Anion chemistry on Titan: A possible route to large N-bearing hydrocarbons

The reaction of CN^? with cyanoacetylene (HC₃N), has been studied as a function of the HC₃N pressure in a quadrupole tandem mass spectrometer. The mass spectra revealed the fast depletion of the CN^? parent ion and formation of larger anions of rapidly growing size. Most of the ions observed were found to belong to two series of products: (HC₃N)_x·C_2p+1N^? and (HC₃N)_x·C_2pN^? resulting from the sequential additions of HC₃N molecules and loss of HCN or HCCN molecules. The mechanism and energetics of the first two reaction steps are briefly discussed. The laboratory data are compared with those from the Cassini CAPS-ELS spectrometer. It is believed that the reactions observed could account for the growth of anions in Titan’s ionosphere.     

A low temperature X-ray single-crystal diffraction and polarised infra-red study of epidote

The effects of low-temperature on the crystal structure of a natural epidote [Ca_1.925Fe_0.745Al_2.265Ti_0.004Si_3.037O₁₂(OH), a = 8.8924(7), b = 5.6214(3), c = 10.1547(6)? and β = 115.396(8)° at room conditions, Sp. Gr. P2₁ /m] have been investigated with a series of structure refinements down to 100 K on the basis of X-ray single-crystal diffraction data. The reflection conditions confirm that the space group is maintained within the T-range investigated. Structural refinements at all temperatures show the presence of Fe³⁺ at the octahedral M(3) site only [%Fe(M3) = 70.6(4)% at 295 K]. Only one independent proton site was located and two possible H-bonds occur, with O(10) as donor and O(4) and O(2) as acceptors. The H-bonding scheme is maintained down to 100 K and is supported by single crystal room-T polarised FTIR data. FTIR Spectra over the region 4,000–2,500 cm⁻¹ are dominated by the presence of a strongly pleochroic absorption feature which can be assigned to protonation of O(10)–O(4). Previously unobserved splitting of this absorption features is consistent with a NNN influence due to the presence of Al and Fe³⁺ on the nearby M(3) site. An additional relatively minor absorption feature in FTIR spectra can be tentatively assigned to protonation of O(10)–O(2). Low-T does not affect significantly the tetrahedral and octahedral bond distances and angles, even when distances are corrected for “rigid body motions”. A more significant effect is observed for the bond distances of the distorted Ca(1)- and Ca(2)-polyhedra, especially when corrected for “non-correlated motion”. The main low-T effect is observed on the vibrational regime of the atomic sites, and in particular for the two Ca-sites. A significant reduction of the magnitude of the thermal displacement ellipsoids, with a variation of U _eq (defined as one-third of the trace of the orthogonalised U _ij tensor) by ~40% is observed for the Ca-sites between 295 and 100 K. Within the same T-range, the U _eq of the octahedral and oxygen sites decrease similarly by ~35%, whereas those of the tetrahedral cations by ~22%.     

A depth average SPH model including μ(I) rheology and crushing for rock avalanches

Classical depth-integrated smoothed particle hydrodynamics (SPH) models for avalanches are extended in the present work to include a μ(I)− rheological model enriched with a fragmentation law. With this improvement, the basal friction becomes grain distribution dependent. Rock avalanches, where grain distribution tends to change with time while propagating, are the appropriate type of landslide to apply the new numerical proposal. The μ(I)− rheological models considered in the present work are those of Hatano and Gray, combined with two different fragmentation laws, a hyperbolic and a fractal-based law. As an application, Frank avalanche, which took place in Canada in 1903, is analyzed under the scope of the present approach, focusing in the influence of the rheological and fragmentation laws in the evolution of the avalanche.     

Stability at high-pressure,elastic behaviour and pressure-induced structural evolution of CsAlSi<Subscript>5</Subscript>O<Subscript>12</Subscript>, a potential host for nuclear waste

The elastic and structural behaviour of the synthetic zeolite CsAlSi₅O₁₂ (a = 16.753(4), b = 13.797(3) and c = 5.0235(17) Å, space group Ama2, Z = 2) were investigated up to 8.5 GPa by in situ single-crystal X-ray diffraction with a diamond anvil cell under hydrostatic conditions. No phase-transition occurs within the P-range investigated. Fitting the volume data with a third-order Birch–Murnaghan equation-of-state gives: V ₀ = 1,155(4) ų, K _T0 = 20(1) GPa and K′ = 6.5(7). The “axial moduli” were calculated with a third-order “linearized” BM-EoS, substituting the cube of the individual lattice parameter (a ³, b ³, c ³) for the volume. The refined axial-EoS parameters are: a ₀ = 16.701(44) Å, K _T0a = 14(2) GPa (β_a = 0.024(3) GPa^?1), K′ _a = 6.2(8) for the a-axis; b ₀ = 13.778(20) Å, K _T0b = 21(3) GPa (β_b = 0.016(2) GPa^?1), K′ _b = 10(2) for the b-axis; c ₀ = 5.018(7) Å, K _T0c = 33(3) GPa (β_c = 0.010(1) GPa^?1), K′ _c = 3.2(8) for the c-axis (K _T0a:K _T0b:K _T0c = 1:1.50:2.36). The HP-crystal structure evolution was studied on the basis of several structural refinements at different pressures: 0.0001 GPa (with crystal in DAC without any pressure medium), 1.58(3), 1.75(4), 1.94(6), 3.25(4), 4.69(5), 7.36(6), 8.45(5) and 0.0001 GPa (after decompression). The main deformation mechanisms at high-pressure are basically driven by tetrahedral tilting, the tetrahedra behaving as rigid-units. A change in the compressional mechanisms was observed at P ≤ 2 GPa. The P-induced structural rearrangement up to 8.5 GPa is completely reversible. The high thermo-elastic stability of CsAlSi₅O_12, the immobility of Cs at HT/HP-conditions, the preservation of crystallinity at least up to 8.5 GPa and 1,000°C in elastic regime and the extremely low leaching rate of Cs from CsAlSi₅O₁₂ allow to consider this open-framework silicate as functional material potentially usable for fixation and deposition of Cs radioisotopes.     

