Illite crystallinity is expressed as full width half maximum of illite peak at 10 Angstrom measured on (001) plane. it is proposed and quantified by by kubler and webber and expressed in ∆ °2θ units. I am confused about delta in this expression as my xrd IC result shows FWHM values in °2θ. Can anyone explain please ?
A width at half height is simply a difference (delta) in 2 theta values in the Bragg plot. The width or broadening of the line can be used to obtain other information (crystallite size by the Scherrer equation, for example).
To be extra clear:
- identify maximum Illite peak lenght (or intensity);
- trace horizontal line at half peak lenght;
- trace vertical lines from each intersection between half peak horizontal line and curve;
- annotate 2θ diffraction angle value at each vertical line;
- subtract smaller 2θ diffraction angle value from larger 2θ diffraction angle value (Delta 2θ) to obtain ∆ °2θ.
See example attached.
The broader and shorter the peak, the less crystalline the Illite sample is. The narrower and stronger the peak, the more crystalline the illite sample is.
In other words, the shorter the horizontal line at half peak (expressed in ∆ °2θ units), the more crystalline the sample.
The (001) plane, mentioned in the standard, is the basal plane.
Additionally, care must be taken in noting which is the source of the X-Rays. The standard, shown at the bottom of the figure attached to my previous post, is Cu Kalpha (a Copper source). If another source is used, e,g, a Fe or Co source, the diffraction angles are slightly different, and the half-peak angular width also slightly different. A Fe or Co source is useful in case of Fe-, Mn-, or Cr-rich clay minerals (e.g. a member of the Chlorite Group).
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