Identifying and fixing methodical weaknesses by participating the Reynolds Cup competition in quantitative mineral analysis


Identifying and fixing methodical weaknesses by participating the Reynolds Cup competition in quantitative mineral analysis

Kleeberg, R.; Möckel, R.; Kempe, U.

The Reynolds Cup does not only pose a scientific challenge for mineralogists but offers also an independent sight on the weaknesses of the techniques applied in an individual laboratory. Some of the errors seem to be related to well known problems, but nevertheless these mistakes are common in routine as well as in intensive competitive analytical work. The aim of this talk is to highlight problems of mineral analysis using the data of the 7th Reynolds Cup competition. Some exemplary conclusions will be drawn and ideas for methodic improvements will be drafted.
Qualitative identification of minor non-clay minerals in complex mixtures by X-ray powder diffraction techniques is known to be limited due to low peak intensity and peak overlap. Successful strategies are (i) applying mineral enrichment or even single grain separation for XRD and SEM, (ii) performing low-noise measurements, (iii) applying peak identification from difference plots of preliminary Rietveld refinements, and (iv), more unconventional, running measurements on just coarsely powdered samples to enhance peak intensities. Such techniques were successfully applied to identify of < 1 wt% of tourmaline in samples 1 and 3 as well as for the identification of amphibole and its compositional constraint. On the other hand, SEM-EDX analysis can give valuable information for the presence of phases which could not be identified in any XRD pattern, as demonstrated for apatite and nahcolite.
The correct identification of clay minerals from basal reflections measured on oriented samples seems to be straightforward, but practical limitations arise from the limited amount of sample, complicating the application of standard particle size separation techniques. Nevertheless, it is important to apply standard procedures commonly used in clay mineralogy like controlling the interlayer cation occupation, as demonstrated by the improved detection limit for I/S in sample 2.
In fact all quantification procedures successful in the RC competition are based on XRPD patterns but suffer from profile overlap. Especially, the differentiation of disordered 2:1 minerals needs for improved modeling of the diffraction profiles, including the basal reflections in randomly oriented samples. Some minerals call for the application of other techniques than XRD.

Keywords: Reynolds Cup; X-ray diffraction; analytics

  • Lecture (Conference)
    7th Mid-European Clay Conference, 16.-19.09.2014, Dresden, Deutschland

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Publ.-Id: 20368