Metamorphism and aqueous alteration in low petrographic type ordinary chondrites

In order to investigate the relative importance of dry metamorphism and aqueous alteration in the history of chondruies, chondruies were hand-picked from the Semarkona (petrographic type 3.0), Bishunpur (3. 1), Chainpur (3.4), Dhajala (3.8) and Allegan (5) chondrites, and matrix samples were extracted from the first three ordinary chondrites. The thermoluminescence (TL) properties of all the samples were measured, and appropriate subsets of the samples were analyzed by electron-microprobe and radiochemical neutron activation and the water and H-isotopic composition determined. The TL data for chondrules from Semarkona and Bishunpur scatter widely showing no unambiguous trends, although group B1 chondrules tend to have lower sensitivities and lower peak temperatures compared with group A5 chondrules. It is argued that these data reflect the variety of processes accompanying chondrule formation. The chondrules show remarkably uniform contents of the highly labile elements, indicating mineralogical control on abundance and volatile loss from silicates and loss and recondensation of mobile chalcophiles and siderophiles in some cases. Very high D/H values (up to approx. 8000% SMOW) are observed in certain Semarkona chondrules, a confirmation of earlier work. With increasing petrographic type, mean TL sensitivities of the chondrules increase, the spread of values within an individual meteorite decreases, and peak temperatures and peak widths show trends indicating that the TL is mainly produced by feldspar and that dry, thermal metamorphism is the dominant secondary process experienced by the chondrules. The TL sensitivities of matrix samples also increase with petrographic type. Chainpur matrix samples show the same spread of peak temperatures and peak widths as Chainpur chondruies, indicating metamorphism-related changes in the feldspar are responsible for the TL of the matrix. The TL data for the Semarkona and Bishunpur matrix samples provide, at best, only weak evidence for aqueous alteration, but the matrix contains H with approximately terrestrial D/H values, even though it contains much water. Secondary processes (probably aqueous alteration) presumably lowered the D/H of the matrix and certain chondrules. While chondrule properties appear to be governed primarily by formation processes and subsequent metamorphism, the matrix of Semarkona has a more complex history involving aqueous alteration as a meteorite-wide process

In Situ Observations during Chemical Vapor Deposition of Hexagonal Boron Nitride on Polycrystalline Copper.

Using a combination of complementary in situ X-ray photoelectron spectroscopy and X-ray diffraction, we study the fundamental mechanisms underlying the chemical vapor deposition (CVD) of hexagonal boron nitride (h-BN) on polycrystalline Cu. The nucleation and growth of h-BN layers is found to occur isothermally, i.e., at constant elevated temperature, on the Cu surface during exposure to borazine. A Cu lattice expansion during borazine exposure and B precipitation from Cu upon cooling highlight that B is incorporated into the Cu bulk, i.e., that growth is not just surface-mediated. On this basis we suggest that B is taken up in the Cu catalyst while N is not (by relative amounts), indicating element-specific feeding mechanisms including the bulk of the catalyst. We further show that oxygen intercalation readily occurs under as-grown h-BN during ambient air exposure, as is common in further processing, and that this negatively affects the stability of h-BN on the catalyst. For extended air exposure Cu oxidation is observed, and upon re-heating in vacuum an oxygen-mediated disintegration of the h-BN film via volatile boron oxides occurs. Importantly, this disintegration is catalyst mediated, i.e., occurs at the catalyst/h-BN interface and depends on the level of oxygen fed to this interface. In turn, however, deliberate feeding of oxygen during h-BN deposition can positively affect control over film morphology. We discuss the implications of these observations in the context of corrosion protection and relate them to challenges in process integration and heterostructure CVD.P.R.K. acknowledges funding from the Cambridge Commonwealth Trust and the Lindemann Trust Fellowship. R.S.W. acknowledges a research fellowship from St. John’s College, Cambridge. S.H. acknowledges funding from ERC grant InsituNANO (no. 279342), EPSRC under grant GRAPHTED (project reference EP/K016636/1), Grant EP/H047565/1 and EU FP7 Work Programme under grant GRAFOL (project reference 285275). The European Synchrotron Radiation Facility (ESRF) is acknowledged for provision of synchrotron radiation and assistance in using beamline BM20/ROBL. We acknowledge Helmholtz-Zentrum-Berlin Electron storage ring BESSY II for synchrotron radiation at the ISISS beamline and continuous support of our experiments.This is the final version. It was first published by ACS at http://pubs.acs.org/doi/abs/10.1021/cm502603

Acid sites characterization of Al-SBA-15 by a multitechnique approach: microacalorimetry, XPS and FTIR

internationa

Intercalation compounds of Mg-Al layered double hydroxides with dichlophenac: Different methods of preparation and physico-chemical characterization

International audience[2-(2,6-Dichloro-phenylamino)-phenyl]-acetic acid (DPAPAc) was intercalated between layers of Mg-Al hydrotalcites. Several methods of incorporation were applied: direct ion exchange of the layered double hydroxide (LDH) carbonates with the organic ions, direct synthesis by co-precipitation of metal nitrates and sodium DPAPAc, and reconstruction of the Mg-Al oxide solid solution. The LDH reconstruction method led to the highest degree of intercalation and gave rise to an enlargement of the interlayer space of about 6.7 Å, which was observed in the powder X-ray diffraction (PXRD) patterns. The new hybrid phase obtained by this method was partially intercalated, as the LDH-carbonate phase was still mainly present in the material. The characterization of the prepared samples was also achieved with X-ray photoelectron spectroscopy (XPS), which indicated changes in the environment of the elements on insertion of the organic anions. Finally, the collected data indicated that the method of reconstruction was the most effective route to the hybrid materials. © 2004 Elsevier B.V. All rights reserved

Microcalorimetry, TPR and XPS studies of acid-base properties of NiCuMgAl mixed oxides using LDHs as precursors

NiCuMgAl layered double hydroxides (LDHs) with hydrotalcite-like structure contg. different proportions of Ni2+, Cu2+, Mg2+ and Al3+ cations have been prepd. Thermogravimetry and X-ray diffraction data indicate that the transformation of LDH into mixed oxides is effective after calcination at 450°C. The acid-base properties of these mixed oxides have been investigated using adsorption microcalorimetry and XPS with SO2 (for basicity) and NH3 (for acidity) as probe mols. The basicity depends principally on the Ni/Cu ratio and increases with the Ni proportion. The strength (strong and medium) of the basic sites is particularly homogeneous. On the surface of the oxides, they are both Broensted (.apprx.60%) and Lewis sites (.apprx.40%). The acidity (only Lewis type) is rather weak and dependant, as the basicity, of the Ni/Cu ratio. The obsd. heterogeneity of these sites can be related to the heterogeneity of the interactions between the nickel atoms and the other elements