Reynolds Cup 10 was organized by Steve Hillier, Helen Pendlowski and Ian Phillips of the James Hutton Institute. The Clay Minerals Society, and the DTTG are acknowledged for direct financial support, the James Hutton Institute for contributions in kind, and the ICDD for helping to advertise the competition.
Three samples were carefully prepared for RC10 and nominally identified as a ‘hydrothermally altered shale’ a ‘muddy limestone’ and a ‘Martian volcaniclastic sediment/soil’.
Ninety-nine labs registered for RC10, two withdrew.
Samples were dispatched to participants at the beginning of March 2020 and because of COVID restrictions participants were given longer than normal to make their analyses of the samples. Despite restrictions 76 labs, spread over 23 countries around the world returned results to RC10. As in previous Reynolds Cups, most participants relied on X-ray powder diffraction (XRD) as the primary method of quantification, supported by various complimentary methods. Indeed, no method other than XRD has ever figured in any Reynolds Cup as the primary method of quantification by a top placed (named) contestant.
As usual the Reynolds cup is judged on cumulative bias as the sum of the absolute difference between the known concentration and submitted concentration for all phases across all three samples, with mis-identified phases also counting towards bias.
The distribution of bias for the 76 participating labs in RC10 is shown below.
Close runners up in joint fourth place were three labs all with a total bias of near 83.
The first of these was the team, from left to right, of Stanislav Jelavić, Tonči Balić Žunić, and Marie-Louise Siggaard-Andersen from the University of Copenhagen with a bias score of 83.8.
These we just bettered by the team from the BGR in Hannover consisting of Kristian Ufer (colored mask), Stephan Kaufhold (grey mask), and Reiner Dohrmann (black mask) who achieved a bias of 83.6.
And the final joint fourth place mention is to the French team from BRGM, based in Orléans with a bias of 83.0.
Contestants who finish in the top three positions are asked to give some details and especially tips on how they did it to help other contestants improve their methods in subsequent Reynolds Cups.
Third place in RC10 was also joint, with the team from the Polish Institute of Geological Sciences (PAS) scoring a bias of 77.0.
he team from the PAS began by analyzing the as-received samples in side-loading mode using XRD. Then portions were milled in a McCrone mill and side-loaded for full random powder XRD results. Another portion of the samples were sent to Activation Laboratories in Canada for chemical analysis of major and some minor and trace elements. Fractions < 2 µm were extracted using Stokes’ Law and saturated with Ca. These were mounted as oriented specimens and analyzed with XRD in air-dry form and after saturation in ethylene glycol.
Both the bulk samples and the < 2 µm fractions were also analyzed using mid-infrared spectroscopy (ATR) and thermogravimetry combined with mass spectrometry of the evolved vapors and gases. For bulk rock XRD interpretation and mineral quantification, BGMN and Q-Min software were used. Mineral quantification was finally compared against chemical analyses.
The other joint third place contestants were Bruno Lanson and Nathaniel Findling from ISTerre, Université Grenoble Alpes who achieved a bias of 75.1.
Another aliquot (~2.5g) was used to extract the <2µm clay fraction. Potential carbonates and organic matter were removed first by using a Na acetate/acetic acid buffer and hydrogen peroxide, respectively. The extracted clay fraction was then saturated (Na) and oriented slides were prepared by pipetting a clay slurry onto a glass slide. XRD data was collected in both Air Dried and Ethylene Glycol states. Identification was performed first and then “refined” using the Sybilla software to fit the data. The suspected presence of halloysite in sample RC10-2 was confirmed with a quick (1h) DMSO (vapor) treatment to differentiate halloysite versus kaolinite.
Additionally, part of the extracted <2µm fraction was used to prepare a randomly oriented powder to differentiate di- and tri-octahedral clay minerals. Part of this fraction was also used for SEM/EDX analyses to assess the actual composition of the clay minerals identified for the same purpose.
The >2µm fraction “residue” of this extraction was used for SEM/EDX observations i) to confirm the chemical composition of phases identified with XRD and ii) to possibly identify minor phases that were overlooked in the 1st identification/refinement round. This fraction was also X-rayed, including as oriented preparations (RC10-2 and RC10-3).
Total bias was reduced significantly further to 63.2 by the team from CSIRO in Adelaide, Australia, who were clear second place finishers.
Left to right – Rodrigo Gomez-Camacho, Peter Self, Mark Raven, Rong Fan (Inset – Nathan Webster)
The CSIRO team also began with the samples initially back pressed into sample holders for XRD analysis in their as received state. Semi-quantitative elemental analysis was also performed on the as received samples to help identify minor phases that may otherwise be difficult to confirm by XRD alone.
