Smectite growth is of importance across various fields due to its abundance on the surface of both Earth and Mars. However, the impact of the crystallinity of initial materials on smectite growth processes remains poorly understood. In this study, the kinetic processes of smectite growth were examined via experimental synthesis of trioctahedral Mg-Ni saponites. Mg-Ni saponites were synthesized using mixed precursors, specifically end-member Mg-saponite and Ni-saponite, which exhibit different crystallinities. The crystal chemistry and morphology of samples were analyzed using X-ray diffraction, Fourier-transform infrared spectroscopy, and high-angle annular dark-field scanning transmission electron microscopy. The experimental results converge towards these main conclusions: (i) the formation of Mg-Ni saponite solid solutions are promoted when the precursors are small particles, whereas large-particle precursors limit their own dissolution and do not yield Mg-Ni saponite solid solutions under the experimental conditions; (ii) because Ni exhibits a greater stability within the saponite structure compared to Mg, the Mg-Ni-saponite solid solutions formed more easily from the mixture of Ni-saponite germs and well-crystallized Mg-saponite precursors than from the mixture of Mg-saponite germs and well-crystallized Ni-saponite precursors; (iii) the dissolution extent (DE) of precursor mixtures increases with longer synthesis time, higher synthesis temperature, and larger gap between synthesis temperature of precursors and of samples, and stabilizes once it reaches a certain value. Thus DE can be used to estimate the kinetics of Mg-Ni saponite crystallization from precursor mixtures. These results obtained from the experimental Mg-Ni saponite system are useful for predicting the evolution processes of smectite in natural systems.

The rheological properties of three Na-activated, trioctahedral Mg-bentonites (hectorite clay from the CMS Source Clay Project repository, saponite clay from Spain and stevensite clay from Rhassoul, Morocco) and a sepiolite clay from Greece were examined after dynamic ageing at temperatures up to 230°C. The 5% w/v suspensions were prepared by dispersing the clay mineral samples in distilled water. The suspensions underwent dynamic, thermal ageing for 16 h before determination of the viscosity, filtration loss, filter cake thickness and pH and the concentration of dissolved Na+ and Mg2+. Thermal ageing contributed to the dispersion of clay particles, with a direct effect on plastic and apparent viscosity, introducing pseudoplastic behaviour. With the exception of the stevensite clay at 230°C that displayed limited dissolution at 230°C and partial conversion to kerolite, the clays were stable at high temperatures. The Na-activation of all clays except for stevensite was not adversely affected by thermal ageing. Thermal ageing of stevensite at 230°C facilitated Na exchange and yielded suspension with high viscosity and low filtrate loss. Only the suspensions of hectorite and those of stevensite aged at 230°C met with American Petroleum Institute specifications. The thermal behaviour and rheological properties of the clays might be interpreted according to the intrinsic properties of the clay minerals, such as layer charge and charge distribution.

An assemblage of clay minerals, Na-zeolites, Fe- and Sb-sulfides, and borates occurs in cavities in the basalt flow that underlies the main borax ore body exposed by the U.S. Borax open pit at Boron, Kern County, California. Analcime and saponite are widespread in cavities and fractures near the top of the basalt. Part of the basalt recently exposed, although restricted in distribution, contains a much more diverse assemblage of diagenetic minerals. Although only a few different minerals occur in each cavity, the composite order of deposition is: ferroan saponite, pyrrhotite, saponite, phillipsite, gmelinite, clinop-tilolite, herschelite, analcime, greigite, rhodochrosite, searlesite, borax, calcite, and colemanite. In some cavities early-formed zeolites appear to have dissolved as later ones crystallized.

Microprobe analyses of the zeolites yielded the following compositions: phillipsite, Na4.28K0.08(Al4.36-Si11.64O32)·xH2O (Si/Al = 2.67); gmelinite, Na6.64K0.10(Al6.80Si17.22O48· xH2O (Si/Al = 2.53); herschelite, Na2.96K0.24(Al3.21Si8.79O24)· xH2O (Si/Al = 2.75); clinoptilolite, Na6.48K0.56(Al7.15Si28.77O72) · xH2O (Si/Al = 4.02); analcime, Na14.02K0.09(Al13.33Si34.44O96)· xH2O (Si/Al = 2.58).

