Clay people

I think the single crystal analysis of clays is interesting. I have not read
the literature but in determining the intensities is overlap of the dots
considered as I would have expected the dots to be very much smeared (5 to
10 degrees 2Th in my experience). If yes the fitting in two dimension would
be better.

Thus the question to ask is how accurate can QPA be for clays if the
intensities can be accurately obtained; is this an open question or is the
book closed on this. If as Reinhard Kleeberg mentioned that some directions
are unaffected then it would seem plausible that something can be gained
especially if one of "those models" work. 

Also, TOPAS simply offers a means of describing the peak shapes using a hkl
dependent spherical harmonics. From my experiences it seems to work. Like
Lubomir Smrcok remarked getting the intensities is critical. 

Another important point, again as Lubomir Smrcok mentioned, is preferred
orientation. If there's very strong preferred orientation then the peak
shapes will be affected due to axial divergence as well; it best to remove
preferred orientation.

Cheers
Alan



-----Original Message-----
From: Reinhard Kleeberg [mailto:[EMAIL PROTECTED] 
Sent: Wednesday, 21 March 2007 7:48 PM
To: rietveld_l@ill.fr
Subject: Re: Problems using TOPAS R (Rietveld refinement)

Dear colleagues,
sorry, my mail should go directly to Leandro, but I used this damned reply
buttom...
My answer was related to Leandro's questions regarding these line broadening
models. I realised that Leandro is going on to apply a Rietveld program for
phase quantification, including kaolinite and later other clay minerals. I
only tried to express my personal experience, that any inadequate profile
description of a clay mineral will surely cause wrong QPA results, nothing
else. This is a practical issue, and it is only partially related to
structure refinement. Lubomir Smrcok is definitely right that other things
like PO are frequently biasing a QPA result, but for the most of these
problems working solutions do exist. 
But I disagree that anisotropic line broadening is a "noble problem". In
clay mineral mixtures, it is essentially to fit the profiles of the single
phases as best as one can, to get any reasonable "QPA" result in a +-5 wt%
interval. On the other hand, for the QPA purpose it is not so much important
to find any sophisticated description of the microstructure of a phase. But
the "model" should be flexible enough to cover the variablility of the
profiles in a given system, and, on the other hand, stabil enough (not
over-parametrised) to work in mixtures. 
The balancing out of these two issues could be the matter of an endless
debate. And here I agree again, a better, more stable minimisation algorithm
can help to keep a maximum of flexibility of the models.
Best regards
Reinhard Kleeberg

Lubomir Smrcok schrieb:

