C. H. Pons

Stacking order in a 14.30-Aa Mg-vermiculite

The stacking order of a 14.30-Å Mg-vermiculite from Santa-Olalla, Spain, has been determined from Weissenberg photographs. The results prove that the Mg-vermiculite structure is not a 2-layer polytype structure, but a semi-ordered one. Because the structure is semi-ordered, its resolution needed a dual approach: (1) a direct approach using an electron density projection along the y axis in conjunction with a one-dimensional electron-density projection onto the z axis; and (2) an indirect approach in which the observed intensities along the (0,k) and (1,k) reciprocal rods were compared to the calculated intensities given by model defect structures. The semi-ordered structure of the Mg-vermiculite results from ±b/3 shifts in the stacking of the silicate layers. The shifts are randomly either along +b or -b.

A CRYSTAL-CHEMICAL STUDY OF NATURAL AND SYNTHETIC ANIONIC CLAYS

A comparative crystallochemical study was performed on natural and synthetic hydrotalcite-like compounds with similar compositions. The nature of the brucite-like sheet stacking was addressed by means of powder X-ray diffraction. From the resulting electron diffraction patterns it was possible to establish the order-disorder of the cations in the brucite-like sheet. The results show that a natural sample from Snarum is an intergrowth of hydrotalcite (3R1 polytype) and manasseite (2H1 polytype) at a ratio of 77:23 (wt.%). An aluminian serpentine is associated with the hydrotalcite and manasseite minerals. The structure of a synthetic sample, Mg:Al = 2:1, was determined as space group

Small angle scattering setting at LURE: Description and results

R\,\overline 3 \,m

Interstratification in Malawi vermiculite; effect of bi-ionic K-Mg solutions

R3¯m. For a few crystals in this sample, the octahedral cation distribution is compatible with the observed supercell (a = a′ √3). A second synthetic sample showed the presence of stacking faults and was described as a random layer sequence of two polytypes (3R and 2H).

Quantification curves for XRD analysis of mixed-layer 14Aa/10Aa clay minerals

Abstract A device has been assembled on the X-Ray beam line of D.C.I. (Orsay) which permits to record small angle scattering diagrams and/or large angle diffraction patterns. Experiments are made with a monochromatic beam provided by a germanium crystal whose curvature is adjustable to focus at 2 m from the monochromator. Scattering data are collected with a wire chamber. The experiments which have been yet performed at LURE use three of the characteristics of the Synchroton radiation: the low vertical divergence of the beam, the high intensity, and the fact that the wavelength can be easily changed. On the basis of preliminary results it will be discussed about the improvement of the resolution in the small angle scattering curves and the possibility of studying kinetics.

STACKING ORDER IN A K/Mg INTERSTRATIFIED VERMICULITE FROM MALAWI

The conversion of Malawi vermiculite into K-vermiculite by treatment with bi-ionic K-Mg solutions of 1 N total ion concentration (KCl and MgCl2 mixed solutions of ionic strength equal to 0.5) was studied by following the 00l X-ray powder diffraction (XRD) reflections. Flakes of Mg-saturated samples were treated at 160°C during 24 hr with bi-ionic solutions, with the K concentration varying from zero to pure 1 N KCl solution. The K-Mg interlayer exchange began at a critical value xK = .0196 (K/Mg = 1/100) of the molar fraction of K in the solution. Above the critical concentration and extending to pure 1 N KCl, the XRD diagrams were characteristic of a 10-Å/14-Å interstratification that had a marked tendency towards regularity. Experiments with KCl and MgCl2 mixed solutions of ionic strength equal to 0.75 and 1.0 showed that the exchange began at the same critical value xK as the experiments with ionic strength equal to 0.5, if the K added was equivalent. X-ray fluorescence analysis further showed that the amount of K adsorbed was proportional to the molar fraction xK and to the proportion of K-saturated layers (10 Å) in the interstratification. To explain the mechanism of this quasi-regular interstratification, a crystallochemical rather than a thermodynamic mechanism is proposed.

Evolution of benzylammonium-vermiculite and ornithine-vermiculite intercalates

Using theoretical profiles of diffracted X-ray intensity for interstratification between layers having d-spacings around 14.3 Å and 10.1 Å, a series of diagrams was derived from which the proportion of 14.3 Å layers (W14) and the probability of passing from a 14.3 Å layer to a 10.1 Å layer (P14/10) can be derived. W14 can be derived independently of P14/10 using the angular distance between reflections situated at 18.2° and 25.4° 2θ (CuKα). Once W14 is determined, P14/10 may be obtained using the angular width of the diffuse reflections between 27° and 34° 2θ. In this case, two different diagrams are proposed for P14/10 determination because experimental X-ray patterns show either one or two diffuse reflections. Comparison of five experimental patterns with theoretical patterns calculated using W14 and P14/10 obtained using these diagrams indicates that the method can be useful for determining W14 and P14/10 in unknown samples. Moreover, the method described is independent of the Lorentz polarization factor and the layer type. The d-spacings associated with the two kinds of layers, however, should be similar (± 1%) to those for which the determinative diagrams were calculated.

Utilisation du rayonnement synchrotron en diffusion aux petits angles pour l'etude du gonflement des smectites; I, Etude du systeme eau-montmorillonite-Na en fonction de la temperature

The stacking order of a bi-ionic K/Mg vermiculite from Malawi (Nyasaland), has been determined from Weissenberg data. The sample corresponds to a K/Mg interstratified vermiculite containing 50% K layers (dK = 1.01 nm) and 50% Mg layers with two layers of water (dMg = 1.44 nm). The observed intensities along (0, 0), (h, 0), (0, k) and (1, k) reciprocal rods were compared to the calculated intensities given by model defect structures. It was found that: 1) The (h, 0), (0, k) and (1, k) rods reveal the interstratification phenomenon which was previously observed on the (0, 0) rod; 2) The exchange of Mg by K does not occur randomly in a single interlayer, but occurs in interlayer domains sufficiently extensive to allow the reorganisation of the layer stacking and development of the ordered K-vermiculite structure from the semi-ordered structure characteristic of magnesium vermiculites; 3) For this sample, the interlayer water of the Mg phase occurs in two coexisting configurations; one configuration has water molecules forming octahedral coordination around the Mg2+, the other has water which is not linked to the Mg2+ cation forming two planes.

Utilisation du rayonnement synchrotron en diffusion aux petits angles pour l'etude du gonflement des smectites; II, Etude de differents systemes eau-smectites en fonction de la temperature

This report consists of a study of l-ornithine hydrochloride-vermiculite and of benzylammon-ium hydrochloride-vermiculite complex. The evolution of these organo-vermiculite structures upon heating is studied by X-ray diffraction (XRD) as well as infrared spectroscopy.After heating vermiculite saturated with 1-ornithine cations, it shows condensation of interlayer ornithine molecules (peptide complexes). The stacking mode, opposing ditrigonal cavities, is not modified between aminoacid complex and peptide complex.For vermiculite saturated with benzylammonium cations, the stacking sequence changes through heating by changing benzylammonium to NH4+. This transformation implies a sliding of the layers over each other. The ditrigonal surface cavities become face to face, as in the original mica. There are no random translations as in the starting complex.