These zeolites formed by reactions between the basalt and evolving fluids, which were controlled by the development of an overlying, Na-borate lake. The wide distribution of saponite, analcime, and searlesite suggests that these minerals formed by the diagenetic reaction between basaltic glass and Na- borate water. The localized occurrence of the complex mineral assemblage, including some of the saponitic clay and the zeolites, phillipsite, gmelinite, and clinoptilolite, formed through a different process, perhaps deuteric or hydrothermal alteration of the basalt, before the lake developed. Because these zeolites are extraordinarily rich in Na, cation exchange may have taken place as the pore fluids became increasingly Na rich. In the latest stages of mineral growth, each of these zeolites was apparently partially dissolved and epitaxially overgrown by a second generation crystal, phillipsite on phillipsite, herschelite on gmelinite, and sodian heulandite on clinoptilolite.

The rehydration properties and behavior of interlayer cations of Ca-, Mg-, Na-, and K-saturated homoionic saponite and vermiculite heated at various temperatures were examined and their rehydration mechanisms elucidated. The most notable features of saponite were (1) except for the Mg-saturated specimen, all saponite samples rehydrated until the crystal structure was destroyed by heating; (2) the rehydration rate in air after heating decreased in the order: K+ > Na+ > Ca2+ > Mg2+; (3) the interlayer cations apparently migrated into hexagonal holes of the SiO4 network on thermal dehydration; and (4) the b-parameter expanded on thermal dehydration. The rehydration properties and behavior of interlayer cations of vermiculite were: (1) except for the K-saturated specimen, all vermiculite samples rehydrated until the crystal structure was destroyed by heating; (2) the rehydration rate in air after heating decreased in the order: Mg2+ > Ca2+ > Na+ > K+; (3) the interlayer cations apparently did not migrate into the hexagonal holes, but remained at the center of the interlayer space, even after thermal dehydration; and (4) except for the K-saturated specimen, the 6-parameters of the samples contracted on thermal dehydration. The different rehydration properties of saponite and vermiculite were apparently due to the behavior of the interlayer cations during thermal dehydration. For rehydration to occur, the interlayer cations of saponite had to migrate out of the hexagonal holes. Consequently, saponite saturated with a large cation rehydrated rapidly, whereas saponite saturated with a small cation rehydrated slowly. On the other hand, the interlayer cations of vermiculite remained in the interlayer space; therefore, the rehydration properties of vermiculite were strongly affected by the hydration energies of the interlayer cations. Furthermore, electron diffraction patterns suggested that the saponite and vermiculite consisted of random stacking and ordered stacking of adjacent 2:1 layers, respectively. The nature of the stacking of the minerals seemed to be the most important factor controlling the behavior of interlayer cations in the thermal dehydration process.

To understand the genetic relationship between hisingerite material in the joints of an overlying grey basalt and nontronite and Fe-rich saponite in the joints and matrix of a more deuterically altered, underlying green basalt, the hisingerite material was treated in a series of hydrothermal experiments. No well-ordered clay mineral was produced at temperatures <340°C, although extended treatment for 445 days and at 110°C or 42 days at 180°C resulted in the formation of materials that gave broad, weak, basal X-ray powder diffraction (XRD) reflections characteristic of 2:1 phyllosilicates. Hematite did not form at 110°C, but it did form at 180°C in 1- and 6-week runs. Treatments at 340°C in Pt, Ag-Pd, and Au containers resulted in mixtures of Fe-rich saponite + hematite, but the same starting material treated at 340°C in stainless steel yielded, in addition, some chlorite, probably due to the more reducing conditions in the stainless steel container. Treatment of the unaltered grey basalt at 340°C and 50 MPa for 10 days resulted in complete alteration of olivine (and probably glass) to a trioctahedral smectite.

