Widespread alteration in the Miocene lacustrine volcanic/sedimentary rocks of the Ankara-Çankiri basin of central Anatolia has resulted in the formation of sizeable (economic) quantities of bentonite deposits. No detailed characterization of the geological, mineralogical, and geochemical properties or the depositional environments of these primary and secondary bentonite deposits has been carried out to date. The present study was undertaken to close this knowledge gap. Two possible hypothetical processes were examined to explain the genesis of the bentonites: 1) The bentonites were formed by the devitrification of volcanic glass in a lacustrine setting; and 2) The bentonites were formed by the chemical weathering of previously deposited volcaniclastic sediments and ophiolitic materials. The characteristics of the bentonites were examined using X-ray diffractometry, scanning and transmission electron microscopy, energy dispersive spectroscopy, and chemical analyses of major and trace elements. The Ankara-Çankırı bentonites are found gradationally interbedded with parent Miocene volcanic and volcaniclastic rocks. These bentonites were deposited in a shallow lacustrine setting based on observed desiccation cracks, locally enclosed coal seams, plant rootlets, gypsum lenses, yellow sulfate-like fracture infillings, and ferric iron oxide stains. Smectite resulted from the chemical weathering of feldspar and possibly also the weathering of biotite and hornblende. This smectite was precipitated in situ on partially dissolved glass and feldspar. The average major-element composition of the smectite-rich clay fractions yielded the following montmorillonitic smectite structural formula: (Na0.33Ca0.31K0.18) (Al2.35Fe0.80Mg0.78)(Si7.79Al0.21)O20(OH)4.

The interlayer cation occupancy in the smectite-rich clay fractions was based on the use of Na+/(Na++Ca2+) ratios and showed a composition between a Ca-smectite and a Na-smectite. The relative increases in several groups of elements according to the LREE/(MREE+HREE) ratio, Al2O3, the sum of Ni+Co+Cr, the sum of Fe2O3+MgO+TiO2, the positive correlation between Rb+Ba and Na2O+K2O, Sr and Rb, Rb/Sr and Zr, Zr/Sm and SiO2, the negative Eu anomaly, and the field and petrographic observations further showed that the Si, Al, Fe, and Mg required to form smectite were mainly supplied from the decomposition of feldspars, amphiboles, and volcanic glass from volcanic materials and were partially supplied from the chemical weathering of ophiolitic basement units. The bentonite deposits examined in this study are mainly primary bentonites derived from volcanics and local secondary bentonites from previously deposited volcaniclastic sediments and ophiolitic materials.

Atomic force microscopy (AFM) is a novel method for measuring changes in clay swelling in situ at the tactoid level in an aqueous environment. While the swelling process has been directly observed at the mesoscale level for multi-tactoid aggregates and the associated pores, no method to date has allowed the direct observation of swelling dynamics at the nanoscale. In initial proof-of-concept studies, individual tactoids of a Na-exchanged nontronite (NAu-1) were imaged in a solution of 5 mM NaCl. When multiple line profiles were examined on the same tactoid, the changes in height varied and depended on which layers of the profile were transected, and demonstrated that AFM analyses can be used to directly probe intratactoid heterogeneity in the swelling process. To better visualize this heterogeneity, a method was developed to restrict AFM images to include only the portions of a tactoid above a threshold height. A comparison of the changes in these images for multiple threshold values revealed that swelling in one part of a tactoid may occur simultaneously with compression in another portion, which suggests that the encroachment of layers into intra-tactoid micropores can partially compensate for the overall volume change. Finally, to demonstrate the ability of this technique to monitor in situ swelling changes as the surrounding aqueous environment is modified, a tactoid of K-montmorillonite (SWy-2) was monitored during cation exchange as a KCl solution was replaced with NaCl. After exchange, a transition from the crystalline swelling regime to the osmotic regime was observed. Subsequent height profiles were unchanged for a period of several hours and indicated that the AFM measurements were stable in the absence of changes to the aqueous phase composition. Because this technique is the first method that allows the swelling of a single tactoid to be monitored in an aqueous solution, it complements the ensemble-averaged data obtained from diffraction and scattering techniques.

