Defect Mechanisms for the Solid State Reduction of Olivine
- Robert N. Schock
Published Online: 18 MAR 2013
Copyright 1985 by the American Geophysical Union.
Point Defects in Minerals
How to Cite
Boland, J. N. and Duba, A. (1985) Defect Mechanisms for the Solid State Reduction of Olivine, in Point Defects in Minerals (ed R. N. Schock), American Geophysical Union, Washington, D. C.. doi: 10.1029/GM031p0211
- Published Online: 18 MAR 2013
- Published Print: 1 JAN 1985
Print ISBN: 9780875900568
Online ISBN: 9781118664070
- Mineralogical chemistry—Congresses;
The development of the microstructures in experimentally reduced single crystals of San Carlos olivine (Fo92) has been studied by optical and electron microscopy. At 1400°C in a reducing atmosphere of nearly pure CO, reduction proceeds by propagation of a reaction zone from the surface to the center of the crystals. This zone consists of metallic precipitates of Fe-Ni in a matrix of a more forsterite-rich olivine. No silica-rich phase has been detected. The reduction process can be divided into a number of stages. There is an incubational pre-precipitation period followed by two micro-structurally-distinct stages. In the first stage, the reaction zone is clearly delineated by its optically dense rim. Based on a mass transfer model, the kinetics of this stage is controlled by oxygen vacancy diffusivity. The next stage, with its increased reduction rate, has a more diffuse reaction-zone front accompanied by coarsening of the metallic precipitates. Dislocations, whether free or bound to subboundaries, act as high diffusivity pathways for oxygen vacancies. The ratio of dislocation pipe diffusion to lattice diffusion (Dp/Dl) is 2.5 x 105 at 1400°C.
Additionally, dislocations are preferred nucleation sites for the metallic precipitates. Nickel is selectively reduced ahead of the advancing reaction zone, producing metallic precipitates with up to 80 weight percent nickel. This reaction, which is unrelated to pre-existing dislocations, is at least an order of magnitude faster than the oxygen vacancy mechanism and indicates that electronic defects are responsible. Pores are observed close to the gas-mineral interface. Their formation represents a precipitation reaction involving vacancy clustering in olivine that has become supersaturated with vacancies.