The Fe-rich saponite produced by the hydrothermal treatment of the hisingerite material has a composition and XRD pattern similar to the Fe-rich saponite found in the green basalt and an XRD pattern similar to that produced by the hydrothermal treatment of the grey basalt; thus these clays may have had a similar origin. The compositions and XRD patterns of these clays are not similar, however, to those of the nontronite in the joints of the green basalt. The nontronite probably formed during a subsequent low-temperature alteration.

Iddingsite rimming olivine in a basanite from the Limberg, Germany, is composed of saponite and goethite. Transmission electron microscopy of ion-thinned, oriented crystals suggests a two-stage alteration process. At first, the olivine breaks into a mosaic of 50-Å diameter {110} bounded needle-shaped domains which change to a metastable hexagonal phase having a = 3.1 Å and c = 4.6 Å, probably of close-packed, metal-oxygen octahedra. This reaction opens solution channels in the olivine which are detectable from about 20-Å diameter and are parallel to the olivine y-axis. Laths of smectite, one or two layers thick, 20 Å wide, and as much as 100 Å long parallel to their y-axis nucleate from the metastable phase and begin to fill in the solution channels. The laths orient with smectite (001) parallel to olivine (100). As the channels widen, prismatic {110} goethite crystals form directly from the metastable hexagonal phase. This first stage thus provides heterogeneous nuclei of smectite and goethite, formed epitactically and perhaps topotactically from a metastable intermediary.

In a second stage,these nuclei enlarge by deposition from solution as water migrates readily through the solution channels. A reduction in total volume allows smectite veins to form, misoriented with respect to the olivine. The reaction conserves iron, requires the addition of aluminum and water, and releases magnesium and silicon. Electron microprobe analyses of the iddingsite indicate that the smectite is saponite.

The adsorption of tris(1,10-phenanthroline)-ruthenium(II) (Ru(phen)32+) by two kinds of colloidally dispersed clays, sodium montmorillonite and synthetic saponite, was studied by spectrophotometric and electron-optical methods. Montmorillonite adsorbed this complex stronger than saponite. The electronic spectrum measurements suggested that the electronic states of the complex were more perturbed on adsorption by montmorillonite than by saponite. High-performance liquid chromatography was attempted on an ion-exchanged adduct of optically active Ru(phen)32+ and these clays. 1,1′-Binaphthol was eluted with a 1:1 (v/v) water-methanol mixture as an eluent. The compound was resolved with a separation factor of 15 and 1.4 on the saponite and montmorillonite columns, respectively. If tris(acetylacetonato)-chromium(III) was eluted with water, the compound was resolved with separation factors of 1.9 and 11 on the saponite and montmorillonite columns, respectively. These separation tendencies were probably due to the difference in the external surface area and the density of the bound chelates.

Saponites and vermiculites may assume at least 11 ordered or semi-ordered layer stacking sequences. For a given relative humidity, the layer stacking type assumed is a function of the nature of the interlayer cation, the layer charge density, the mean size of the particles, and the di- or trioctahedral character of the sheets. For each interlayer cation, a succession of layer stacking types can be observed as relative humidity increases. For high relative humidity, some particular layer stacking types exist, but only for low-charge minerals. No other differences have been found for saponites and vermiculites in each successive layer stacking type. The degree of order that these layer stacking types imply is probably due to the existence of electrostatic bonds between hydrated interlayer cations and surface oxygens of the substituted tetrahedra. For octahedrally substituted 2:1 phyllosilicates, however, the disorder of the layer stacking sequences is related to a highly delocalized distribution of negative charges on the surface oxygens of the layers.

A study of the superstructures detected in saponites and vermiculites indicates that the interlayer cations tend to be located as far as possible from one another. The superstructures exist only with some cations and some layer stacking types and if the layer charge density is compatible with the charge produced by the cation distribution in this kind of superstructure.

