Victor A. Drits

Investigation of smectite hydration properties by modeling experimental X-ray diffraction patterns: Part I. Montmorillonite hydration properties

Abstract Hydration of the <1 μm size fraction of SWy-1 source clay (low-charge montmorillonite) was studied by modeling of X-ray diffraction (XRD) patterns recorded under controlled relative humidity (RH) conditions on Li-, Na-, K-, Mg-, Ca-, and Sr-saturated specimens. The quantitative description of smectite hydration, based on the relative proportions of different layer types derived from the fitting of experimental XRD patterns, was consistent with previous reports of smectite hydration. However, the coexistence of smectite layer types exhibiting contrasting hydration states was systematically observed, and heterogeneity rather than homogeneity seems to be the rule for smectite hydration. This heterogeneity can be characterized qualitatively using the standard deviation of the departure from rationality of the 00l reflection series (ξ), which is systematically larger than 0.4 Å when the prevailing layer type accounts for ~70% or less of the total layers (~25% of XRD patterns examined). In addition, hydration heterogeneities are not distributed randomly within smectite crystallites, and models describing these complex structures involve two distinct contributions, each containing different layer types that are interstratifed randomly. As a result, the different layer types are partially segregated in the sample. However, these two contributions do not imply the actual presence of two populations of particles in the sample. XRD profile modeling also has allowed the refinement of structural parameters, such as the location of interlayer species and the layer thickness corresponding to the different layer types, for all interlayer cations and RH values. From the observed dependence of the latter parameter on the cation ionic potential (v/r; v = cation valency and r = ionic radius) and on RH, the following equations were derived: Layer thickness (1W) = 12.556 + 0.3525 × (v/r . 0.241) × (v × RH . 0.979) Layer thickness (2W) = 15.592 + 0.6472 × (v/r . 0.839) × (v × RH . 1.412) which allow the quantification of the increase of layer thickness with increasing RH for both 1W (one water) and 2W (two water) layers. In addition, for 2W layers, interlayer H2O molecules are probably distributed as a unique plane on each side of the central interlayer cation. This plane of H2O molecules is located at ~1.20 Å from the central interlayer cation along the c* axis

Structure of synthetic monoclinic Na-rich birnessite and hexagonal birnessite; I, Results from X-ray diffraction and selected-area electron diffraction

Abstract Synthetic Na-rich birnessite (NaBi) and its low pH form, hexagonal birnessite (HBi), were studied by X-ray and selected-area electron diffraction (XRD. SAED). SAED patterns were also obtained for synthetic Sr-exchanged bimessite (SrBi) microcrystals in which Sr was substituted for Na. XRD confirmed the one-layer monoclinic stmcture of NaBi and the one-layer hexagonal structure of HBi with subcell parameters a = 5.172 Å, b = 2.849 Å, c = 7.34 Å, β = 103.3° and a = 2.848 Å, c = 7.19 Å, γ = 120°, respectively. In addition to super-reflection networks, SAED patterns for NaBi and SrBi contain satellite reflections. On the basis of these experimental obervations, structural models for NaBi and HBi are proposed. NaBi consists of almost vacancy-free Mil octahedral layers. The departure from the hexagonal symmetry of layers is caused by the Jahn-Teller distortion associated with the substitution of Mn3+ for Mn4+. The supercell A = 3a parameter arises from the ordered distribution of Mn3+-rich rows parallel to [010] and separated from each other along [100] by two Mn4+ rows. The superstructure in the b direction of NaBi type II (B = 3b) comes from the ordered distribution of Na cations in the interlayer space. The maximum value of the layer negative charge is equal to 0.333 v.u. per Mn atom and is obtained when Mn3+-rich rows are free of Mn4+. The idealized structural formula proposed for NaBi type II is Na0.333(Mn4+0.222Mn3+0.222Mn2+0.055)O2. NaBi type I has a lower amount of Mn3+ and its ideal composition would vary from Na0.167(Mn4+0.833Mn3+0.167)O2 to Na0.25(Mn4+0.25Mn3+0.25)O2. Satellites in SAED patterns of NaBi crystals result from the ordered distribution of Mn4+ and Mn2+ pairs in Mn3+-rich rows with a periodicity of 6b. The structure of HBi consists of hexagonal octahedral layers containing predominantly Mn4+ with variable amounts of Mn3+ and layer vacancies. The distribution of layer vacancies is inherited from the former Mn3+ distribution in NaBi. Interlayer Mn cations are located above or below vacant layer sites. The driving force of the NaBi to HBi transformation is probably the destabilization of Mn3+-rich rows at low pH.

