Showing papers by "Bruno Lanson published in 2002"

Authigenic kaolin and illitic minerals during burial diagenesis of sandstones: a review

Abstract: The diagenetic evolution of kaolin and illitic minerals in sandstones is described here. The structural characterization of these minerals, the possible reaction pathways leading to their crystallization, and the origin of the fluids involved are discussed specifically. While early precipitation of kaolinite is in general related to flushing by meteoric waters, subsequent diagenetic kaolinite-to-dickite transformation probably results from invasion by acidic fluids of organic origin. Dickite is the stable polytype in most sandstone formations and the kaolinite-to-dickite conversion is kinetically controlled. The conventional model of kaolin illitization, assuming a thermodynamic control in a closed system, is discussed and compared to an alternative model in which illitization of kaolin is not coupled to dissolution of K-feldspar (Berger et al. , 1997). In the latter model, illite crystallization at the expense of kaolin implies that an energy barrier is overcome either by an increased K+/H+ activity ratio in solution or by a considerable temperature increase.

Site occupancies by iron in nontronites

Abstract: Twelve nontronites and two ferruginous smectites have been characterized with respect to Fe3+ occupancy of tetrahedral sites. The techniques used were near infrared, Fe-K X-ray absorption near-edge and X-ray absorption fine-structure spectroscopies, along with two X-ray diffraction techniques. The results show that calculations of the structural formulae of many nontronites should be adjusted to include Fe3+ in tetrahedral sites. The nontronite from Spokane County, Washington, (∼44% Fe2O3) is essentially an end-member with its non-siliceous tetrahedral sites occupied by Fe3+. Samples with chemical compositions similar to Garfield nontronite (∼36.5% Fe2O3) may have small amounts (<5% of total Fe3+) of tetrahedral Fe3+. Tetrahedral Fe3+ is unlikely to be present in samples containing less than ∼;34% Fe2O3.

Structure of synthetic Na-birnessite: Evidence for a triclinic one-layer unit cell

Abstract: The structure of a synthetic analogue of Na-birnessite (NaBi) was studied by powder X-ray diffraction (XRD). It is shown that NaBi has a one-layer triclinic structure with sub-cell parameters: a P = 2.9513(4) A, b P = 2.9547(4) A, c P = 7.334(1) A, αP = 78.72(2) A, βP = 101.79(1) A, γP = 122.33(1) A, and space group P1 . This sub-cell is equivalent to the base-centered sub-cell with parameters: a = 5.174 A, b = 2.848 A, c = 7.334 A, α = 90.53 A, β = 103.20 A, and γ = 90.07 A. A structure model has been refined using the Rietveld technique. NaBi consists of vacancy-free Mn-bearing octahedral layers whose negative charge arises mostly from the substitution of Mn3+ for Mn4+. The departure from the hexagonal symmetry of layers results from Jahn-Teller distortion of Mn3+ octahedra, which are elongated along the a axis, segregated in Mn3+-rich rows parallel to the b axis, and separated from each other along the a axis by two Mn4+-rows. Structural sites of interlayer Na cations and H2O have been determined as well as their occupancies. The sub-cells of the two NaBi modifications described by Drits et al. (1997) as types I and II likely contain four sites for interlayer species, two of which are occupied by Na and the other two by H2O molecules. In the two NaBi varieties, these pairs of sites are split along the c axis and related by a center of symmetry. This splitting is consistent with the modulated structure of both NaBi types, which arises from the periodic displacement of interlayer species along the b axis with a periodicity λ = 6 b (Drits et al. 1997).

