Norberto,
if the magnetite crystals in a natural sample are so big that you can
observe their octahedral morphology and this shape really gives rise to
any significant PO, than they are also definitely too big for any
quantitative XRPD phase analysis. Therefore, reducing the particle size
is definitely necessary, simply to minimize the effects of microabsorption.
Milling of magnetite is no problem in any usual type of mills, e.g.
McCrone micronizing mill, using agate, corundum, or zirconia grinding
elements. Even hand-grinding in an agate mortar can be tried. As
magnetite has no extreme tendency for cleavage, milled magnetite powder
should not tend to PO in powder samples and no PO correction is necessary.
Reinhard
Am 21/03/2014 10:59, schrieb Norberto Masciocchi:
Dear Friends,
I am facing a problem in quantifying magnetite in a complex natural
mixture, using the conventional Rietveld method (in its quantitative
analysis approach). Apparently, the octahedral morphology of
(difficult to grind) magnetite crystals affects itd diffraction pattern.
To my knowledge, as certified in the original paper (JAC 1986,
267-272), the ubiquitous and highly performing March-Dollase equation,
holding for "any" crystal symmetry, only applies to inequant
crystallites (i.e. crystallites with unequal sides), or, better said,
to "effectively rod- or disk-shaped specimens" (provided that the
geometry of the experiments keeps cylindrical symmetry). Neither
octahedral nor cubic (e.g., NaCl) crystals can be considered "inequant".
So, the question is:
Is there any way do get around this problem (without resorting to
spherical hamonics or to grind the specimen in a WC, SiC or BN mill)?
Thank you for your patience.
Norberto
--
TU Bergakademie Freiberg
Dr. R. Kleeberg
Mineralogisches Labor
Brennhausgasse 14
D-09596 Freiberg
Tel. ++49 (0) 3731-39-3244
Fax. ++49 (0) 3731-39-3129
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