Bentonites are candidate materials for encapsulating radioactive waste within barrier systems in crystalline rocks. In the ‘Alternative Buffer Material’ (ABM) test in the hard rock laboratory in Äspö, Sweden, six packages of eleven different buffer materials (mainly bentonites) with various exchangeable cation populations were packed vertically with an iron tube used as a heater in the center. After installation, the second ‘ABM package’ (ABM-II) was first allowed to saturate with water for approximately 2.5 years. The blocks were then exposed to a temperature of up to 141°C for approximately 3–4 years. The hypotheses for the present study were: (1) no horizontal gradient of the cation exchange population was present in the individual blocks of ABM-II because ABM-II had a longer reaction time in comparison to the ABM-I package, which did not have horizontal gradients; (2) the exchangeable cation Ca2+:Na+:Mg2+ ratio was equal in all blocks of ABM-II and was independent of block position in the package. As expected from ABM-I, all blocks in the ABM-II experiment showed large differences between the measured values of the reference materials and the reacted samples. The exchangeable Na+ and Mg2+ values in ABM-II were reduced by up to 55% to 59% in comparison to the reference material. Contrary to the first hypothesis, horizontal gradients were observed in ABM-II; and, contrary to the second hypothesis, the exchangeable cation ratios differed markedly in the different reacted buffer materials. The largest total Na+ loss was observed in the middle part (-67%), whereas Mg2+ values decreased by 79% in the upper part. The exchangeable Ca2+ values increased strongly in ABM-II, particularly in the upper part. The most useful parameter to distinguish between ion exchange equilibria of ABM-I and ABM-II was the Na+/Mg2+ ratio. This ratio was constant in ABM-I (3.0) and had a similar ratio (3.5) in the lower part of ABM-II; however, the ratio strongly increased (5–10) in the upper part of the ABM-II package. The large Na+/Mg2+ ratios in the upper part of ABM-II could possibly be explained by water loss into the rock (caused by a pressure drop and boiling) and subsequent water uptake.

In 2000, The Clay Minerals Society established a biennial quantitative mineralogy round robin. The so-called Reynolds Cup competition is named after Bob Reynolds for his pioneering work in quantitative clay mineralogy and exceptional contributions to clay science. The first contest was run in 2002 with 40 sets of three samples, which were prepared from mixtures of purified, natural, and synthetic minerals that are commonly found in clay-bearing rocks and soils and represent realistic mineral assemblages. The rules of the competition allow any method or combination of methods to be used in the quantitative analysis of the mineral assemblages. Throughout the competition, X-ray diffraction has been the method of choice for quantifying the mineralogy of the sample mixtures with a multitude of other techniques used to assist with phase identification and quantification. In the first twelve years of the Reynolds Cup competition (2002 to 2014), around 14,000 analyses from 448 participants have been carried out on a total of 21 samples. The data provided by these analyses constitute an extensive database on the accuracy of quantitative mineral analyses and also has given enough time for the progression of improvements in such analyses. In the Reynolds Cup competition, the accuracy of a particular quantification is judged by calculating a “bias” for each phase in an assemblage. Determining exactly the true amount of a phase in the assemblage would give a bias of zero. Generally, the higher placed participants correctly identified all or most of the mineral phases present. Conversely, the worst performers failed to identify or misidentified phases. Several contestants reported a long list of minor exotic phases, which were likely reported by automated search/match programs and were mineralogically implausible. Not surprisingly, clay minerals were among the greatest sources of error reported. This article reports on the first 12 years of the Reynolds Cup competition results and analyzes the competition data to determine the overall accuracy of the mineral assemblage quantities reported by the participants. The data from the competition were also used to ascertain trends in quantification accuracy over a 12 year period and to highlight sources of error in quantitative analyses.

Shales have undergone a complex burial diagenesis that involved a severe modification of the pore structure. Reconstituted shales can provide new insights into the nature of the pore structure in natural materials. The effects of diagenesis on the microfabric, pore size distribution, and porosity of Opalinus shale were measured by comparing the behavior of natural and reconstituted specimens. The parent material (Opalinus shale) was reconstituted through multiple grinding operations, sedimentation from a dispersed slurry, and one-dimensional isothermal consolidation. This process produced uniform specimens that were not cemented and had replicable microfabric and engineering properties. The microfabric and mineralogy of the materials were examined using high-resolution scanning/backscattered electron microscopy (SEM/BSEM) and energy-dispersive X-ray spectroscopy (EDS) for specimens with broken and milled surfaces. Mercury intrusion porosimetry (MIP) and N2 adsorption were used to assess the pore size distributions and specific surface areas of the materials. The microstructure of natural shale was characterized to be highly heterogeneous with significant concentrations of calcareous microfossils, calcite, and quartz particles embedded within the clay matrix. The microfossils were observed to be locally infilled and rimmed by a calcite cement that showed evidence of dissolution. The reconstituted specimens showed a double-structure microfabric that evolved with the level of consolidation stress and converged into a single-structure material (comparable to the natural shale) at a consolidation stress of more than twice the estimated maximum in situ effective stress. The natural shale had a lower specific surface area in comparison to the reconstituted material, which was consolidated at large effective stresses. These differences can be attributed to cementation at a submicron pore scale and highlight chemical diagenesis effects that were not replicated in the reconstituted specimens.