Defect Mechanisms for the Solid State Reduction of Olivine

  1. Robert N. Schock
  1. J. N. Boland1 and
  2. A. Duba2

Published Online: 18 MAR 2013

DOI: 10.1029/GM031p0211

Point Defects in Minerals

Point Defects in Minerals

How to Cite

Boland, J. N. and Duba, A. (2013) 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

Author Information

  1. 1

    Institute of Earth Sciences, State University of Utrecht, 3508 TA Utrecht, the Netherlands

  2. 2

    Lawrence Livermore National Laboratory, University of California

Publication History

  1. Published Online: 18 MAR 2013
  2. Published Print: 1 JAN 1985

ISBN Information

Print ISBN: 9780875900568

Online ISBN: 9781118664070

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Keywords:

  • Mineralogical chemistry—Congresses;
  • Crystals—Defects—Congresses

Summary

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.