Structure of birnessite obtained from decomposition of permanganate under soft hydrothermal conditions. I. Chemical and structural evolution as a function of temperature.

International audienceThe structure of a synthetic K-rich birnessite (KBi) prepared by hydrothermally reacting (4 days at 170°C) a finely ground KMnO4 powder in acidified water was determined. At room temperature the structure of KBi corresponds to a 3R - polytype which can be described as using the close-packing formalism. Assuming an orthogonal base-centered unit cell, KBi has a = b√3 = 4.923 A, b = 2.845 A, γ = 90° and c = 21.492 A. The layer charge deficit originates exclusively from the presence of vacant layer sites as only Mn AbC...BcACaBAbC c' b' a' c' a' b' 4+ cations are present in the octahedral layers, and the following structural formula can be proposed

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

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 A 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 randomly distributed within smectite crystallites, and models describing these complex structures involve two distinct contributions, each containing different layer types that are randomly interstratifed. 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 has allowed also 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 ( r/v , v = cation valency and r = ionic radius) and on RH, the following equations were derived: 36 37 Layer thickness (1W) = 12.556 + 0.3525 × ( r/v - 0.241) × (v × RH - 0.979) Layer thickness (2W) = 15.592 + 0.6472 × ( 38 r/v - 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 A from the central interlayer cation along the c* axis

Diagenetic smectite-to-illite transition in clay-rich sediments: A reappraisal of X-ray diffraction results using the multi-specimen method

International audienceSmectite illitization is a common mineralogical reaction occurring during the burial diagenesis of clay-rich sediments and shales, and has thus attracted sustained interest over the last fifty years. Prior studies have concluded that smectite illitization proceeds through a steady set of homogeneous reactions involving intermediate mixed layers of varying compositions. In these intermediate structures, illite and smectite, or, more generally, expandable layers (I and Exp layers, respectively) coexist among the same crystallites giving rise to non-periodic structures (I-Exp) characterized by specific diffraction effects. Consistent with this model, reaction progress was characterized by the simultaneous increase in the illite content in I-Exp and in their stacking order leading to the following mineralogical sequence: smectite → randomly interstratified I-Exp with high smectite contents (> 50% Exp layers) → ordered I-Exp with high illite contents (> 50% I layers) → illite. Although reaction mechanisms have been extensively debated, this structural characterization has not been challenged, possibly due to a methodological bias. In the present study, X-ray diffraction patterns typical of the diagenetic illitization of smectite are interpreted using modern approaches involving profile fitting (multi-specimen method). Novel insights into the structure of intermediate reaction products are thus obtained. In particular, original clay parageneses are described including the systematic presence of illite, kaolinite, chlorite and a mixed layer containing kaolinite and expandable layers (K-Exp). In contrast to previous descriptions, the early stages of smectite illitization are characterized by the coexistence of discrete smectite and of a randomly interstratified I-Exp with a high content of illite layers (>50% I layers). Both the smectite and the I-Exp are authigenic and form under shallow burial, that is at low temperature conditions. With increasing burial depth, the relative proportion of I-Exp increases, essentially at the expense of discrete smectite, and the composition of I-Exp becomes slightly more illitic. In the second stage of smectite illitization, two illite-containing mixed layers are observed. They result from two parallel reaction mechanisms affecting the randomly interstratified I-Exp present in the shallow section of the series. The first reaction implies the dissolution of this randomly interstratified I-Exp and leads to the crystallization of an ordered I-Exp without significant illitization, possibly because of the low K-availability. The second reaction affecting the randomly interstratified I-Exp implies the growth of trioctahedral (Mg, Al) hydroxide sheets in Exp interlayers, thus developing di-trioctahedral chlorite layers (Ch layers) in the initial I-Exp to form an I-Exp-Ch. A layer-by-layer mechanism is hypothesized for this reaction. In this scheme, Mg cations released by the dissolution-recrystallization reaction of I-Exp likely represent the source of Mg for the formation of brucite-like sheets in expandable interlayers, and thus of the I-Exp-Ch. The reported structural characterization of smectite illitization intermediate products contradicts the conventional wisdom of a homogeneous reaction through a series of pure mixed layers of variable composition. In contrast, the coexistence of different phases implies a heterogeneous reaction via a sequence of intermediate phases and requires reassessing the reaction mechanisms proposed in the literature. The compositional range (relative proportion of the different layer types) of these phases is limited and smectite illitization proceeds essentially as relative proportions of the different phases vary. In addition, reaction kinetics and stability of the different intermediate products also need to be reconsidered

