I'm not really arguing with you here as I prefer capillary data myself as it 
gives better data in many circumstances - however it can sometimes lead you up 
the garden path (another old saying!).  I suppose what it boils down to is that 
needles are a pain as they can orientate whatever you do them (reflection or 
transmission).  
 
The texture versus preferred orientation difference has some signficant blurry 
edges from a practical point of view.  
 
Anyway - I'm on holiday so I'm going to put my brain to sleep and go and do 
some gardening!
 
Pam
________________________________

From: Martin [mailto:[EMAIL PROTECTED]
Sent: Fri 16/05/2008 11:47 AM
To: Whitfield, Pamela; rietveld_l@ill.fr
Subject: RE: Preferred orientation?


Hi, happy Rietvelders

Some elements of confusion creeping in here. I think you said, Pam that 
transmission wont help much if it's wollastonite and what I'm saying is that it 
does and gave a pointer to a study that shows it. Indeed I don't claim 
transmission gets rid of PO either, but it does reduce it hugely which, if one 
reads back, is my claim here. The PO function in this case is merely to 
illustrate the point: 0.9 and a refined model vs. 1.6 and a bad refinement. The 
merits of various PO functions aren't important when it's the data that really 
count (or the counts that count, if you like). Why start out with bad data in 
the first place? As my old dad says, you can't make a silk purse out of a pigs 
ear.
 
regards, Martin 
PS Just to clear up another possible point of confusion: large particles lead 
to texture effects, not preferred orientation.  






________________________________

        Subject: FW: Preferred orientation?
        Date: Fri, 16 May 2008 10:43:21 -0400
        From: [EMAIL PROTECTED]
        To: rietveld_l@ill.fr
        
        
         
        I don't remember saying that reflection would work well with 
wollastonite only that a capillary won't get rid of the orientation (or at 
least that was my intention). As always this is going to vary from sample to 
sample, i.e. how large the particles are, aspect ratio of the particles, the 
diameter of the capillary, pure or mixture, etc.  Orientation with needles is 
going to be more of a problem in a 0.3mm versus a 0.8mm capillary.  Particle 
statistics are also a potential issue with samples like this.  If they are 
large enough to orientate badly then the crystallites are probably large and 
the capillary will do a better job with the statistics
         
        The MD correction doesn't work very well for alot of these systems, SH 
is better as long as the correlations don't get out of hand (which they can 
quite easily).  However 0.9 is still significantly orientated so it doesn't get 
rid of it by any means.  With platey particles you can pretty much eliminate 
the preferential orientation with a capillary versus flat plate with 
significant impact of quantitative analysis results (paper published in Powder 
Diffraction a couple of years ago).
         
        I do have reflection data from 400 mesh wollastonite (albeit with MoKa 
from a high pressure gas cell) which can be fitted quite nicely with SH (lousy 
with MD).  The quantitative analysis results from the carbonation are good 
enough to extract a rate constant which suits me nicely.  The additional 
penetration of the MoKa should help with the stats in this case even if 
transparency becomes a problem - you can't win eh?  
         
        Unfortunately it's quite difficult to completely decouple orientation, 
statistcs and microabsorption as they are all related to size.
         
        Pam
         
          

________________________________

        From: Martin [mailto:[EMAIL PROTECTED]
        Sent: Fri 16/05/2008 10:04 AM
        To: Whitfield, Pamela; rietveld_l@ill.fr
        Subject: RE: Preferred orientation?
        
        
        In fact I think you might find it helps quite a bit. Have a look at:
         
        http://img.chem.ucl.ac.uk/www/vickers/po/po.htm
        
         
        Martin 
        
________________________________

        Subject: RE: Preferred orientation?
        Date: Thu, 8 May 2008 10:55:12 -0400
        From: [EMAIL PROTECTED]
        To: rietveld_l@ill.fr
        
        
        

                I do that myself but it doesn't always help much if you've got 
something like wollastonite! J

                 

                From: Martin [mailto:[EMAIL PROTECTED] 
                Sent: May 8, 2008 10:51 AM
                To: rietveld_l@ill.fr
                Subject: RE: Preferred orientation?

                 

                Forget all that long winded stuff. Just collect the data on 
capillary transmission geometry and avoid all (well, most of) the fuss.
                 
                Martin Vickers
                
                

                
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