Gabbroic saprolite at Mont Mégantic, Quebec, was studied in detail mineralogically to gain a better understanding of the origin of ferruginous smectite reported previously from these rocks. The parent rock is a ferrogabbro composed of plagioclase, augite, calc-alkalic amphiboles, biotite, olivine, magnetite, ilmenite, and apatite. Extensive weathering has decomposed most of the mafic minerals and magnetite to goethite, lepidocrocite, and iron-rich clay minerals, which occur in numerous microcracks distributed irregularly in the outer shells of boulders and in cracks and fissures in the bedrock. Some of the felsic minerals have altered to kaolinite. The secondary minerals and the more resistant primary minerals, such as plagioclase, ilmenite, and apatite, have subsequently moved to the lower part of the saprolite. The major ferruginous clay minerals present are smectite and vermiculite, which are compositionally similar, except that the smectite is slightly richer in SiO2 and MgO and poorer in Fe2O3 than the vermiculite.

To establish a structural formula for the ferruginous smectite, the oxidation state of Fe in a sample treated with dithionite-citrate-bicarbonate was examined by Mössbauer spectroscopy. All structural iron was found to be ferric. The calculated structural formula of a Na-saturated sample is:

$$N{a_{0.61}}\left( {M{g_{1.39}} + Fe_{0.85}^{3 + }A{l_{0.17}}M{n_{0.03}}} \right)\left( {S{i_{3.49}}A{l_{0.51}}} \right){O_{10}}{\left( {OH} \right)_2},$$

To determine the dependency of rheological properties on mineralogical characteristics, eleven organophilic hectorite and nine organophilic saponite samples were prepared from hydrothermally synthesized smectites and a quaternary ammonium salt containing mainly octadecyl groups as alkyl chains and were examined by X-ray powder diffraction (XRD), particle size, chemical, and rheological procedures. The rheological properties of the organophilic products dispersed in toluene were found to depend chiefly on (1) particle size, (2) the expansion behavior of the stacked clay platelets, in which the amount and location of negative charge of the silicate layer affected the expandability, and (3) the amount of methanol added as a polar activator. By the addition of 2–4% methanol, apparent viscosities of the hectorite clays were drastically increased at low shear rate, although even a 10% addition did not significantly affect the saponite clays. Under the most suitable gellation conditions, the saponite clays showed lower apparent viscosity than the hectorite clays. Inasmuch as the former had its negative charge near the surface of the silicate layer and formed strong bonds to the organic ion, the expansion of the platelets was apparently difficult. An optimum layer charge for increasing the viscosity was found; specifically 0.45–0.50 and 0.50–0.55 equivalents per half unit cell for saponite and hectorite clays, respectively. Because the saponite clays contained a denser population of organic ions than the hectorite clays, the ion density in the interlayer of both clays was considered to be the same, i.e., about 0.4 organic ions per unit ab area.

Phyllosilicates are major components of the <2-μm fraction (1–3 wt. % of most bulk specimens) in more than 50 samples of air-fall tephra from several 1980 eruptions of Mount St. Helens. In all samples, trioctahedral smectite is the major clay mineral. The integral series of 00ℓ reflections in ethylene glycol-treated samples indicates a lack of interstratification; absence of a peak near 5 Å after heat treatment, the 060 peak at 1.54 Å, and energy dispersive chemical analyses indicate that the smectite is a Mg- and Fe- rich, trioctahedral saponite. Minor mica and chlorite are present in the <2-μm fraction of most samples, and some samples show a peak near 12 Å after heating to 550°C which is probably due to the presence of an interstratifled chlorite/collapsed smectite or chlorite/collapsed vermiculite. The tephra contains glass and crystals originating from new magma and lithic fragments incorporated from the pre-existing cone. The clay minerals in the tephra are lithic components stripped from older, hydrothermally altered rocks during explosive ejection. Cleaned pumice fragments, which are new magmatic components, lack smectite, but contain rare biotite in xenoliths. Old, hydrothermally altered rocks from the volcano's summit and from the debris-avalanche (former north flank) contain saponite together with chlorite and chlorite/smectite which may have formed from it. Saponite and zeolites that precipitated from neutral to alkaline hydrothermal solutions line cavities in some of these rocks. The saponite was probably not subjected to magmatic temperatures because heating this material for 5 min at 750°C collapses it irreversibly to 10 Å. Kaolinite, alunite, and opal, indicative of acid-sulfate alteration, were found only in the pre-1980 summit crater and the southwest thermal area, but were not evident in the lithic components of the 1980 deposits.

