Microabsorption

[Whitfield, Pamela]

Strictly speaking you shouldn't trust Rietveld results with that particle size whether there is microabsorption or not.  If you look at a cement sample with a microdiffraction system it becomes really obvious how grainy some of the stuff really is. If you have a Co system microabsorption itself won't be a problem.    There was a paper some time ago by Deane Smith regarding particle statistics in powder diffraction.  The general conclusion was that less than about 5 microns was required to get a good random powder, at which point microabsorption isn't as much of an issue.  Many people use 20 microns as a maximum, which is a 600 mesh sieve I believe, but that is barely adequate.  The usual mill for reducing particle sizes without ruining the sample is a McCrone microniser.  You have to be careful with cement systems as the grinding process can affect the gypsum, bassanite phases if the sample gets too hot.  Micronising in isopropyl alcohol is the usual way of prepping good cement samples.   Pam Dr Pamela Whitfield CChem MRSC Energy Materials Group Institute for Chemical Process and Environmental Technology Building M12 National Research Council Canada 1200 Montreal Road Ottawa  ON   K1A 0R6 CANADA Tel: (613) 998 8462         Fax: (613) 991 2384 Email: <mailto:[EMAIL PROTECTED]> ICPET WWW: http://icpet-itpce.nrc-cnrc.gc.ca -----Original Message----- From: Maria Chrysochoou [mailto:[EMAIL PROTECTED] Sent: June 3, 2005 11:20 AM To: Rietveld_l@ill.fr Subject: Microabsorption Hi,   I am a new member of the mailing list. My name is Maria Chrysochoou and I am a ph.D. student in Environmental Engineering at Stevens Institute of Technology, USA. I deal with soils, cements and cementitious systems in my research and currently use Rietveld to study the mineralogy of a cementitious waste. I use the Jade software by Materials Data Inc. I have the following problem, if anyone can provide some insight: The cementitious system I study contains common cement phases, such as ferrite and periclase, and several alteration products, which have much lower absorption coefficients. The problem is that the system contains temperature sensitive phases and we are forced to hand-grind our samples down to a #400 sieve (37 µm). We have not managed to reduce the particle size further manually. I have two questions: 1)       Would you recommend the micronizing mill for a high-pH, temperature-sensitive system that contains a lot of Al-phases? How would you suggest to deal with the microabsorption problem? 2)       Would you trust Rietveld results without microabsorption correction at this particle size (37 µm)?   Thanks,   Maria Chrysochoou W.M. Keck Geoenvironmental Laboratory Stevens Institute of Technology Tel.: +1 201 216 8773 Fax: +1 201 216 8212 E-mail: [EMAIL PROTECTED]