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Line broadening in chromites from Zimbabwe using high-resolution X-ray diffraction

Published online by Cambridge University Press:  10 January 2013

T. R. C. Fernandes  and
J. I. Langford
T. R. C. Fernandes
Affiliation:
Institute of Mining Research, University of Zimbabwe, PO Box 167, Mount Pleasant, Harare, Zimbabwe
J. I. Langford
Affiliation:
School of Physics & Astronomy, University of Birmingham, Birmingham B15 2TT, United Kingdom
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Abstract

Chromite grains in ores from the Great Dyke, Zimbabwe, exhibit varying degrees of shearing when viewed by optical microscopy. High resolution diffraction data revealed that line broadening from powder samples of sheared chromites is largely due to two or more spinel phases with slightly different cell parameters, the number of phases increasing with the degree of shearing. The dominant or “parent” phase, with parameters ranging from 8.3123(2) to 8.2676(2) Å, constitutes 57% to 76% of most samples. The cell parameters of secondary phases are generally less than that of the parent phase by δa/a0 in the range −1.3 to −4.0×10−3, the difference again tending to increase with shearing. Most reflections for the parent phase are relatively sharp whereas those for the secondary phases exhibit line broadening that could be analyzed in terms of crystallike (domain) size and rms strain. The crystallite diameters, assuming spherical particles, are relatively large for the parent phases [260(2)–100(5) nm], while those for the secondary phases range from 165(70) to 70(25) nm. The rms strain is not large for any sample and is negligible or small for the unsheared and weakly sheared material. The microstrain in secondary phases is greater than that in the parent phase and tends to increase with shearing, the maximum rms strain being 1.6(3)×10−3. Two new forms of chromite in highly sheared material are reported, one with a slight (0.33%) tetragonal distortion and the other with a cell parameter of 17.561(2) Å, greater than that of normal spinel chromites by a factor of about 3/√2, and a lowering of symmetry from Fd3¯m to Fm3¯m. Changes in chemical composition, indicated by a range in cell parameter, are attributed to tectonics that affected much of the Great Dyke. The strain is partitioned between failure by brittle deformation of the chromite grains and stress-induced cation migration leading to cells with slightly different composition.

Keywords

chromitespinelhigh resolution X-ray powder diffractioncell parametersline broadening
Type
Research Article
Information
Powder Diffraction , Volume 14 , Issue 2 , June 1999 , pp. 93 - 105
Copyright
Copyright © Cambridge University Press 1999

