Organisation de l'espace interlamellaire dans les vermiculites monocouches et anhydres

Abstract

RésuméLes vermiculites Mg, Ca, Sr et Ba ont été étudiées par diffraction des rayons X et absorption infra-rouge.Pour les vermiculites monocouches, l'espacement interréticulaire est compris entre 12·2 et 11·45 Å.On distingue: (i) les vermiculites Ba; le cation, entouré de 5 à 6 molécules d'eau, est à environ 1Å du plan médian et il intéragit avec un hydroxyle structural du feuillet le plus proche, (ii) les vermiculites Mg, Ca et Sr; le cation, entrouré de 3 ou 4 molécules d'eau,est dans le plan médian ou dans son voisinage (0·3 Å), et il n'interagit pas avec les hydroxyles des feuillets.Pour les vermiculites anhydres, l'espacement interréticulaire est inférieur à 10 Å, le cation est dans le plan médian et il intéragit avec les hydroxyles de 2 feuillets opposés.A partir de ces résultats, complétés par les données de la bibliographie, des modéles sont proposés pour représenter l'organisation de l'espace interlamellaire.

Hydration Properties and Interlayer Organization of Water and Ions in Synthetic Na-Smectite with Tetrahedral Layer Charge. Part 1. Results from X-ray Diffraction Profile Modeling

“…These values are also consistent with the three-dimensional (3D) structure determination obtained for Ca-, Sr-, Li-, Ba-, or Nasaturated vermiculite minerals in which the cation hydration shell was found to not exceed 4 H 2 O molecules per cation with a coordination similar to a flat tetrahedron. [45][46][47][69][70][71] Additional information can be obtained by examining the evolution of the interlayer water configuration, as seen through the width of the Gaussian distribution of water molecules located close to the midplane of the interlayer space (fwhm , Tables 1 and 2). Figure 5a displays the evolution of fwhm values as a function of water content.…”

“…In this regime, the low fwhm values and the marginal effect of dehydration on layer thickness are consistent with a planar arrangement of H 2 O molecules surrounding sodium cation, similar to that evidenced for vermiculite minerals. [45][46][47][69][70][71] Bihydrated Layers. The evolution in thickness of 2W layers as a function of relative humidity (Figure 6a) is correlated also to the water content (Figure 6b).…”

“…28 This configuration allows reproducing 00l reflections of a variety of dioctahedral smectites saturated with monovalent and divalent cations, without introducing significant positional scatter. 20 Positional disorder of interlayer H 2 O molecules is, however, suggested by the 3D structure refinement of vermiculites, [45][46][47] and by computational studies. 36,37,40,42,44,[48][49][50][51] The present XRD modeling study thus integrates this positional disorder in the determination of the fine interlayer structure of smectite and of its evolution upon dehydration using low-and high-charge synthetic saponites.…”

Vermiculite: Structural Properties and Examples of the Use

“…The CEC varies between the 120 and 200 cmol+/kg (air dried vermiculites) or 140 and 240 cmol(+)/kg (dehydrated vermiculites) [14,15]. Water molecules associated with internal surfaces evokes hydration of interlayer cations.…”

Interaction of Vermiculite with Aliphatic Amides (Formamide, Acetamide and Propionamide): Formation and Study of Interstratified Phases in the Transformation of Mg- to NH4-Vermiculite

“…The TG curve shows a 1st step of weight loss of 9.87% from room temperature to 180 ~ with a DTG peak centered at 150 ~ followed immediately by a 2nd weight loss of 4.45% that is finished at 300 ~ with a DTG peak at 260 ~ The 1st weight loss is associated with interlayer water that is lost by heating, producing a change from the 2-layer hydrate to the l-layer hydrate (Suquet et al 1991). The 2nd one is associated with water loss from dehydroxylation of OH groups coordinated to Mg ions, producing a zero-layer state, according to Rausell-Colom et al (1980), which needs a higher temperature to be completely eliminated. It is clear that these thermal steps can be separated from each other.…”