We studied a set of 15 reference clays from The Clay Minerals Society (CMS) Source Clays repository. Our aim was to use them as reference materials in our version of the QUAX mineral database. The QUAX software (Quantitative Phase-Analysis with X-ray Powder Diffraction) has been used successfully at the KTB site (German Continental Deep Drillling) to determine mineral assemblages quickly, in an automatic fashion, on a large number of samples (∼40,000). It was also applied to Quaternary marine sediments of the Japan Sea. Our current research focuses on marine and lacrustrine sediments from the Arctic Ocean and Siberia.

QUAX is a full-pattern method using a reference materials database. The quality of a particular quantification depends on the availability of the relevant mineral phases in the database. Our aim is to extend and improve the database continuously with new data from our current projects, particularly from clay and feldspar minerals.

A reference material in the QUAX software must be monomineralic. Before X-ray diffraction (XRD) patterns of CMS clays could be added to the database, quantification of any impurities was necessary. After measuring the bulk material by XRD, the <2 µm fraction was separated because we assumed it would contain the smallest amount of impurities. Here we present grain-size data, XRD data and X-ray fluorescence (XRF) data for this clay-sized fraction. The results of chemical and mineralogical preparation techniques and (elemental) analysis methods were combined. For XRD, random and oriented clay-aggregate samples as well as pressed pellets for QUAX analysis were prepared. Semi-quantitative clay mineral determinations were run for comparison.

The influence of clay preparation procedure on sorption and hydrolysis of carbaryl (1-naphthyl, A-methyl carbamate) by the reference smectite SWy-2 was examined. For research purposes, reference clays are sometimes used without purification, or more commonly, the <2 μm size fraction is obtained by gravity sedimentation or low-speed centrifugation. We determined that these common methods did not remove all the inorganic carbonate impurities present in SWy-2, and that these impurities caused alkaline conditions in aqueous clay suspensions leading to the alkaline hydrolysis of carbaryl to 1-naphthol. The hydrolytic activity of homoionic K-SWy-2 disappeared once carbonates were eliminated. Two methods were evaluated for preparing K-SWy-2 devoid of inorganic carbonates. In Method A, inorganic carbonates were first removed by incremental additions of a 0.5 M sodium acetate buffer (pH 5.0) until the clay suspension reached pH 6.8, followed by low-speed centrifugation to obtain the <2 μm size fraction; in Method B, the order of these steps was reversed. Carbaryl hydrolysis was used as a probe to determine the effectiveness of the two methods in terms of the removal of carbonate accessory minerals. Homoionic K-SWy-2 obtained by Methods A and B produced near neutral pH when suspended in water and hydrolysis of carbaryl in these suspensions was not evident. In this regard, both clay preparation methods were acceptable. However, there were procedural advantages with Method B, which is therefore recommended for the partial purification of reference clays, as detailed in this paper.