Dear Maria,
indeed the relative intensities of the first basal reflection (you
expressed this as RIR) of smectites/vermiculites are quite high. There
are several reasons:
1) because of high d spacings / low diffraction angle peaks always cause
high L and P factors
2) because the layered structure with in-plane cations in the interlayer
gives really high structure factors for 001
3) because the "ideal" structures of smectite presented in some papers
do not take into account the positional disorder of the water molecules
in the interlayer and therefore unrealistic intensities are calculated
from such "ideal" structure models.
Moreover, you should take into account that the intensity of these first
basal reflection is highly sensitive to variations in site (cation)
occupation of octahedra and interlayer positions, the hydration stage,
the uncertainties of z coordinates, and of course to experimental
effects like preferred orientation, beam overflow, sample roughness...
The peak profiles are often complicated by the slope of LP and by maybe
inhomogeneous hydration stages...
Thus, any quantification of smectites/vermiculites based on the integral
intensities of their first basal reflections is in general a dangerous
undertaken (even if some frequently used techniques in soil and sediment
research are based on this). See also Reynolds, R.C. (1989) in Pevear &
Mumpton (1989) and many other textbooks on clay mineral analysis.
Best regards
Reinhard
Am 02/04/2014 11:47, schrieb Leonid Solovyov:
The Rietveld-QPA doesn’t require RIR, as the peak intensities of phases are
calculated from the structure models. What do you mean under “doing Rietveld”
and ”structural files” in your case?
*******************************************************
Leonid A. Solovyov
Institute of Chemistry and Chemical Technology
660036, Akademgorodok 50/24, Krasnoyarsk, Russia
http://sites.google.com/site/solovyovleonid
*******************************************************
----- Original Message -----
From: "Chrysochoou, Maria" <mchry...@engr.uconn.edu>
To: "rietveld_l@ill.fr" <rietveld_l@ill.fr>
Cc:
Sent: Wednesday, April 2, 2014 4:21 PM
Subject: Reference Intensity Ratio of clay minerals
Hello,
I am wondering why the RIR of clay minerals such as vermiculite and
montmorillonite is so high in the structural files (e.g. around 22-28). I am
not a crystallographer, but I have noticed that high RIRs are usually
associated with dense structures and heavy elements and neither is the case
with clays. And for example, kaolinite and illite have much lower RIRs, 1 and
0.5, even though the structures are not that much different. This results in
very very low concentrations when doing Rietveld, even though there is
substantial intensity associated with vermiculite and it does not make
intuitive sense to me.
I would appreciate any insight into this question.
Thank you,
Maria Chrysochoou
Associate Professor
Department of Civil and Environmental Engineering
University of Connecticut
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