Structure of the Synthetic K-rich Phyllomanganate Birnessite Obtained by High-Temperature Decomposition of KMnO4. Substructures of K-rich Birnessite from 1000°C Experiment

International audienceThe structure of a synthetic potassium-rich birnessite prepared from the thermal decomposition of KMnO4 at 1000°C in air has been refined by Rietveld analysis of the powder X-ray diffraction (XRD) data, and the structure model shown to be consistent with extended X-ray absorption fine structure data. K-rich birnessite structure is a two-layer orthorhombic polytype (2O) with unit-cell parameters a = 5.1554(3) Ä, b = 2.8460(1) Ä, c = 14.088(1) Ä, α = β = γ = 90°, a/b = √3.281, and was refined in the Ccmm space group. The structure is characterized by the regular alternation of octahedral layers rotated with respect to each other by 180°. Octahedral layers are essentially devoid of vacant sites, the presence of 0.25 Mn 3+ layer cations within these layers being the main source of their deficit of charge, which is compensated for by interlayer K + cations. Mn3+ octahedra, which are distorted by the Jahn-Teller effect, are systematically elongated along the a axis (cooperative Jahn-Teller effect) to minimize steric strains, thus yielding an orthogonal layer symmetry. In addition, Mn 3+ octahedra are segregated in Mn3+-rich rows parallel to the b axis that alternate with two Mn 4+ rows according to the sequence ...-Mn3+-Mn4+-Mn4+-Mn3+-... along the a direction, thus leading to a A = 3a super-periodicity. At 350°C, the structure partially collapses due to the departure of interlayer H2O molecules and undergoes a reversible 2O-to-2H phase transition. This transition results from the relaxation of the cooperative Jahn-Teller effect, that is from the random orientation of elongated Mn 3+ octahedra

Structural change in celadonite and cis-vacant illite by electron radiation in TEM

High-resolution transmission electron microscopy (HRTEM) images of two dioctahedral micas, celadonite and cis-vacant (cv) illite, were examined in detail to understand the effects of electron radiation on their structures during image acquisition. Celadonite, a dioctahedral mica with Fe and Mg as major octahedral cations, originally has a trans-vacant (tv) octahedral sheet but the contrast in the high- resolution transmission electron microscopy (HRTEM) images indicates a different cation distribution in the sheet. Furthermore, the β angle for the 1M polytype derived from the HRTEM images is ~98.5°, which is considerably smaller than that (~100.5°) reported for celadonite. In previous works, cation migration from the tv to cv-like configurations and a decrease in the β angle after dehydroxylation of celadonite/ glauconite by heating were reported. The same phenomenon, dehydroxylation and subsequent cation migration, probably occurs by electron radiation in TEM. However, the new cation-distribution models derived from HRTEM images along the [100] and [110] directions are not in agreement. On the other hand, the contrast in a number of HRTEM images rom an illite specimen in which cv-illite is dominant is the same as that for the tv-dioctahedral layer. This result is also interpreted as cation migration accompanied by dehydroxylation in TEM, as reported in heated cv-illite. The increased β angle (~102.5°) from that in the natural state (101.5°) estimated from the HRTEM images also supports this interpretation. This phenomenon is a large obstacle to the investigation ofphyllosilicates containing Al-rich cv and Mg,Fe-rich tv 2:1 layers, using HRTEM

Trans-vacant and cis-vacant 2:1 layer silicates : Structural features, identification, and occurrence