XRD Measurement of Mean Crystallite Thickness of Illite and Illite/Smectite : Reappraisal of the Kubler Index and the Scherrer Equation

The standard form of the Scherrer equation, which has been used to calculate the mean thickness of the coherent scattering domain (CSD) of illite crystals from X-ray diffraction (XRD) full width data at half maximum (FWHM) intensity, employs a constant, Ksh, of 0.89. Use of this constant is unjustified, even if swelling has no effect on peak broadening, because this constant is valid only if all CSDs have a single thickness. For different thickness distributions, the Scherrer “constant” has very different values.Analysis of fundamental particle thickness data (transmission electron microscopy, TEM) for samples of authigenic illite and illite/smectite from diagenetically altered pyroclastics and filamentous illites from sandstones reveals a unique family of lognormal thickness distributions for these clays. Experimental relations between the distributions’ lognormal parameters and mean thicknesses are established. These relations then are used to calculate the mean thickness of CSDs for illitic samples from XRD FWHM, or from integral XRD peak widths (integrated intensity/maximum intensity).For mixed-layer illite/smectite, the measured thickness of the CSD corresponds to the mean thickness of the mixed-layer crystal. Using this measurement, the mean thickness of the fundamental particles that compose the mixed-layer crystals can be calculated after XRD determination of percent smectitic interlayers. The effect of mixed layering (swelling) on XRD peak width for these samples is eliminated by using the 003 reflection for glycolated samples, and the 001, 002 or 003 reflection for dehydrated, K-sa-turated samples. If this technique is applied to the 001 reflection of air-dried samples (Kubler index measurement), mean CSD thicknesses are underestimated due to the mixed-layering effect.The technique was calibrated using NEWMOD©-simulated XRD profiles of illite, and then tested on well-characterized illite and illite/smectite samples. The XRD measurements are in good agreement with estimates of the mean thickness of fundamental particles obtained both from TEM measurements and from fixed cations content, up to a mean value of 20 layers. Correction for instrumental broadening under the conditions employed here is unnecessary for this range of thicknesses.

Structure of synthetic monoclinic Na-rich birnessite and hexagonal birnessite: II. Results from chemical studies and EXAFS spectroscopy

Abstract Solution chemical techniques were used to study the conversion of synthetic Na-rich buserite (NaBu) to hexagonal (H+-exchanged) birnessite (HBi) at low pH. The low-pH reaction is broadly characterized by the exchange of structural Na+ with solution H+ and the partial loss of Mn2+ to the aqueous phase. The desorption of Na+ in two temporally distinct steps indicates the existence of two types of binding sites for this cation. Mn2+ appears to originate from a partial disproportionation of Mn31 in the NaBu layers, according to the sequence Mn3+layer + Mn3+layer → Mn4+layer + Mn2+layer → Mn4+layer + Vacancy + Mn2+aq. EXAFS measurements on Na-rich birnessite (NaBi) show that this mineral is primarily a layered structure formed by edge-sharing MnO6 octahedra, with no evidence for triplecorner (TC) sharing Mn. HBi is significantly different with strong evidence for TC-sharing Mn and therefore layer vacancies. The relative numbers of edge (E)-sharing and TC-sharing neighbors determined from EXAFS measurements on HBi is consistent with SAED results (Drits et al. 1997), which suggest that the layer vacancies are restricted to every third row of Mn cations, with 50% of the Mn sites along these rows vacant. The density of vacancies in the entire layer is therefore one in six of layer Mn sites. Polarized EXAFS measurements on orientated films of NaBi and HBi confirm the absence of TC-sharing Mn in NaBi and indicate that Mn adsorbed at layer vacancy sites in HBi at pH 4 is dominantly Mn3+. The intensity of the TC-sharing contribution to the Mn EXAFS spectra of HBi samples increases with increasing pH from pH 2 to 5, and supports a mechanism of formation involving both the direct migration of layer Mn3+ to interlayer TC-sharing positions and re-adsorption of Mn2+ from solution onto layer vacancy sites. The migration of Mn31 cations into the interlayer releases the steric strain associated with the Jahn-Teller distortion of these octahedra. This model of the NaBu-to-HBi conversion explains the transformation from orthogonal to hexagonal layer symmetry, respectively, as reported by Drits et al. (1997). Analysis of the Zn EXAFS spectrum of Zn21-exchanged birnessite shows that Zn2+ also occupies TC-sharing positions at layer vacancy sites. The results of this study strongly suggest that lattice cation vacancies are of critical importance in adsorption and electron transfer processes occurring at the surface of this mineral.

