb> of 0.039 J/bar.
