The clay mineral palygorskite and the common dye known as indigo form spontaneously upon mild heating a very resistant hybrid pigment with a vivid blue color.  Near-infrared spectroscopy (NIR), turned out to be the ideal tool for studying the structure of the hybrid, which is relevant to the famous meso-American pigment known as Maya blue. Palygorskite particles are lath-like. They have a chessboard cross-section of alternating aluminosilicate ribbons and hydrated channels, as well as abundant hydrophilic silanol groups on their outer surface.  At the reaction temperature (up to 130 ^oC) both the tunnels and the surface SiOH dehydrate, thereby inducing well-defined spectral changes in the NIR, which are reversible upon cooling at ambient temperature and relative humidity.  How can the binding site of indigo be identified?  Why indigo, among many organics, is so unique in forming a stable complex with palygorskite at loadings up to 10 wt%?  Is the formation of the complex limited to the (nearly ideal) variety found in Yucatan, Mexico? Answers to these (and other) questions can be found in Tsiantos et al., 2014, Tsiantos et al., 2012, Sanchez del Rio et al., 2009.

Sepiolite and palygorskite, the two members of a clay group with a discontinuous octahedral sheet and modulated layer structure, undergo structural "folding", i.e. the collapse of their tunnels over a narrow temperature range. Locally, folding is induced by the removal of half of the coordinated H₂O molecules lining the "side walls" of the tunnels. On a crystallite scale, a huge number of these individual dehydration events must correlate along the length of several tunnels to produce a cooperative effect:  all tunnels in the cross-section of the crystallite must fold simultaneously.  As these minerals are in the form of laths or fibers, cooperative folding necessitates both, the 1D diffusion of H₂O out of extremely long tunnels, as well as the integrity of the chemical bonds ("hinges") keeping the structural modules together throughout the process.  Tsampodimou et al., 2015 studied and discussed sepiolite folding as a cooperative structural transformation based on complementary NIR and TGA experiments.

Attenuated total reflectance (ATR) spectroscopy in the mid-infrared, as well as diffuse reflectance via optical fibers in the near-infrared (NIR) are the least invasive vibrational spectroscopic techniques for the study of clay minerals.  Combining these two spectroscopic techniques in real-time and on the same sample is an extremely powerful method for the structural study of processes involving clays.  Due to the different extinction coefficients of the fundamental and higher order vibrations, the two techniques operate ideally on different sample scales.  Bukas et al. 2013 developed a simple environmental cell allowing for the independent synchronous collection of the NIR and ATR spectra under optimum sampling conditions and demonstrated its potential with the study of sepiolite upon drying from the H- and D-form.  Additional designs for non-invasive infrared sampling are reviewed by Chryssikos, 2017.

Smectites are expandable phyllosilicates owing to the negative charge of their layers which is compensated by hydrated cations in the interlayer.  The layer charge, a key determinant of the technological properties of smectite, is extremely difficult to measure by the standard structural formula and alkylammonium exchange methods.  In collaboration with the Institute of Geological Sciences of the Polish Academy of Sciences in Cracow, we have developed and calibrated a new infrared-based methodology for the determination of layer charge. The so-called O-D method exploits the spectroscopic properties of interlayer H₂O which presents “dangling” O-H bonds protruding towards and weakly interacting with the charged siloxane layer. The interlayer is H₂O is exchanged by D₂O in situ (to avoid spectral interference with the structural OH of the mineral) and the exact wavenumber of “dangling” O-D bonds is linearly correlated to layer charge.  The new method is based on an intensive property measurement of an intrinsic interlayer and requires less than 5 mg of sample with essentially no preparation.  It is fast, accurate, precise and requires a modest single-reflection ATR cell with an environmental cup for performing   layer charge measurements on a routine basis. Its use and merits can be appreciated in several recent publications (e.g. Tsiantos et al. 2018, Christidis et al. 2018, Kuligiewicz et al. 2018, Kaufhold et al. 2019 etc.).  A critical assessment of the various methods for measuring the layer charge of smectites can be found here.