Structure of heavy metal sorbed birnessite. Part III: Results from powder and polarized extended X-ray absorption fine structure spectroscopy

The local structures of divalent Zn, Cu, and Pb sorbed on the phyllomanganate birnessite (Bi) have been studied by powder and polarized extended X-ray absorption fine structure (EXAFS) spectroscopy. Metal-sorbed birnessites (MeBi) were prepared at different surface coverages by equilibrating at pH 4 a Na-exchanged buserite (NaBu) suspension with the desired aqueous metal. Me/Mn atomic ratios were varied from 0.2% to 12.8% in ZnBi and 0.1 to 5.8% in PbBi. The ratio was equal to 15.6% in CuBi. All cations sorbed in interlayers on well-defined crystallographic sites, without evidence for sorption on layer edges or surface precipitation. Zn sorbed on the face of vacant layer octahedral sites (□), and shared three layer oxygens (Olayer) with three-layer Mn atoms (Mnlayer), thereby forming a tridentate corner-sharing (TC) interlayer complex (Zn-3Olayer-□-3Mnlayer). TCZn complexes replace interlayer Mn2+ (Mninter2+) and protons. TCZn and TCMninter3+ together balance the layer charge deficit originating from Mnlayer4+ vacancies, which amounts to 0.67 charge per total Mn according to the structural formula of hexagonal birnessite (HBi) at pH 4. At low surface coverage, zinc is tetrahedrally coordinated to three Olayer and one water molecule ([IV]TC complex: (H2O)-[IV]Zn-3Olayer). At high loading, zinc is predominantly octahedrally coordinated to three Olayer and to three interlayer water molecules ([VI]TC complex: 3(H2O)-[VI]Zn-3Olayer), as in chalcophanite ([VI]ZnMn34+O7·3H2O). Sorbed Zn induces the translation of octahedral layers from −a/3 to +a/3, and this new stacking mode allows strong H bonds to form between the [IV]Zn complex on one side of the interlayer and oxygen atoms of the next Mn layer (Onext): Onext…(H2O)-[IV]Zn-3Olayer. Empirical bond valence calculations show that Olayer and Onext are strongly undersaturated, and that [IV]Zn provides better local charge compensation than [VI]Zn. The strong undersaturation of Olayer and Onext results not only from Mnlayer4+ vacancies, but also from Mn3+ for Mn4+ layer substitutions amounting to 0.11 charge per total Mn in HBi. As a consequence, [IV]Zn,Mnlayer3+, and Mnnext3+ form three-dimensional (3D) domains, which coexist with chalcophanite-like particles detected by electron diffraction. Cu2+ forms a Jahn-Teller distorted [VI]TC interlayer complex formed of two oxygen atoms and two water molecules in the equatorial plane, and one oxygen and one water molecule in the axial direction. Sorbed Pb2+ is not oxidized to Pb4+ and forms predominantly [VI]TC interlayer complexes. EXAFS spectroscopy is also consistent with the formation of tridentate edge-sharing ([VI]TE) interlayer complexes (Pb-3Olayer-3Mn), as in quenselite (Pb2+Mn3+O2OH). Although metal cations mainly sorb to vacant sites in birnessite, similar to Zn in chalcophanite, EXAFS spectra of MeBi systematically have a noticeably reduced amplitude. This higher short-range structural disorder of interlayer Me species primarily originates from the presence of Mnlayer3+, which is responsible for the formation of less abundant interlayer complexes, such as [IV]Zn TC in ZnBi and [VI]Pb TE in PbBi.

