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Full width at half maximum of low-angle basal phyllosilicate X-ray diffraction reflections: fitted peaks vs. diffraction traces

Published online by Cambridge University Press:  27 July 2018

Hanan J. Kisch
Hanan J. Kisch*
Affiliation:
Department of Geological and Environmental Sciences, Ben-Gurion University of the Negev, Beer-Sheva, Israel
*
*E-mail: kisch@bgu.ac.il
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Abstract

Bernard Kübler measured illite ‘crystallinity’, the half-height width or full width at half maximum (FWHM) of the X-ray diffraction line of illite/mica at 10 Å, directly on the diffraction traces; this procedure has since been followed by the vast majority of workers. However, some workers have recently measured the FWHM of the fitted Pearson VII function rather than on the diffraction traces. The FWHM of this function for low-angle phyllosilicate diffraction peaks (FWHM*PVII) is almost consistently ‘broader’ than those measured directly on the diffraction trace profiles (FWHMtrace) by up to 0.08°Δ2θ for the broader peaks. The Pearson VII function shows gentle curvature (‘smoothing’) at its tops and fast fading of the tails relative to virtually all 10 Å diffraction traces. The broad FWHM*PVII results from the consequent lowering/’under-fitting’ of the peak tops and the upper tails and compensatory broadening/’over-fitting’ of the intermediate peak flanks. FWHM*PVII ‘contraction’ with respect to FWHMtrace and enhancement of the peak maximum is found on traces of muscovite strips. The fitting reliabilities of the Cauchy function are almost invariably better than those of the Pearson VII function. Their FWHM*Cauchy values are narrower for both the illite/mica 10 Å and the chlorite 7 Å reflections; although they still differ somewhat from the FWHMtrace, they are much closer, usually within 0.02°Δ2θ. This markedly lesser broadening of FWHM* of the Cauchy of the Pearson VII function is the result of its stronger top curvature and notably faster tail fading (less ‘smoothening’). For higher-angle mica peaks, the FWHM* values of the Pearson VII and Cauchy functions converge, usually differing only by 0.01–0.03°Δ2θ for the 5 Å peak, and even less for the 3.3 Å peak. It is therefore strongly recommended that FWHM values of the illite/mica 10 Å reflections be measured on the diffraction traces rather than on fitted functions. Where peak fitting is unavoidable (e.g. in order to separate the contributions of adjoining, partly resolved or unresolved reflections on broadened 10 Å reflections), Cauchy rather than Pearson VII functions should be used.

Keywords

FWHMillite/muscovite crystallinity10 Å illite/muscovite peak7 Å chlorite peakpeak fittingPearson VII functionCauchy function
Type
Article
Information
Clay Minerals , Volume 53 , Issue 3 , September 2018 , pp. 325 - 338
Copyright
Copyright © Mineralogical Society of Great Britain and Ireland 2018 

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Footnotes

This paper was presented during the session: ‘GG01 – Clay mineral reaction progress in very low-grade temperature petrologic studies' of the International Clay Conference 2017.

Guest Associate Editor: Blanca Bauluz

References

REFERENCES

Árkai, P. (1991) Chlorite crystallinity: an empirical approach and correlation with illite crystallinity, coal rank and mineral facies as exemplified by Palaeozoic and Mesozoic rocks of northeast Hungary. Journal of Metamorphic Geology, 9, 723–734.Google Scholar
Battaglia, S., Leoni, L. & Sartori, F. (2004) The Kübler index in late diagenetic to low-grade metamorphic pelites: a critical comparison of data from 10 Å and 5 Å peaks. Clays and Clay Minerals, 52, 85–105.Google Scholar
Biscaye, P.E. (1964) Distinction between kaolinite and chlorite in recent sediments by X-ray diffraction. American Mineralogist, 49, 1281–1289.Google Scholar
Ferreiro-Mählmann, R. & Frey, M. (2012) Standardization, calibration and correlation of the Kübler-index and the vitrinite/bituminite reflectance: an inter-laboratory and field related study. Swiss Journal of Geosciences, 105, 163–170.Google Scholar
Himeda, A. (2012) Size-strain analysis using the fundamental parameter (FP) method. The Rigaku Journal, 28(2), 11–14.Google Scholar
Hosokawa, M., Naito, M., Nogi, K. & Yokoyama, T., editors (2012) Nanoparticle Technology Handbook, second edition. Elsevier, Amsterdam, The Netherlands.Google Scholar
Kisch, H.J. (1980) Incipient metamorphism of Cambro-Silurian clastic rocks from the Jämtland Supergroup, central Scandinavian Caledonides, western Sweden: illite crystallinity and ‘vitrinite’ reflectance. Journal of the Geological Society London, 137, 271–288.Google Scholar
Kisch, H.J. (1990) Calibration of the anchizone: a critical comparison of illite ‘crystallinity’ scales used for definition. Journal of Metamorphic Geology, 8, 312–346.Google Scholar
Kisch, H.J. (1991) Illite crystallinity: recommendations on sample preparation, X-ray diffraction settings, and interlaboratory samples. Journal of Metamorphic Geology, 9, 665–670.Google Scholar
Kisch, H.J., Árkai, P. & Brime, C. (2004) On the calibration of the illite Kübler index (illite ‘crystallinity’). Schweizerische Mineralogische und Petrographische Mitteilungen, 84, 323–331.Google Scholar
Kisch, H.J. & Nijman, W. (2010) Metamorphic grade and gradient from white K-micas of Na-mica bearing sedimentary rocks in the Mosquito Creek Basin, East Pilbara Craton, Western Australia. Precambrian Research, 176, 11–26.Google Scholar
Krumm, S. (1994) WINFIT1.0 – a computer program for X-ray diffraction line profile analysis. XIIIth Conference on Clay Mineralogy and Petrology, Praha (1994), Acta Universitatis Carolinae Geologica, 38, 253–261.Google Scholar
Kübler, B. (1967) La cristallinité de l'illite et les zones tout a fait supérieures du métamorphisme. Pp. 105–120 in: Étages tectoniques, Colloque de Neuchâtel 18–21 Avril 1966. A la Baconnière, Neuchâtel, Switzerland.Google Scholar
Warr, L.N. & Ferreiro Mählmann, R. (2015) Recommendations for Kübler index standardization. Clay Minerals, 50, 282–285.Google Scholar
Warr, L.N. & Rice, A.H.N. (1994) Interlaboratory standardization and calibration of clay mineral crystallinity and crystallite size data. Journal of Metamorphic Geology, 12, 141–152.Google Scholar
Weaver, C.E. (1961) Clay minerals of the Ouachita structural belt and adjacent foreland. Pp. 147–162 in: The Ouachita System (Flawn, P.T. et al., editors). University of Texas Press, Austin, Texas, USA.Google Scholar
Woldemichael, S. (1998) Low Grade Metamorphism and Hydrothermal Alteration in the Basement Greywacke Terranes of the Northern and Central North Island, New Zealand: Reconnaissance Study. PhD thesis, University of Auckland, New Zealand.Google Scholar