Sub-samples of ~ 1.5 g of each of the samples were then micronized under ethanol with a McCrone micronizing mill then oven dried at 60° C. After drying, the micronized samples were thoroughly mixed in an agate mortar and pestle to ensure homogeneity. The fine powders were lightly back pressed to minimise preferred orientation and XRD patterns collected on a PANalytical X’Pert Pro MPD using iron filtered cobalt K alpha radiation. Patterns were collected from 3 to 80° 2 theta at 0.017° steps. Total data collection time was ~30 minutes. The process of micronizing the samples under ethanol followed by oven drying partially dehydrates any swelling clay minerals resulting in broad asymmetric 00l peaks. The micronized samples were therefore calcium saturated to restore the 001 peaks of the swelling clay minerals to ~15 Å. This was achieved by washing the micronized samples twice with 1M CaCl2, washing with deionised water followed by ethanol (centrifuged at 6000rpm after each step) before oven drying at 60° C. The Ca saturated samples were again thoroughly mixed in an agate mortar and pestle to ensure homogeneity and lightly back pressed into sample holders for XRD measurement. XRD patterns were then re-collected. Comparison of the XRD patterns before and after Ca saturation confirmed there were no water soluble phases present in any of the samples. Ca saturation of sample RC10-3, however, shifted the sharp 14.3 Å peak to 14.8 Å confirming the presence of vermiculite.
A further 2 g sub-sample of the as received materials were dispersed with 1M NaCl and centrifuged at various speeds to separate <0.2 µm, 0.2-2 µm and >2 µm fractions. The fractions were again Ca saturated and pressed powder and oriented, calcium and magnesium saturated and glycerolated specimens were prepared to help identify the clay mineral species. Oriented Mg and glycerolated XRD patterns of the <0.2 µm fraction of RC10-2 seemed to show an unusual interstratified species which was modelled in NEWMOD with an interstratified chlorite-smectite clay. This was subsequently judged as a misidentified mineral, however, it is likely that the amorphous aluminium hydroxide phase, also in RC10-2, may have entered the exchange sites in nontronite and/or illite-smectite clays during the clay separation procedure leading to the misidentification. The 7.3 Å peak in the 0.2-2 µm fraction of RC10-2 was identified as halloysite after treating oriented patterns before and after exposure to formamide. The 7.3 Å peak shifting to ~10.2 Å after formamide treatment.
Quantification was primarily performed using SIROQUANT version 4 with calibrated HKL files prepared from several in-house clay mineral standards. TOPAS version 6 was used to help determine the composition of numerous carbonate minerals in RC10-2 and feldspars in RC10-3. Regardless of any evidence of amorphous material features, amorphous content was determined using the internal standard method by difference after the addition of 20% by weight of a fine grained corundum standard to each micronized Ca saturated sample.
And finally – the winners of the 10th Reynolds Cup (2020) were the team from QMINERAL in Belgium, with an impressive total bias of 51.9.
When asked how they did it, Rieko Adriaens commented –
“Our approach consisted of performing an XRD measurements on random oriented powders and oriented clay specimens measured in air-dry and glycolated conditions. As emphasized in the ‘Key Recommendations’ section of the Reynolds Cup, appropriate sample preparation and correct identification are the key to success in QXRD.
The powders were prepared by micronizing a representative part of each sample and ensuring optimal random orientation when filling the measurement holder. Identification of the phases was performed using Diffrac.Eva software and the subsequent quantification using TOPAS software (Bruker AXS).
A second part of each sample was used to isolate a fine fraction in which clay minerals are concentrated. Slides with oriented clay aggregates were measured in air-dry and glycolated conditions and finally heated and remeasured. These measurements allowed to identify all clay minerals present in the samples. Sample RC10-2 was particularly interesting as we strongly suspected the presence of dioctahedral smectite based on the powder diffraction data. Our observations on the XRD pattern of the oriented slide however showed a very limited swelling and no collapse after heating which made us drift away from the identification of smectite. We suspect the smectite interlayer was occupied by another phase present in the mixture, possibly the amorphous Al(OH)3.
The QXRD results were complemented by chemical analyses. The chemical data were however not very helpful as the mixtures were composed of many phases with a strongly variable and overlapping chemical compositions. Hence, we trusted our data obtained by QXRD.
We are very proud to win the Reynolds Cup for the second time!”