OK, that question is a FAQ. But, preferred orientation
is the main pain for Rietveld users, isn't it ?-)
I have some mineral samples to study (biotite and
amphibole), and they are separated from the rocks,
but not milled. In fact I have 100-500mg of big crystals.
(so why not to study them as single crystals ? Yes this
is a good question, but we have only an old overloaded
4-circle diffractometer, with different priorities...). So that
obtaining an ideal powder is not yet so easy. Hand milling
is not efficient. 5mn of ball-milling provides the good
grain size but starts to distort the biotite structure.
And then, problems with preferred orientation occured.
I had never tried the technique of mixing the sample with
Cab-osil before this time and was mainly using the
technique of dusting the sample through a sieve. Even
after mixing 25% biotite and 75% Cab-osil, in volume,
one cannot expect to be able to pack the sample : the
75% Cab-osil tends to decrease to a few percent after
packing, so that the preferred orientation is not reduced.
However, by using a side-loading sample
holder, and never packing, the result tends to be as
good as that obtained by the dusting technique, maybe with
a better sample surface (more plane).
Comments about this, similar or different experiences,
reference of studies giving ideal percentages for mixing
a clay mineral with Cab-osil, etc ?
Armel Le Bail
PS : Please, no advice on the spray technique because here
we are a small unit doing academic research with a small
budget, even if we obtain times to times a new
diffractometer :).