An improved model for structural transformations of heat-treated aluminous dioctahedral 2:1 layer silicates

An improved model for the interpretation of thermal effects during dehydroxylation in aluminous dioctahedral 2:1 layer phyllosilicates considers trans-vacant (tv) and cis-vacant (cv) 2:1 layers and leads to very different temperatures of dehydroxylation for these tv and cv vacant modifications. In particular, smectites and illites consisting of cv 2:1 layers are characterized by dehydroxylated temperatures which are higher by 150°C to 200°C than those for the same minerals consisting of the tv 2:1 layers. A considerable lengthening of the OH-OH edges in cv 2:1 layers in comparison with the OH-OH edges in the tv 2:1 layers is postulated as the reason for the higher dehydroxylation.Dehydroxylation in aluminous cv 2:1 layer silicates should occur in two stages. Initially, each two adjacent OH groups are replaced by a residual oxygen atom and the Al cations, which originally occupied cis -and trans-sites, become 5- and 6-coordinated, respectively. The structure of 2:1 layers corresponding to this stage of the dehydroxylation is unstable. Thus the Al cations migrate from the former trani-sites to vacant pentagonal prisms. The resulting dehydroxylated structure of the original cv 2:1 layers is similar to that of the former tv 2:1 layers.Diffraction and structural features of the cv dehydroxylates predicted by the model are in agreement with X-ray diffraction effects observed for cv illite, illite-smectite and montmorillonite samples heated to different temperatures. In particular, the diffusion of Al cations to empty five-fold prisms during dehydroxylation of the tv 2:1 layers explains why dehydroxylation of reheated cv montmorillonites occurs at temperatures lower by 150°C to 200°C than samples that were not recycled.

Structure of H-exchanged hexagonal birnessite and its mechanism of formation from Na-rich monoclinic buserite at low pH

Abstract The structural transformation of high pH Na-rich buserite (NaBu) to H-exchanged hexagonal birnessite (HBi) at low pH was studied by simulation of experimental X-ray diffraction patterns. Four HBi samples were prepared by equilibration of NaBu at constant pH in the range pH 5-2. The samples differ from each other by the presence of one (at pH 2 and 3) or two (at pH 4 and 5) phases, and by the structural heterogeneity of these phases which decreases with decreasing pH. The sample obtained at pH 5 is a 4:1 physical mixture of a 1H phase (a = 4.940 Å, b = a/√3 = 2.852 Å, c = 7.235 Å, β = 90°, γ = 90°) and of a 1M phase (a = 4.940 Å, b = a/√3 = 2.852 Å, c = 7.235 Å, β = 119.2°, γ = 90°) in which successive layers are shifted with respect to each other by +a/3 along the a axis as in chalcophanite. Both the 1H and 1M phases contain very few well-defined stacking faults at pH 5. At pH 4, the sample is a 8:5 physical mixture of a 1H phase containing 15% of monoclinic layer pairs and of a 1M phase containing 40% of orthogonal layer pairs. Any further decrease of the pH leads to the formation of a single defective 1H phase. This 1H phase contains 20% and 5% of monoclinic layer pairs at pH 3 and 2, respectively. Independent of pH, all phases contain 0.833 Mnlayer cations, 0.167 vacant layer sites, and 0.167 interlayer Mn cations located either above or below layer vacancies per octahedron. A structural formula is established at each pH. The origin of the observed phase and structural heterogeneities has been analyzed. 1H and 1M phases are assumed to inherit their specific structural and crystal chemical features from the two distinct NaBu modifications. NaBu type I, with a high proportion of Mn4+layer cations, is thought to be responsible for the monoclinic layer stacking because this configuration allows Mn cations from adjacent layers to be as far as possible from each other, thus minimizing the electrostatic repulsion between these high charge cations. In contrast, NaBu type II has a high interlayer charge induced by Mn3+layer for Mn4+layer substitutions. Consequently, the 1H phase has a high amount of interlayer protons and achieves compensation of the unfavorable overlap of layer and interlayer Mn cations, in projection on the ab-plane, by the presence of strong hydrogen bondings between layers. The higher proportion of defined stacking faults in both 1H and 1M phases at pH 4 compared to pH 5 can be attributed to the increase in reaction rate with decreasing pH. At lower pH (3 and 2) the formation of strong hydrogen bonds between adjacent layers controls the layer stacking mode and leads to the formation of a unique 1H phase. The proportion of well-defined stacking faults in this phase decreases from pH 3 to 2.

