Jump to ContentJump to Main Navigation
Show Summary Details
More options …

American Mineralogist

Journal of Earth and Planetary Materials

Ed. by Baker, Don / Xu, Hongwu / Swainson, Ian


IMPACT FACTOR 2018: 2.631

CiteScore 2018: 2.55

SCImago Journal Rank (SJR) 2018: 1.355
Source Normalized Impact per Paper (SNIP) 2018: 1.103

Online
ISSN
1945-3027
See all formats and pricing
More options …
Volume 86, Issue 10

Issues

Composition and temperature dependence of cation ordering in Ni-Mg olivine solid solutions: a time-of-flight neutron powder diffraction and EXAFS study

C. Michael B. Henderson
  • Department of Earth Sciences, University of Manchester, Manchester M13 9PL, U.K. and Daresbury Laboratory, CLRC, Warrington WA4 4AD, U.K.
  • Email
  • Other articles by this author:
  • De Gruyter OnlineGoogle Scholar
/ Simon A.T. Redfern
  • Department of Earth Sciences, Downing Street, University of Cambridge, Cambridge CB2 3EQ, U.K. 3ISIS, Rutherford Appleton Laboratory, CLRC, Chilton, Oxfordshire OX11 0QX, U.K.
  • Other articles by this author:
  • De Gruyter OnlineGoogle Scholar
/ Ronald I. Smith / Kevin S. Knight / John M. Charnock
  • Department of Earth Sciences, University of Manchester, Manchester M13 9PL, U.K. and Daresbury Laboratory, CLRC, Warrington WA4 4AD, U.K.
  • Other articles by this author:
  • De Gruyter OnlineGoogle Scholar
Published Online: 2015-03-26 | DOI: https://doi.org/10.2138/am-2001-1007

Abstract

The non-convergent ordering of Mg and Ni over the M1 and M2 sites of synthetic olivines has been studied using “time of night” 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 (Mg1-xNix)2SiO4. where X= 0.15, 0.2, 0.25,0.3, 0.5, and 0.8 atoms Ni (XNi; i.e.. mole fraction of Ni-olivine end-member). Ni orders into M1 with KD = (Ni/Mg in M1)/ (Ni/Mg in M2) reaching a maximum of 9.5 at a composition of Mg1.0Ni0.4SiO4. The temperature dependence of order/disorder at up to 1100 °C was determined for two samples (XNi = 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 Mg1.6Ni0.4SiO4 and Mg1.0Ni1.0SiO4 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 X-edge EXAFS data show that samples with XNi = 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 A. 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 Mg1.6Ni0.4SiO4 at room temperature. Comparison of our data with earlier studies at high P and T on Mg1.0Ni1.0SiO4 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 ΔVdisorder of 0.039 J/bar.

About the article

Received: 2000-09-27

Accepted: 2001-06-11

Published Online: 2015-03-26

Published in Print: 2001-10-01


Citation Information: American Mineralogist, Volume 86, Issue 10, Pages 1170–1187, ISSN (Online) 1945-3027, ISSN (Print) 0003-004X, DOI: https://doi.org/10.2138/am-2001-1007.

Export Citation

© 2015 by Walter de Gruyter Berlin/Boston.

Comments (0)

Please log in or register to comment.
Log in