“Dogged” Search of Fresh Nakhla Surfaces Reveals New Alteration Textures

Special Issue: 74th Annual Meeting of the Meteoritical Society, August 8-12, 2011, London, U.K.International audienceCarbonaceous chondrites are considered as amongst the most primitive Solar System samples available. One of their primitive characteristics is their enrichment in volatile elements.This includes hydrogen, which is present in hydrated and hydroxylated minerals. More precisely, the mineralogy is expected to be dominated by phyllosilicates in the case of CM chondrites, and by Montmorillonite type clays in the case of CI. Here, in order to characterize and quantify the abundance of lowtemperature minerals in carbonaceous chondrites, we performed thermogravimetric analysis of matrix fragments of Tagish Lake, Murchison and Orgueil

Structure development during shear flow induced crystallization of i-PP: In situ wide-angle X-ray diffraction study

In situ synchrotron wide-angle X-ray diffraction (WAXD) was used to monitor crystallization of isotactic polypropylene (i-PP) in the subcooled melt at 140 °C after step shear. The melt was subjected to a shear strain of 1430% at three different shear rates (10, 57, and 102 s-1) using a parallel-plate shear apparatus. WAXD results were used to determine the type (α- and β-crystals), orientation, and corresponding mass fractions of i-PP crystals. It was found that formation of oriented α-crystals occurred immediately after application of the shear field. Subsequently, growth of primarily unoriented β-crystals was observed. WAXD patterns clearly showed that β-crystals grew only after the formation of oriented α-crystals in the sheared i-PP melt. The contribution of β-crystals to the total crystalline phase was as high as 65-70% at high shear rates (57 and 102 s-1) and low (20%) at low shear rates (10 s-1), which was attributed to the different amount of surface area of oriented α-crystal cylindrites generated at different shear rates. The growth of β-crystals which is related to the surface area of the oriented α-form crystalline assembly has been proposed earlier. Also, the unoriented nature and fast growth of the β-crystals determined from WAXD experiments provide an explanation for the 2 orders of magnitude increase in the kinetics of crystallization of the unoriented structures, which was previously observed (but not explained) in our crystallization study by small-angle X-ray scattering (SAXS). © 2001 American Chemical SocietyNSF DMR-0098104 and in part by ExxonMobil.Peer Reviewe

Shear-induced crystallization of isotactic polypropylene with different molecular weight distributions: In situ small- and wide-angle X-ray scattering studies

In situ synchrotron wide-angle X-ray scattering (WAXS) and small-angle X-ray scattering (SAXS) were used to monitor the structural and morphological developments during crystallization of a series of isotactic polypropylene (iPP) blends with different molecular weight distributions (MWD). The experiments were carried out in the undercooled melt at 150°C subjected to a high shear strain (1428%) at a fixed shear rate (57 s-1). The final WAXS patterns showed arcing of the main Bragg reflections, indicating the presence of oriented crystallites. These oriented crystallites, in the form of lamellae, were also revealed by two strong meridional reflections in the SAXS patterns. The crystallization kinetics exhibited a strong dependence on the molecular weight distribution, which were due to the production of primary nuclei induced by orientation. A method to deconvolute the total integrated scattered intensity (SAXS and WAXS) into the isotropic and anisotropic contributions from the crystallized polymer was used to analyze the data. The fraction of oriented morphology determined by SAXS and that of oriented crystallinity determined by WAXS showed good agreement with each other. However, it has been demonstrated that the oriented fraction from SAXS is more suitable for determining the 'critical orientation molecular weight' (M*) value. Only polymer chains above M* in the distribution can become oriented at a given shear rate. It is observed that, regardless of the MWD, a similar value for M* is obtained under our experimental conditions. © 2001 Elsevier Science Ltd.NSF DMR-9732653, US-Spain Science and Technology program 1999 and ExxonMobil. DGICYT, Spain, grant PB94-0049Peer Reviewe

Structure development during shear flow-induced crystallization of i-PP: in-situ small-angle X-ray scattering study

In-situ synchrotron small-angle X-ray scattering (SAXS) was used to follow orientation-induced crystallization of isotactic polypropylene (i-PP) in the subcooled melt at 140°C after step shear under isothermal conditions. The melt was subjected to a shear strain of 1428% at three different shear rates (10, 57, and 102 s-1) using a modified Linkam shear stage. The SAXS patterns showed strong meridional reflections due to the rapid development of oriented polymer crystallites within the melt. On the basis of the SAXS data, a schematic representation of nucleation and growth in orientation-induced crystallization of i-PP is proposed. During flow, orientation causes alignment of chain segments of polymer molecules and results in the formation of primary nuclei in the flow direction. These nuclei facilitate the growth of oriented crystal lamellae that align perpendicular to the flow direction. The half-time of crystallization was calculated from the time evolution profiles of the total scattered intensity. The crystallization kinetics was found to increase by 2 orders of magnitude as compared to quiescent crystallization. A method was used to deconvolute the total integrated scattered intensity into contributions arising from the isotropic and anisotropic components of the crystallized chains. The fraction of oriented crystallites was determined from the ratio of the scattered intensity due to the oriented (anisotropic) component to the total scattered intensity. At low shear rates (approx. 10 s-1) the oriented fraction in the polymer bulk was lower than at high shear rates (57 and 102 s-1). It was shown that only the polymer molecules above a 'critical orientation molecular weight' (M*) could become oriented at a given shear rate. The M* values at different shear rates were determined from the area fractions of the molecular weight distribution of the polymer.© 2000 American Chemical SocietyUS-Spain Science & Technology Program 1999. NSF DMR-9732653, DGICYT, Spain (Grant PB-0049), and ExxonMobil Chemical CompanyPeer Reviewe