Layer charge measurements by the O-D method are provided by the Applied Spectroscopy Laboratory of TPCI/NHRF in the context of academic partnerships or industrial service collaborations.

High through-put vibrational spectroscopic techniques, such as NIR diffuse reflectance spectroscopy, are able to generate large "hyperspectral" datasets which can capture the compositional variability of very large samples, e.g. a deposits. Multivariate techniques can interrogate these datasets and extract different kinds of information from them. For a recent review on multivariate analysis of clays, see Chryssikos and Gates 2017. They can produce unsupervised hierarchical classification schemes, identify the minimum number of latent key players needed to describe meaningfully the variability of the spectra, and they can be trained to correlate the spectra with independently collected data to produce quantitative chemometric tools. Published examples of chemometric algorithms developed in our laboratory can be found here, here, and here.

Multivariate and chemometric methodologies for the clay deposit management and processing are provided as services to the relevant industry by the Applied Spectroscopy Laboratory of TPCI/NHRF.

We use NIR spectroscopy to study the well-known sigmoidal kinetics of the intercalation of kaolinite at the level of chemical bonding and compare with XRD.  NIR allows for the convenient real-time monitoring of the heterogeneous reaction which takes place in a closed reactor as a function of temperature and in the presence of excess amide.  The results were surprising.  NIR recorded exactly the same sigmoidal kinetics as XRD despite the different scale of the two techniques.  The shape of the sigmoidal depended on kaolinite but not on temperature.  At each temperature, a single environment of intercalated amide was observed regardless of reaction progress and type of kaolinite studied.  This meant that the interlayers are observed either empty or fully intercalated.  Instead of the true intercalation reaction, the sigmoidals represent the temporal distribution of instantaneous intercalation events following exposure to the amide and can be fitted as such, e.g. by a cumulative log-normal function.  New, intriguing aspects of kaolinite intercalation can be found in the recent papers by Andreou et al. (2021) and (2022).

The widely used halloysite nanotubes are formed by the irreversible drying of the natural hydrated halloysite-(10Å). The latter is unstable at ambient humidity and temperature, and commonly contaminated with 7Å impurities.  Joining forces with The James Hutton Institute in Scotland, we study the drying of polygonal and cylindrical halloysite-(10Å) with infrared spectroscopy and XRD. For the spectroscopic part, we combine ATR with an environmental cell and a D₂O humidity generator to provide the first detailed vibrational study of the halloysite dehydration process.  The key idea was to H/D exchange selectively the pristine -(10Å) material and then measure it as it equilibrates step-wise to progressively decreasing %RD. The drying and collapse of the interlayers induces local ordering into kaolinite-like domains but accumulates disorder and interlayer detachment in other parts of the structure. The original structure cannot be recovered by re-wetting because the kaolinite-like interlayers are not expandable. Halloysite nanotubes have an open imperfect structure with at least two different kinds of stacking defects, which are decorated by H₂O and accessible to H/D exchange. A detailed account of the structural changes accompanying the dehydration of halloysite-(10Å) was published recently by Siranidi et al. (2024)

The aluminol groups in the interlayer of kaolinite resist D/H exchange at near ambient conditions, thereby suggesting that the stable H-isotope composition of this clay mineral may have been be preserved over geological times. A new experimental setup based on NIR spectroscopy via a diffuse-reflectance powder probe enabled the monitoring of D/H exchange kinetics in kaolinite exposed to a flow of D₂O-saturated gas at temperatures/times in the range from 125^οC/ 5 days to 275^οC/ 3 hours. At any temperature, the degree of exchange was found to depend strongly on the stacking disorder of kaolinite (KGa-2 > KGa-1b) and was below detection limit in the most ordered hydrothermal sample (Keokuk). The results were modelled by colleagues at the Institute of Geological Sciences of the Polish Academy of Sciences in Krakow who demonstrated that the preservation of the pristine H-isotope composition is unlikely in kaolinites of low structural order, such as those of soil/weathering origin. The full story can be found in Derkowski et al. (2024).