Fe2+-Mg order-disorder in orthopyroxene

Equilibrium fractionation between the octahedral sites and thermodynamic analysis

M. Stimpfl, Jibamitra Ganguly, G. Molin

Research output: Contribution to journalArticle

51 Citations (Scopus)

Abstract

The equilibrium intracrystalline distribution coefficient, k(D)(*), of Fe(*) (i.e. Fe2+ + Mn) and Mg between the M1 and M2 sites of three natural nearly binary Fe2+-Mg orthopyroxene crystals (Fs14, Fs15 and Fs49) were determined by annealing experiments at several temperatures between 550 and 1000 °C and single crystal X-ray structure refinements. In addition, the X-ray data of an orthopyroxene crystal (Fs23), which were collected earlier by Molin et al. (1991) between 700 and 1000 °C, were re-refined. The data were processed through two different refinement programs (SHELXL93 and RFINE90) using both unit and individual weights and also both ionic and atomic scattering factors. The calculated site occupancies were found to agree within their estimated standard errors. However, the use of ionic scattering factors led to significantly better goodness of fit and agreement index, and smaller standard deviations of the site occupancies than those obtained from the use of atomic scattering factors. Furthermore, the weighted refinements yielded significantly smaller standard deviations of the site occupancies than the unweighted refinements even when the same set of reflections was used in the two procedures. The site occupancy data from this study were combined with selected published data to develop expressions of k(D)(*) as a function of temperature and composition. Calculation of the excess configurational entropy, ΔS(XS), suggests that orthopyroxene should be treated as a two parameter symmetric solution instead of as a 'simple mixture'. The calculated ΔS(XS) values and the excess Gibbs free energy of mixing suggested by available cation exchange data lead to a slightly negative enthalpy of mixing in the orthopyroxene solid solution.

Original languageEnglish (US)
Pages (from-to)297-309
Number of pages13
JournalContributions to Mineralogy and Petrology
Volume136
Issue number4
DOIs
StatePublished - Sep 1999

Fingerprint

Order disorder transitions
Lattice vibrations
Fractionation
orthopyroxene
crystal structure
fractionation
thermodynamics
Thermodynamics
disorders
X rays
Crystals
standard deviation
Electronic data interchange
Gibbs free energy
scattering
crystals
Cations
Enthalpy
Solid solutions
goodness of fit

ASJC Scopus subject areas

  • Geochemistry and Petrology
  • Geophysics

Cite this

@article{a528f5ae9a2f4e82ab9c07bd41b6a7eb,
title = "Fe2+-Mg order-disorder in orthopyroxene: Equilibrium fractionation between the octahedral sites and thermodynamic analysis",
abstract = "The equilibrium intracrystalline distribution coefficient, k(D)(*), of Fe(*) (i.e. Fe2+ + Mn) and Mg between the M1 and M2 sites of three natural nearly binary Fe2+-Mg orthopyroxene crystals (Fs14, Fs15 and Fs49) were determined by annealing experiments at several temperatures between 550 and 1000 °C and single crystal X-ray structure refinements. In addition, the X-ray data of an orthopyroxene crystal (Fs23), which were collected earlier by Molin et al. (1991) between 700 and 1000 °C, were re-refined. The data were processed through two different refinement programs (SHELXL93 and RFINE90) using both unit and individual weights and also both ionic and atomic scattering factors. The calculated site occupancies were found to agree within their estimated standard errors. However, the use of ionic scattering factors led to significantly better goodness of fit and agreement index, and smaller standard deviations of the site occupancies than those obtained from the use of atomic scattering factors. Furthermore, the weighted refinements yielded significantly smaller standard deviations of the site occupancies than the unweighted refinements even when the same set of reflections was used in the two procedures. The site occupancy data from this study were combined with selected published data to develop expressions of k(D)(*) as a function of temperature and composition. Calculation of the excess configurational entropy, ΔS(XS), suggests that orthopyroxene should be treated as a two parameter symmetric solution instead of as a 'simple mixture'. The calculated ΔS(XS) values and the excess Gibbs free energy of mixing suggested by available cation exchange data lead to a slightly negative enthalpy of mixing in the orthopyroxene solid solution.",
author = "M. Stimpfl and Jibamitra Ganguly and G. Molin",
year = "1999",
month = "9",
doi = "10.1007/s004100050540",
language = "English (US)",
volume = "136",
pages = "297--309",
journal = "Contributions of Mineralogy and Petrology",
issn = "0010-7999",
publisher = "Springer Verlag",
number = "4",

}

TY - JOUR

T1 - Fe2+-Mg order-disorder in orthopyroxene

T2 - Equilibrium fractionation between the octahedral sites and thermodynamic analysis

AU - Stimpfl, M.