Burial diagenetic reactions of di- and trioctahedral clay minerals were compared in Brazilian offshore, basinal sediment sequences of Cretaceous age. Originally dioctahedral smectite-rich shales of three basins—Potiguar,Ceara, and Ilha de Santana—exhibited the classical smectite-to-illite burial pattern. Trioctahedral clay-rich shales and trioctahedral clay-mineral cements in sandstones, however, showed a burial sequence of saponite to mixed-layer chlorite/saponite with progressive increase in the percentage of chlorite layers with increasing burial depth.

The change from disordered to ordered interstratifications of chlorite/saponite occurred in the temperature range 60°-70°C at a vitrinite reflectance value of about 0.65. These values are lower than those found for the ordering of illite/smectite clays. Increasing substitution of Al for Si in tetrahedral sites, followed by fixation of interlayer hydroxide sheets was found to be the major chemical change promoting transformation of saponite to chlorite via corrensite.

Oxygen isotope analyses (δO18) of micas that were artificially depleted in K+ indicate little or no isotope exchange during the transformation. The oxidation of iron in K-depleted, iron-rich micas by H2O2 treatment resulted in 1.6 to 4.6% decrease in SO18 due to the fact that the equilibrium fractionation factor is less than the initial difference between the starting δO18 of the fluid and micas. The oxygen isotope ratio of a saponite formed by the weathering of phlogopite showed a 9.7% increase in δO18 due to authigenic recrystallization. These results suggest that oxygen isotope ratios can be used to determine the nature of chemical transformation during the weathering of mica to vermiculite and/or smectite.

White saponite occurs in joints and open fracture zones in metamorphosed dolomitic limestone near Ballarat, California. The saponite appears to have formed by hydrothermal alteration, possibly during Pliocene times. The material shows a 06l X-ray powder diffraction peak at 1.529 Å, A12O3 and MgO contents of about 4.4 and 23.5%, respectively, and a half-cell octahedral atoms summation value of about 2.82. The saponite appears to consist of a single, uniform clay species; the main impurities are fine shards of diopside and tremolite. The infrared and thermal properties of the Ballarat saponite are similar to those of the Allt Ribhein saponite. It has a lower water-holding capacity than montmorillonite and is characterized by lower Atterberg limits and expansion pressures and higher compaction densities. The apparent density of the saponite, 2.865 g/cm3, is greater than that of montmorillonite. This saponite is available from the Source Clays Repository of The Clay Minerals Society.

Solutions containing AlCl3 and Si(OH)4 (concentrations ≤ 1.5 mM with molar Si:Al ratios of 1:2, 1:1 and 3:1) and FeCl2 (0, 0.5 and 1.0 mM) were adjusted to pH 8 with Ca(OH)2, and incubated at 23°C and 89°C without exclusion of air in the presence of CaCO3 for 8–12 weeks. The products were characterized by infrared spectroscopy and X-ray diffraction. Systems with 3:1 and 1:1 Si:Al ratios without Fe gave hydrous feldspathoids at 23° and 89°C. Systems with 3:1 Si:Al ratios containing Fe gave aluminous nontronites at 89°C and noncrystalline, nontronite-like products at 23°C. Systems with 1:1 Si:Al ratios with added Fe gave Fe(III)-substituted hydrous feldspathoids at 23°C. At 89°C, the system with 1:1 Si:Al ratios and 0.5 mM Fe produced a “protohalloysite,” while that with 1.0 mM Fe gave a poorly ordered nontronite-like layer silicate. In systems with 1:2 Si:Al ratios, the formation of “protoimogolite” at 23°C was little affected by additions of Fe. At 89°C, the “protoimogolite” decomposed to boehmite and poorly-ordered layer silicate phases. Inclusion of 1 mM MgCl2 in the above systems had no effect on the products at 23°C, but at 89°C produced saponites and a mixed layer saponite-chlorite in the 3:1 Si:Al systems, and saponite-like layer structures in the 1:1 and 1:2 Si:Al systems.