Structure of heavy-metal sorbed birnessite: Part 1. Results from X-ray diffraction

Abstract: Selected-area electron diffraction (SAED) and energy dispersive analysis were used to study the structure of synthetic heavy-metal sorbed birnessites (MeBi). Samples were prepared by equilibrating a suspension of Na-rich buserite (NaBu) at pH4 in the presence of various heavy metal cations (Me), including Pb, Cd, Zn, and Cu. Five main types of SAED patterns were observed. Types I and II were observed only for ZnBi micro-crystals, and they both consist of two super-cell reflection networks related by a mirror plane parallel to the a *c* plane. In direct space, these twinned networks correspond to the hexagonal supercells with AH = BH = 7b/ 3, and AH = BH = 7b, for ZnBi type I and II, respectively. In the two varieties, the supercells result from an ordered distribution of vacant layer octahedra capped by interlayer Zn in ZnBi layers. This distribution is described by a hexagonal cell with AH = 7b. In ZnBi micro-crystals of type I, interstratified twinned right- and left-handed fragments are similar to chalcophanite (ZnMn3O7-3H2O - Wadsley 1955; Post and Appleman 1988), and distributions of vacant layer octahedra from adjacent layers are regularly shifted with respect to each other by 1/3 of the long diagonal of the hexagonal layer unit cell. In ZnBi micro-crystals of type II, distributions of vacant layer octahedra are not regularly shifted from one layer to the adjacent one. SAED patterns of types III and IV occur for PbBi, ZnBi, and CdBi micro-crystals and contain super-cell reflections distributed parallel to [100] * with a periodicity which is not commensurate with that of the MeBi sub-structure (a */2.15 and a*/5.25, respectively). The super-cell reflections result from the ordered distribution within MeBi layers of vacant layer sites capped by Me as pairs along the a axis. Within each pair, vacant sites are separated by 2a for type III, and by 5a for type IV. In one-layer monoclinic structures, the apparent incommensurability arises from the +a/3 shift between adjacent layers having a similar one-dimensional periodic distribution of interlayer Me located above and below vacant octahedra sharing three corners with Mnlayer octahedra (TC sites). Tetrahedral coordination of these Me cations in TC sites, as in ZnBi, leads to the formation of strong H-bonds between adjacent layers. A similar incommensurate effect occurs in one layer hexagonal MeBi if octahedrally coordinated Me cations periodically distributed along the a axis are located above and/or below empty tridentate cavities sharing three edges with Mnlayer octahedra ( VITE sites, PbBi). SAED patterns of type V contain only sub-cell reflections and were observed mostly for PbBi and CuBi micro-crystals. Three different conditions can lead to the absence of supercell reflections: (1) a low amount of sorbed Me (PbBi); (2) the presence of Me having a similar scattering power as that of Mn on a single side of vacant layer sites (CuBi); or (3) a random distribution of interlayer species.

EXPERIMENTAL INVESTIGATION OF THE INTERACTION OF CLAYS WITH HIGH-pH SOLUTIONS: A CASE STUDY FROM THE CALLOVO-OXFORDIAN FORMATION, MEUSE-HAUTE MARNE UNDERGROUND LABORATORY (FRANCE)

Abstract: The impact of alkaline solutions (pH = 13.2) on the clay mineralogy of the Callovo-Oxfordian formation hosting the French underground laboratory for nuclear waste disposal investigation (Meuse- Haute Marne site) has been studied experimentally. Initially, each of the four samples selected as representative of the mineralogical transition in this Callovo-Oxfordian formation consists of a mixture of three main clay phases: discrete illite, discrete smectite and a randomly interstratified mixed-layered mineral (MLM) containing ~65% of non-expandable layers. Clay separates were altered in batch reactors at 608C using high solution:solid ratios. The mineralogy of this clay fraction and solution chemistry were monitored as a function of reaction time. In addition, the interactions between organic matter and clay particles were investigated using scanning transmission X-ray microscopy (STXM). The clay mineralogy is little affected even though the pH is still high after 1 y reaction time. The only significant mineralogical evolution is the partial dissolution of the discrete smectite component leading to the formation of a new randomly interstratified illite-expandable MLM. Additional mineralogical transformations lead, for one sample, to the dissolution of micro-crystalline quartz and, for another sample, to the crystallization of a tobermorite-like phase. The low reactivity of clay minerals may be attributed to the presence of organic matter in the samples. In their initial state, all outer surfaces of clay particles are indeed covered with organic matter. After 1 y reaction time, STXM studies showed the basal surfaces of clay particles to be devoid of organic matter, but their edges, which are the most reactive sites, were still protected. Key Words—High pH, French Underground Laboratory, Organic Matter, STXM, XRD.