I have to disagree with that; at least on a practical front with lab XRD. I
have done measurements myself with samples containing large portlandite plates
(granted, not a silicate but lovely-looking plates in a SEM) for quantitative
analysis. The whole point of the work was to see if capillary measurements
would be worth it if the normal sample prep techniques would change the nature
of the sample. The reflection measurements had awful orientation, but the
portlandite spherical harmonics PO coefficients for the capillary data gave a
texture index of 1, i.e. an ideal powder. The diffraction optics and detector
were identical for reflection and transmission. The capillary data actually
gave better quantitative results than the reflection. A reason might be that
if the grains are large enough to orientate significantly they might be big
enough to cause microabsorption effects that are best avoided (the Brindley
correction assumes spherical particles so plates are a bit of a headache).
It's just a thought, but the orientation in neutron and X-ray data might differ
due to the difference in sample container size and orientation. Neutron cans
are often mounted vertically and are pretty big so there won't be much
advantage over reflection as the material settles. Lab capillaries are usually
mounted horizontally and the capillary diameter is often quite small in
relation to the grain size (compared to neutron sample cans). All bets are off
for wollastonite though!
I will shut up at this point as I trying to avoid doing clay analysis!
Pam
-----Original Message-----
From: David L. Bish [mailto:[EMAIL PROTECTED]
Sent: March 21, 2007 9:11 AM
To: rietveld_l@ill.fr
Subject: Re: Problems using TOPAS R (Rietveld refinement)
One often hears of attempts to "eliminate" preferred orientation in
diffraction patterns of layer silicates using transmission measurements. Keep
in mind that if PO is a problem in reflection geometry, it will also affect
transmission measurements, in a manner potentially similar to flat-plate
samples. We did some TOF neutron measurements on phyllosilicates a few years
ago with what amounts to capillary sample holders, and preferred orientation
was a significant problem. If a material orients, it will do so in all mounts
unless steps are taken to minimize it.
Dave
At 07:50 AM 3/21/2007 +0100, you wrote:
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
>
