Recently, significant advances have been made in the theory and application of acoustic and electroacoustic spectroscopies for measuring the particle-size distribution (PSD) and zeta potential (ζ potential) of colloidal suspensions, respectively. These techniques extend or replace other techniques, such as light-scattering methods, particularly in concentrated suspensions. In this review, we summarize acoustic and electroacoustic theory and published results on clay mineral suspensions, detail theoretical constraints, and indicate potential applications for the study of environmentally significant clay mineral suspensions. Using commercially available instrumentation and suspension concentrations up to 45 vol.%, acoustic spectroscopy can characterize particle sizes from 10 nm to 10 µm, or greater. Electroacoustic spectroscopy can determine the ζ potential of a suspension with a precision and accuracy in the mV range. Despite the clear potential for their use in environmental settings, to date, acoustic methods have been used mainly on clay mineral colloids with industrial application, typically combined with similar measurements such as isoelectric point (IEP) determined from shear yield stress or ζ potential from electrophoretic mobility measurements. Potential applications in environmentally relevant suspension concentrations are significant, as PSD and ζ potential are important factors influencing the transport of mineral colloids and associated contaminants through porous media. Applications include determining the effects of suspension concentration, surfactants, electrolyte strength, pH and solution composition on soil clay properties and colloidal interactions, and determining changes in PSD, aggregation and ζ potential due to adsorption or variations in the clay mineralogy.

Acoustic and electroacoustic measurements of particle-size distribution (PSD) and zeta potential (ζ potential), respectively, were used to obtain in situ measures of the effects of suspension concentration and pH on interactions between mixed-charge clays and clay minerals from a highly weathered sediment. Measurements were obtained in concentrated suspensions as a function of weight fraction and as a function of pH during titrations. Standard dispersion and centrifugation methods were used to obtain a comparative measure of PSD. Thermogravimetric analysis and X-ray diffraction patterns were used to obtain semi-quantitative and descriptive analyses, respectively, of the sediment, which is composed of Fe oxide minerals, kaolinite, gibbsite, quartz, crandallite, chlorite and traces of other clay minerals. Acoustic measurements showed that the PSD of the clay fraction varied with suspension concentration, and electroacoustic measurements showed the ‘bulk’ ζ potential increased in absolute value as the suspension concentration decreased. Titration results were also sensitive to suspension concentration. Acoustic measurements indicated that the suspensions became unstable at ∼pH 7.5–8.0, as the attenuation spectra changed character near this pH and the calculated PSD shifted to a larger particle size. This pH value is near the points of zero charge of goethite and gibbsite, as verified by titrations on mineral standards. The results confirm the central role oxide minerals play in regulating clay mineral interactions in highly weathered sediments, and indicate that the average ζ potential of a suspension may be a poor indicator of controls on suspension stability.

Interactions of hydrothermal and deformation processes in active fault zones are important in light of seismic and geothermal activities. This study investigates different tectonites of the Grimselpass breccia fault, a hydrothermally active strike-slip structure in the Central Alps (Switzerland). We combine microstructural and geochemical investigations to decipher the interaction of grain-size reduction and associated dilation (volume gain) during frictional deformation with geochemical processes such as mass gain via precipitation, grain growth and healing. Three types of tectonites were investigated in the fault zone: (1) cockade-bearing tectonites, (2) cataclasites and (3) fault gouges. They differ in terms of microstructure of clasts, mass and volume gain/loss, as well as type and degree of cementation. Clast size distributions show decreasing median values from low-strain cataclasites to high-strain cataclasites. Mass/volume gains are up to 20 %, with samples of smallest clast sizes often showing the highest mass gain. In the case of cockade-bearing tectonites large volume and mass gain occur. In particular, the precipitation of SiO2-rich cements is inferred to (i) reduce permeability and (ii) induce a gain in strength of the respective fault rocks. Location, timing and degree of healing directly control the fault architecture by deactivating fault strands from the circulation of hydrothermal fluids. In this case, new rupturing events are necessary to reactivate locked fault strands. Such feedback cycles of episodic deformation, mass/volume and clogging preserved hydrothermal circulation and deformation over the last 3.4 Ma, representing a process chain for the long-term preservation of orogenic hydrothermal systems.