CLAY MINERALS IN THE MEUSE-HAUTE MARNE UNDERGROUND LABORATORY (FRANCE): POSSIBLE INFLUENCE OF ORGANIC MATTER ON CLAY MINERAL EVOLUTION

A clay-rich Callovo-Oxfordian sedimentary formation was selected in the eastern Paris Basin (MHM site) to host an underground laboratory dedicated to the assessment of nuclear waste-disposal feasibility in deep geological formations. As described initially, this formation shows a mineralogical transition from an illite-smectite (I–S) mixed-layered mineral (MLM), which is essentially smectitic and randomly interstratified (R = 0) in the top part of the series to a more illitic, ordered (R ⩾ 1) I–S in its deeper part.This description has been challenged by using the multi-specimen method developed by Drits et al. (1997a) and Sakharov et al. (1999). It is shown that all samples contain a physical mixture of an unusually (?) illitic (∼65% I) randomly interstratified I-Exp (illite-expandable MLM) and of a discrete smectite, in addition to discrete illite, kaolinite and chlorite. Structural parameters of the different clay phases vary little throughout the series. According to the proposed model, the mineralogical transition corresponds to the disappearance of smectite with increasing burial depth.Comparison with clay minerals from formations of similar age (Oxfordian-Toarcian) throughout the Paris Basin shows that the clay mineralogy in the deeper part of the series originates from a smectite-to-illite transition resulting from a low-temperature burial diagenesis. The anomalous lack of evolution of clay minerals in the upper part of the series is thought to be related to specific interactions between organic matter and clay minerals.

SUMMARY OF RECOMMENDATIONS OF NOMENCLATURE COMMITTEES RELEVANT TO CLAY MINERALOGY: REPORT OF THE ASSOCIATION INTERNATIONALE POUR L'ETUDE DES ARGILES (AIPEA) NOMENCLATURE COMMITTEE FOR 2006

Brindley et al. (1951) reported the earliest efforts to obtain international collaboration on nomenclature and classification of clay minerals, initiated at the International Soil Congress in Amsterdam in 1950. Since then, national clay groups were formed, and they proposed various changes in nomenclature at group meetings of the International Clay Conferences. Most of the national clay groups have representation on the Nomenclature Committee of the Association Internationale pour l’Etude des Argiles (AIPEA, International Association for the Study of Clays), which was established in 1966. The precursor committee to the AIPEA Nomenclature Committee was the Nomenclature Subcommittee of the Comite International pour l’Etude des Argiles (CIPEA, International Committee for the Study of Clays). The AIPEA Nomenclature Committee has worked closely with other international groups, including the Commission on New Minerals and Mineral Names (CNMMN) of the International Mineralogical Association (IMA), which is responsible for the formal recognition of new minerals and mineral names, and the International Union of Crystallography (IUCr), which considered extensions to the nomenclature of disordered, modulated and polytype structures (Guinier et al. , 1984) published earlier by a joint committee with the IMA (Bailey, 1977). In contrast to the other national clay groups, however, The Clay Minerals Society (CMS) Nomenclature Committee, which was established in 1963 at the same time as the CMS and predates the AIPEA Nomenclature Committee, remains in existence and occasionally produces recommendations. The precursor to this committee was the Nomenclature SubCommittee, which was organized in 1961 by the (US) National Research Council. The Chair of the AIPEA Nomenclature Committee is a standing member of the CMS Nomenclature Committee so that the committees are in close contact. The purpose of the AIPEA Nomenclature Committee has been to make general and specific recommendations concerning: (1) definitions of mineralogical and crystallographic clay-related terms; (2) classification and terminology …

Refined relationships between chemical composition of dioctahedral fine-grained mica minerals and their infrared spectra within the OH stretching region; Part I, Identification of the OH stretching bands

A large and representative collection of clay-size dioctahedral mica minerals differing in their chemical compositions has been studied by infrared (IR) spectroscopy in the OH stretching vibration region. Decomposition of the IR spectra in the individual OH bands has provided unambiguous identification of the band positions for each defined pair of octahedral cations bonded to OH groups. The presence of pyrophyllite-like local structural environments in samples having a deficiency of K in interlayers has been established. A set of the relationships between the OH frequencies corresponding to pairs of cations having different valency and mass has been found.