AU - Ganguly, Jibamitra

AU - Molin, G.

PY - 1999/9

Y1 - 1999/9

N2 - The equilibrium intracrystalline distribution coefficient, k(D)(*), of Fe(*) (i.e. Fe2+ + Mn) and Mg between the M1 and M2 sites of three natural nearly binary Fe2+-Mg orthopyroxene crystals (Fs14, Fs15 and Fs49) were determined by annealing experiments at several temperatures between 550 and 1000 °C and single crystal X-ray structure refinements. In addition, the X-ray data of an orthopyroxene crystal (Fs23), which were collected earlier by Molin et al. (1991) between 700 and 1000 °C, were re-refined. The data were processed through two different refinement programs (SHELXL93 and RFINE90) using both unit and individual weights and also both ionic and atomic scattering factors. The calculated site occupancies were found to agree within their estimated standard errors. However, the use of ionic scattering factors led to significantly better goodness of fit and agreement index, and smaller standard deviations of the site occupancies than those obtained from the use of atomic scattering factors. Furthermore, the weighted refinements yielded significantly smaller standard deviations of the site occupancies than the unweighted refinements even when the same set of reflections was used in the two procedures. The site occupancy data from this study were combined with selected published data to develop expressions of k(D)(*) as a function of temperature and composition. Calculation of the excess configurational entropy, ΔS(XS), suggests that orthopyroxene should be treated as a two parameter symmetric solution instead of as a 'simple mixture'. The calculated ΔS(XS) values and the excess Gibbs free energy of mixing suggested by available cation exchange data lead to a slightly negative enthalpy of mixing in the orthopyroxene solid solution.

AB - The equilibrium intracrystalline distribution coefficient, k(D)(*), of Fe(*) (i.e. Fe2+ + Mn) and Mg between the M1 and M2 sites of three natural nearly binary Fe2+-Mg orthopyroxene crystals (Fs14, Fs15 and Fs49) were determined by annealing experiments at several temperatures between 550 and 1000 °C and single crystal X-ray structure refinements. In addition, the X-ray data of an orthopyroxene crystal (Fs23), which were collected earlier by Molin et al. (1991) between 700 and 1000 °C, were re-refined. The data were processed through two different refinement programs (SHELXL93 and RFINE90) using both unit and individual weights and also both ionic and atomic scattering factors. The calculated site occupancies were found to agree within their estimated standard errors. However, the use of ionic scattering factors led to significantly better goodness of fit and agreement index, and smaller standard deviations of the site occupancies than those obtained from the use of atomic scattering factors. Furthermore, the weighted refinements yielded significantly smaller standard deviations of the site occupancies than the unweighted refinements even when the same set of reflections was used in the two procedures. The site occupancy data from this study were combined with selected published data to develop expressions of k(D)(*) as a function of temperature and composition. Calculation of the excess configurational entropy, ΔS(XS), suggests that orthopyroxene should be treated as a two parameter symmetric solution instead of as a 'simple mixture'. The calculated ΔS(XS) values and the excess Gibbs free energy of mixing suggested by available cation exchange data lead to a slightly negative enthalpy of mixing in the orthopyroxene solid solution.

UR - http://www.scopus.com/inward/record.url?scp=0033394485&partnerID=8YFLogxK

UR - http://www.scopus.com/inward/citedby.url?scp=0033394485&partnerID=8YFLogxK

U2 - 10.1007/s004100050540

DO - 10.1007/s004100050540

M3 - Article

VL - 136

SP - 297

EP - 309

JO - Contributions of Mineralogy and Petrology

JF - Contributions of Mineralogy and Petrology

SN - 0010-7999

IS - 4

ER -