MANGANESE OLIVINE II - POINT-DEFECT RELAXATION

To investigate the point defect chemistry and the kinetic properties o f manganese olivine Mn2SiO4, the point defect relaxation time (tau) ch aracterizing the rate of re-equilibration of electrical conductivity f ollowing a change in oxygen fugacity was measured for single crystals oriented for electrical conduction along the [010] direction. The expe riments were carried out at temperatures T=1173-1473 K and oxygen fuga cities f(O2)=10(-6.0)-10(3.8)Pa With the MnSiO3 activity controlled at unity. The value of tau, which ranges from 130 to 1463s, increases wi th decreasing temperature. At 1273 K, the value of tau in the high -f( O2) regime is a factor of similar to 1.8 smaller than that in the low -f(O2) regime. The point defect relaxation time was used to calculate the chemical diffusivity ((D) over bar). Values of (D) over tilde lie in the range 2.2x 10(-10) -2.5x10(-9) m(2)/s. For the high -f(O2) regi me, a semi-log plot of (D) over tilde vs 1/T yields a concave downward curve. Based on these results combined with those from Part I of this work for the point defect structure and electrical conductivity in Mn -olivine, it is proposed that the relaxation rate of electrical conduc tion is determined by the coupled diffusion of manganese vacancies V-M n'' and electron holes Mn-Mn, which rate-control the relaxation proces s at low and high temperatures, respectively. Deconvolution of the (D) over tilde-1/T curve into two straight-line segments yields values fo r the mobilities and the diffusivities for Mn vacancies and electron h oles. These results, combined with the measured electrical conductivit y data, were used to calculate the concentrations of Mn vacancies and electron holes. These results in conjunction with those published for other transition-metal silicate olivines reveal the following: The mob ility of electron holes in Mn-olivine is about two orders of magnitude smaller than that in Fe-olivine and is somewhat larger than that in ( Mg0.9Fe0.1)-olivine. The mobilities of metal vacancies in these three olivines are similar both in magnitude and in temperature dependence. The concentrations of the majority point defects are the highest in Fe -olivine crystals and the lowest in Co-olivine crystals, while those i n Mn-olivine and (Mg0.9Fe0.1)-olivine crystals lie in between, The dev iation from stoichiometry in the cation sublattice for a transition-me tal silicate olivine is about two orders of magnitude lower than that for the corresponding transition-metal oxide.