Saponite crystallizes from amorphous gel having an ideal saponite composition within 7 days at all experimental temperatures between 300° and 550°C at 1 kbar pressure. Reactions subsequent to this initial crystallization vary in type and degree, depending on the temperature of reaction and the type of interlayer cation. Above 450°C, the intitially crystallized K-saponite dissolves, and talc and phlogopite nucleate and grow as discrete phases. At 450°C the initial K-saponite reacts to form talc and phlogopite layers, but the reaction proceeds via intracrystalline layer transformations rather than via dissolution and precipitation, producing a mixture of fully ordered, interstratified talc/saponite and fully ordered saponite/phlogopite. The K-saponite shows subtle signs of reaction at 400°C after 200 days: this temperature is at least 150°C lower than experimental reaction temperatures previously reported for saponites. No reactions beyond the initial crystallization of saponite were observed below 400°C. K-saponite reacts more rapidly than either Na-saponite or Ca-saponite above 400°C, and the Na-saponite and Ca-saponite produce no mica layers during their transformation to mixed-layer clays. Interstratified talc/saponite formed in the Na-saponite system, and the Ca-saponite system produced both talc/saponite and chlorite/saponite.

A material resembling what has been described in the literature as hisingerite has been identified in the joints of Cenozoic grey basalt near Geelong, Victoria. The material gives an X-ray powder diffraction pattern with weak broad peaks at 4.49, 2.58, and 1.53 Å and a chemical analysis of SiO2 = 34.2, TiO2 = 0.28, A12O3 = 4.16, Fe2O3 = 17.10, FeO = 3.52, MnO = 0.04, MgO = 5.19, CaO = 0.68, Na2O = 0.26, P2O5 = 0.05, H2O+ = 5.3, H2O− = 25.4, total = 96.18% (3.82% siderite impurity). Assuming a smectite model with 44 negative charges per unit cell, the chemical composition gives a unit-cell formula with −1.14 charges, corresponding to a cation-exchange capacity (CEC) of 135 meq/100 g. The same unit-cell charge is obtained when the structural formula is calculated on the basis of (Si + Al) = 8. The measured CEC of 60 meq/100 g indicates that the smectite structure may not apply to this hisingerite-material, and that in the alternative structure, all of the Al atoms may not be in the tetrahedral positions. Although the specific surface area of this material is between 662 and 758 m2/g (comparable to that of smectite), the infrared spectra do not show the characteristic OH vibrations, and the thermal analysis curves lack the dehydroxylation peak at 550–600°C expected for a smectite. Transmission electron microscopy shows indications of a layer structure and apparently hollow spherical bodies with ‘onion’ structure of 140 to 200 Å internal diameter, which might represent the sites of the large amounts of water contained by the material. The mode of occurrence and the nature of the hisingerite-material suggest precipitation from solution at relatively low temperatures.

The basal spacings of complexes of saponite with five cationic dyes, 1,1′-diethyl-2,2′-cyanine, crystal violet, methylene blue, 1,1′-diethyl-2,2′-carbocyanine, and 1,1′-diethyl-2,2′-dicarbocyanine, varied with degree of saturation of each dye. At low loading of dye to saponite, each cationic dye showed nearly the same absorption spectrum in the UV-visible region as that of its dilute aqueous solution, whereas the spectrum changed distinctly at high loading. With increasing degree of dye loading, the absorption band shifted to longer wavelength for 1,1′-diethyl-2,2′-cyanine (J band) and to shorter wavelength for the others (D, H bands). On the basis of the basal spacing of each respective dye-clay complex, the orientation of the intercalated dye molecules is proposed as follows: the major plane of the cationic dye lies horizontal to the 2:1 layer surface at low loading. With increasing loading, the dye molecules interact with adjacent dye molecules and orient vertically to the 2:1 layer at high loading near the cation-exchange capacity.