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References

Basu, A. R.and MacGregor, I. D. (1975). “Chromite spinels from ultramafic xenoliths,” Geochim. Cosmochim. Acta 39, 937–946.CrossRefGoogle Scholar
Bish, D. L.and Reynolds, R. C. (1989). “Sample preparation for X-ray diffraction,” Rev. Mineral. 20, 73–99.Google Scholar
Boultif, A.and Louër, D. (1991). “Indexing of powder diffraction patterns for low-symmetry lattices by the successive dichotomy method,” J. Appl. Crystallogr. 24, 987–993.CrossRefGoogle Scholar
Brodie, K. H. (1980). “Variations in mineral chemistry across a shear zone in phlogopite peridotite,” J. Struct. Geol. 2, 265–272.CrossRefGoogle Scholar
Cawthorn, R. G. (1994). “Layered igneous rocks: 25 years after Wagner and Brown,” S. Afr. J. Geol. 97, 442–454.Google Scholar
Cernik, R. J., Murray, P. K., Pattison, P., and Fitch, A. N. (1990). “A two-circle powder diffractometer for synchrotron radiation with a closed loop encoder feedback system,” J. Appl. Crystallogr. 23, 292–296.CrossRefGoogle Scholar
Chakraborty, K. L. (1965). “Geology and mineralogical characters of the Indian chromites,” Econ. Geol. 60, 1660–1668.CrossRefGoogle Scholar
de Keijser, T. H., Langford, J. I., Mittemeijer, E. J., and Vogels, A. B. P. (1982). “Use of the Voigt function in a single-line method for the analysis of X-ray diffraction line broadening,” J. Appl. Crystallogr. 15, 308–314.CrossRefGoogle Scholar
de Wolff, P. M. (1968). “A simplified criterion for the reliability of a powder pattern indexing,” J. Appl. Crystallogr. 1, 108–113.CrossRefGoogle Scholar
Fernandes, T. R. C., (1985). “Chromite mineralogy and metallurgical behaviour,” in Proceedings of MINTEK 50 International Conference on Mineral Science and Technology, edited by C. F. Houghton (Council for Mineral Technology, Randburg, South Africa), Vol. 2, pp. 913–922.Google Scholar
Fernandes, T. R. C., Echer, C. J., and Krishnan, K. M. (1997). “Contrasting cation distributions in spinels of similar composition,” in Microscopy and Microanalysis, edited by G. W. Bailey et al. (Springer, Cleveland, OH), Vol. 3, Suppl. 2, pp. 757–758.Google Scholar
Fernandes, T. R. C.and Langford, J. I. (1998). “High-resolution X-ray diffraction study of the chromite (Mg 0.60Fe 0.402+) (Al 0.39Cr 1.50Fe 0.093+)O 4,”Powder Diffr. 13, 96–99.CrossRefGoogle Scholar
Fernandes, T. R. C., Lee, W. E., and Mitchell, T. E. (1994). “Microstructural aspects of the reduction of Zimbabwe chromite to high carbon ferrochromium,” Trans. Inst. Min. Metall. (Sect. C: Mineral Process. Extr. Metall.) 103,C177–C187.Google Scholar
Gresens, R. L. (1966). “The effects of structurally produced pressure gradients on diffusion in rocks,” J. Geol. 74, 307–321.CrossRefGoogle Scholar
Halder, N. C., and Wagner, C. N. J. (1966). “Separation of particle size and lattice strain in integral breadth measurements,” Acta Crystallogr. 20, 312–313.CrossRefGoogle Scholar
Langford, J. I. (1971). “Powder pattern programs,” J. Appl. Crystallogr. 4, 259–260.CrossRefGoogle Scholar
Langford, J. I. (1978). “A rapid method for analyzing the breadths of diffraction and spectral lines using the Voigt function,” J. Appl. Crystallogr. 11, 10–14.CrossRefGoogle Scholar
Langford, J. I. (1992). “The use of the Voigt function in determining microstructural properties from diffraction data by means of pattern decomposition,” in Accuracy in Powder Diffraction II, edited by E. Prince and J. K. Stalick, NIST Special Publ. No. 846 (USCPO, Gaithersburg, MD), pp. 110–126.Google Scholar
Langford, J. I., Cernik, R. J., and Louër, D. (1991). “The breadth and shape of instrumental line profiles in high resolution powder diffraction,” J. Appl. Crystallogr. 24, 913–919.CrossRefGoogle Scholar
Langford, J. I.and Louër, D. (1996). “Powder diffraction,” Rev. Prog. Phys. 59, 131–234.CrossRefGoogle Scholar
Langford, J. I.and Wilson, A. J. C. (1978). “Scherrer after 60 years: a survey and some new results in the determination of crystallite size,” J. Appl. Crystallogr. 11, 102–113.CrossRefGoogle Scholar
Louër, D. and Audebrand, N. (1998). “Profile fitting and diffraction line-broadening analysis,” Adv. X-ray Anal. 41 (in press).Google Scholar
Louër, D.and Langford, J. I. (1988). “Peak shape and resolution in conventional diffractometry with monochromatic X-rays,” J. Appl. Crystallogr. 21, 430–437.CrossRefGoogle Scholar
Maier, W. D.and Teigler, B. (1995). “A facies model for the Western Bushveld Complex,” Econ. Geol. 90, 2343–2349.CrossRefGoogle Scholar
Nusinovici, J. and Rehfeldt-Oskierski, A. (1990). “DIFFRAC-AT search/match and profile fitting programs,” in Collected Abstracts, IUCr Powder Diffraction Satellite Meeting, Toulouse, France, 16–19 July 1990 (unpublished), pp. 315, 316.Google Scholar
Parrish, W.and Hart, M. (1985). “Synchrotron experimental methods for powder structure refinement,” Trans. Am. Crystallogr. Assoc. 21, 51–55.Google Scholar
Prendergast, M. D. and Wilson, A. H. (1989). “The Great Dyke of Zimbabwe. II. Mineralization and mineral deposits,” in Magmatic sulphides—The Zimbabwe Volume, edited by M. D. Prendergast and M. J. Jones (Institute of Mining and Metallurgy, London), pp. 21–42.Google Scholar
Rollinson, H. R. (1995). “The relationship between chromite chemistry and the tectonic setting of Archaean ultramafic rocks,” Sub-Saharan Economic Geology, edited by T. G. Blenkinsop and P. Tromp, Geol. Soc. Zimbabwe, Special Publ. No. 3 (Balkema, Rotterdam) 301 pp.Google Scholar
Slatter, D. de L. (1978). “The physical properties of Rhodesian chromium ores and their significance in ferrochromium production,” Inst. Min. Res. Report No. 28 (Univ. Zimbabwe, Salisbury), 75 pp.Google Scholar
Slatter, D. de L. (1980). “The composition of Zimbabwean chromium ores and the derivation of physico-chemical ratings for smelting the ores to high-carbon ferrochromium,” Inst. Min. Res. Rep. C193 (Univ. Zimbabwe, Harare), 79 pp.Google Scholar
Slatter, D. de L. (1981). “The effects of chemical composition upon the reducibility of Zimbabwean chromium ores,” Inst. Min. Res. Rep. No. 43 (Univ. Zimbabwe, Harare), 39 pp.Google Scholar
Smith, G. S.and Snyder, R. L. (1979). “F N: A criterion for rating powder diffraction patterns and evaluating the reliability of powder-pattern indexing,” J. Appl. Crystallogr. 12, 60–65.CrossRefGoogle Scholar
Stowe, C. W. (1995). “Compositions and tectonic settings of chromite deposits through time—a reply,” Econ. Geol. 90, 2092–2094.CrossRefGoogle Scholar
Wilson, A. H. (1982). “The geology of the Great ‘Dyke,’ Zimbabwe: The ultramafic rocks,” J. Petrol. 23, 240–292.CrossRefGoogle Scholar
Wilson, A. J. C. (1963). Mathematical Theory of X-ray Powder Diffractometry (Philips, Eindhoven).Google Scholar