A comprehensive study of clay minerals should include determination of the vacancy pattern of the dioctahedral sheet. The purpose of this report is to consider the advantages and limitations in various diffraction and non-diffraction methods for the determination of the layer types in clay minerals. Identification of trans-vacant (tv) and cis-vacant (cv) clay minerals reported here is based on powder X-ray diffraction (XRD) patterns calculated for different polytypes consisting of either tv or cv layers, on the simulation of experimental XRD patterns corresponding to illite or illite fundamental particles in which tv and cv layers are interstratified, and on the semi-quantitative assessment of the relative content of the layer types in the interstratified structures by generalized Méring's rules. A simple and effective method for identification of tv and cv layers in dioctahedral 2:1 layer silicates employs thermal analysis and is based on different dehydroxylation temperatures for tv and cv illite and smectite layers. Crystal chemical analysis of various dioctahedral 2:1 layer silicates consisting of tv and cv layers indicates that compositional control is present in the distribution of octahedral cations over trans- and cissites. In dioctahedral smectites the formation of tv and cv layers is related to the layer composition and local order-disorder in the distribution of isomorphous cations. Dioctahedral 1M micas with abundant Fe and Mg occur only as tv varieties. In contrast, 1M-cv illite, as well as cv layers in illite fundamental particles of I-S, can form only as Fe- and Mg-poor varieties. In illites and illite fundamental particles of I-S consisting of tv and cv layers, cv layers prevail when the amounts of Al in octahedra and tetrahedra are >1.55 and >0.35 atoms per O(OH), respectively. The main factors responsible for the stability of cv and tv illites have been established. Monomineral cv 1M illite, its association with tv 1M illite, and interstratified cv/tv illite occur around ore deposits, in bentonites, and in sandstones mostly as a result of different types of hydrothermal activity. The initial material for their formation should be Al-rich, and hydrothermal fluids should be Mg- and Fe-poor. Tv and cv smectites of volcanic origin differ in terms of octahedral cation composition and distribution of isomorphous octahedral cations. Mg-rich cv smectites have random distribution of isomorphous octahedral cations, whereas in Mg-bearing tv smectites octahedral Mg cations are dispersed so as to minimize the amount of Mg-OH-Mg arrangements

The nature of structure-bonded H2O in illite and leucophyllite from dehydration and dehydroxylation experiments

Thermogravimetric analysis combined with mass spectrometry was used to study HO bound to samples of illite-1M, illite-2M and leucophyllite-1M. Samples were heated in a helium atmosphere at different temperatures and after heating at each given temperature were cooled to 35°C. Each cycle in the mass 18 spectrum of each illite sample contains a low-temperature peak at 60-80°C, a medium-temperature peak at 340-360°C, and a high-temperature peak at a temperature that is very close to the maximum temperature of sample heating of a given cycle. Within each heating-cooling cycle, the sample weight at the beginning of cooling is lower than that at the end of the same cooling stage because of HO resorption. However, the number of HO molecules released during each medium-temperature heating cycle is equal to the number of HO molecules resorbed during the corresponding cooling stages. The weight losses, under medium-temperature heating, of the illite samples are related to dehydration when HO molecules located in K-free sites of the illite interlayers are removed. The medium-temperature peak is reproducible for each cycle because during each cooling stage the illite interlayers resorb the same number of HO molecules that were lost during the preceding dehydration. Two distinct features are characteristic of leucophyllite during heating-cooling treatments. First, the number of HO molecules resorbed during cooling is significantly greater than the number of HO molecules lost during dehydration. Second, the medium-temperature peaks in the spectrum appear only in the last five cycles and the maximum-peak temperature is 450-460°C. These data indicate that the heating-cooling treatments are accompanied by partial rehydroxylation. This rehydroxylation occurs during each cooling stage when a small number of resorbed HO molecules are trapped in the interlayers, although most migrate into the octahedral sheet of the 2:1 layers and reform as OH groups. The crystal chemical factors responsible for the dehydration and rehydration as well as for the rehydroxylation reactions are discussed and speculation about the origin of the low- and medium-temperature HO losses is presented