>Gentlemen,
>I've been listening for a week or so and I am really wondering what do 
>you want to get ... Actually you are setting up a "refinement", whose 
>results will be, at least, inaccurate. I am always surprised by 
>attempts to refine crystal structure of a disordered sheet silicate 
>from powders, especially when it is known it hardly works with single 
>crystal data. Yes, there are several models of disorder, but who has ever
proved they are really good ?
>I do not mean here a graphical comparison of powder patterns with a 
>calculated trace, but a comparison of structure factors or integrated 
>intensities. (Which ones are to be selected is well described in the 
>works of my colleague, S.Durovic and his co-workers.) As far as powders 
>are concerned, all sheet silicates "suffer" from prefered orientation 
>along 001. Until you have a pattern taken in a capillary or in 
>transmission mode, this effect will be dominating and you can forget 
>such noble problems like anisotropic broadening.
>
>Last but not least : quantitative phase analysis by "Rietveld" is (when 
>only scale factors are "on") nothing else but multiple linear 
>regression. There is a huge volume of literature on the topic, 
>especially which variables must, which should and which could be a part of
your model.
>I really wonder why the authors of program do not add one option called 
>"QUAN", which could, upon convergence of highly sophisticated 
>non-linear L-S, fix all parameters but scale factors and run standard 
>tests or factor analysis. One more diagonalization is not very time 
>consuming, is it ? To avoid numerical problems, I'd use SVD.
>This idea is free and if it helps people reporting 0.1% MgO (SiO2) in a 
>mixture  of 10 phases to think a little of the numbers they are 
>getting, I would only be happy :-) Lubo
>
>P.S. Hereby I declare I have never used Topas and I am thus not 
>familiar with all its advantages or disadvantages compared to other codes.
>
>
>On Wed, 21 Mar 2007, Reinhard Kleeberg wrote:
>
>  
>
>>Dear Leandro Bravo,
>>some comments below:
>>
>>Leandro Bravo schrieb:
>>
>>    
>>
>>>In the refinement of chlorite minerals with well defined disordering 
>>>(layers shifting by exactly b/3 along the three pseudohexagonal Y 
>>>axis), you separate the peaks into k = 3.n (relative sharp, less 
>>>intensive peak) and k  3.n (broadened or disappeared 
>>>reflections). How did you determined this value k = 3.n and n = 
>>>0,1,2,3..., right?
>>>
>>>      
>>>
>>The occurence of stacking faults along the pseudohexagonal Y axes 
>>causes broadening of all reflections hkl with k unequal 3n (for 
>>example 110, 020, 111..) whereas the reflections with k equal 3n 
>>remain unaffected (001, 131, 060, 331...). This is clear from 
>>geometric conditions, and can be seen in single crystal XRD 
>>(oscillation photographs, Weissenberg
>>photographs) as well in selected area electron diffraction patterns. 
>>The fact is known for a long time, and published and discussed in 
>>standard textbooks, for example *Brindley, G.W., Brown, G.:  Crystal 
>>Structures of Clay Minerals and their X-ray Identification. 
>>Mineralogical Society, London, 1980.*
>>
>>    
>>
>>>First, the chlorite refinement.
>>>
>>>In the first refinement of chlorite you used no disordering models 
>>>and used ´´cell parameters`` and ´´occupation of octahedra``. So you 
>>>refined the lattice parameters and the occupancy of all atoms?
>>>      
>>>
>>Yes, the lattice parameters.
>>Only the occupation/substitution of atoms with significant difference 
>>in scattering power can be refined in powder diffraction. In case of 
>>chlorites, the substitution Fe-Mg at the 4 octahedral positions can be 
>>refined.
>>
>>    
>>
>>>In the second refinement, you use na anisotropic line broadening ´´in 
>>>the traditional way``. So you used a simple ellipsoidal model and/or 
>>>spherical harmonics?
>>>
>>>      
>>>
>>Simple ellipsoidal model, assuming very thiny platy crystals. But it 
>>was clear that this model must fail, see above the known fact of 
>>disorder in layer stacking. And from microscopy it is clear that the 
>>"crystals" are much too large to produce significant line broadening from
size effects.
>>You can see this for a lot of clay minerals: If the "ellipsoidal 
>>crystallite shape" model would be ok, the 00l reflections would have 
>>the broadest lines, and the 110, 020 and so on should be the sharpest
ones.
>>But this is not true in practice, mostly the hkl are terribly 
>>broadenend and smeared, but the 00l are still sharp.
>>
>>    
>>
>>>The last refinement, describing a real structure. You used for the 
>>>reflections k  3.n (broadened peaks) a ´´rod-like intensity 
>>>distribution``, with the rod being projected by the cosine of the 
>>>direction on the diffractogram. You used also the lenghts of the rods 
>>>as a parameter, so as the dimension of the rods for 0k0 with k 
>>> 3.n. I would like to know how did you ´´project`` these rods 
>>>and use them in the refinement.
>>>
>>>For the k = 3.n reflections, you used an anisotropic broadening model 
>>>(aniso crystallyte size) and and isotropic broadening model 
>>>(microstrain broadening). But you said that crystallite size is an 
>>>isotropic line broadening in my kaolinite refinement and I should not 
>>>use it. So I use or not the cry size?
>>>
>>>      
>>>
>>Yes, we used an "additional" ellipsoidal broadening in order to 
>>describe any potential "thinning" of the crystals. But this broadening 
>>model was not significant because the broadening was dominated by the 
>>stacking faults. A "microstrain" makes sense because of natural 
>>chlorits are sometimes zoned in their chemical composition and a 
>>distribution of the lattice constants may occur.
>>In one of your mails you mentioned "crysize gave reasonable numbers 
>>with low error", and from that I assumed you looked only on the errors 
>>of the isotropic crysize as defined in Topas. You must know what model 
>>you did apply. But it is clear that any "crysize" model is inadequate 
>>to describe the line broadening of kaolinite.
>>
>>    
>>
>>>Now the kaolinite refinement.
>>>
>>>In the first refinement was used fixed atomic positions and a 
>>>conventional anisotropic peak broadening. This conventional 
>>>anisotropic peak broadening would be the simple ellipsoidal model 
>>>and/or spherical harmonics?!
>>>      
>>>
>>Only ellipsoidal model, assuming a platy crystal shape, see above. 
>>Only for comparision.
>>
>>    
>>
>>>After that you use the introduced model of disorfering. Is this model 
>>>the same of the chlorite (rods for k  3.n and microstrain 
>>>broadening and anisotropic crystallite size?
>>>
>>>      
>>>
>>Not exactly the same like in chlorite, because the disorder in 
>>kaolinite is much more complicated like in chlorites. See also the 
>>textbook cited above, and extensive works of Plancon and Tchoubar. 
>>Thus, most of the natural kaolinites show stacking faults along b/3 as 
>>well as along a, and additional random faults. Thus, more broadening 
>>parameters had to be defined, and this is not completely perfect until 
>>now. See the presentation I sent you last week.
>>
>>Best regards
>>
>>Reinhard Kleeberg
>>
>>    
>>
>
>  
>




Reply via email to