Spatial and temporal variation of the nekton and hyperbenthos from a temperate European estuary with regulated freshwater inflow

The aquatic macrofauna of the Guadalquivir estuary were sampled (1 mm mesh persiana net) at 5 sampling sites located along the entire (except the tidal freshwater region) estuarine gradient of salinity (outer 50 km). A total of 134 fish and macroinvertebrate species was collected but only 62 were considered common or regularly present in the estuary. Univariate measures of the community structure showed statistically significant differences among sampling sites: species richness, abundance, and biomass decreased in the upstream direction, being positively correlated with the salinity. Temporal differences of these three variables were also statistically significant. While a clear seasonal pattern (minimum densities in winter and maximum in spring-summer) was observed for abundance and biomass, no such pattern existed for the number of species. Mysids was the most dominant group throughout the estuary (96% to 99% of abundance; 49% to 85% of biomass), although fish biomass was also important at the outer estuary (36% to 38%). Multivariate analyses indicated highly significant spatial variation in the macrofaunal communities observed along the salinity gradient. These analyses suggest that the underlying structure was a continuum with more or less overlapping distributions of the species dependent on their ability to tolerate different physicochemical conditions. There were also significant temporal (intermonthly + interannual) variation of the estuarine community; the relative multivariate dispersion indicated that monthly variation was more considerable (relative multivariate dispersion >1) at the outer part of the estuary during the wet year (last 20 km) and was higher in the inner stations during the dry year (32 to 50 km from the river mouth). Since a clear negative exponential relationship was observed between the freshwater input (from a dam located 110 km upstream) and water salinity at all sampling stations, it is concluded that the human freshwater management is probably affecting the studied estuarine communities. While the higher seasonal (long-term) stability of the salinity gradient, due to the human control of the freshwater input, may facilitate the recruitment of marine species juveniles during the meteorologically unstable early-spring, the additional (short-term) salinity fluctuations during the warm period may negatively affect species that complete their lifecycle within the estuary.     

Structural evolution of a 2M 1 phengite mica up to 11 GPa: an in situ single-crystal X-ray diffraction study

The structural evolution at high pressure of a natural 2M ₁-phengite [(K_0.98Na_0.02)_Σ=1.00(Al_1.55Mg_0.24Fe_0.21Ti_0.02)_Σ=2.01(Si_3.38Al_0.62)O₁₀(OH)₂; a = 5.228(2), b = 9.057(3), c = 19.971(6)Å, β = 95.76(2)°; space group: C2/c] from the metamorphic complex of Cima Pal (Sesia Zone, Western Alps, Italy) was studied by single-crystal X-ray diffraction with a diamond anvil cell under hydrostatic conditions up to ~11 GPa. A series of 12 structure refinements were performed at selected pressures within the P range investigated. The compressional behaviour of the same phengite sample was previously studied up to ~25 GPa by synchrotron X-ray powder diffraction, showing an irreversible transformation with a drastic decrease of the crystallinity at P > 15–17 GPa. The elastic behaviour between 0.0001 and 17 GPa was modelled by a third-order Birch–Murnaghan Equation of State (BM-EoS), yielding to K _T0 = 57.3(10) GPa and K′ = ?K _T0/?P = 6.97(24). The single-crystal structure refinements showed that the significant elastic anisotropy of the 2M ₁-phengite (with β(a):β(b):β(c) = 1:1.17:4.60) is mainly controlled by the anisotropic compression of the K-polyhedra. The evolution of the volume of the inter-layer K-polyhedron as a function of P shows a negative slope, Fitting the P–V(K-polyhedron) data with a truncated second-order BM-EoS we obtain a bulk modulus value of K _T0(K-polyhedron) = 26(1) GPa. Tetrahedra and octahedra are significantly stiffer than the K-polyhedron. Tetrahedra behave as quasi-rigid units within the P range investigated. In contrast, a monotonic decrease is observed for the octahedron volume, with K _T0 = 120(10) GPa derived by a BM-EoS. The anisotropic response to pressure of the K-polyhedron affects the P-induced deformation mechanism on the tetrahedral sheet, consisting in a cooperative rotation of the tetrahedra and producing a significant ditrigonalization of the six-membered rings. The volume of the K-polyhedron and the value of the ditrigonal rotation parameter (α) show a high negative correlation (about 93%), though a slight discontinuity is observed at P >8 GPa. α increases linearly with P up to 7–8 GPa (with ?α/?P ≈ 0.7°/GPa), whereas at higher Ps a “saturation plateau” is visible. A comparison between the main deformation mechanisms as a function of pressure observed in 2M ₁- and 3T-phengite is discussed.     

Combined morphological and thermal analysis of lava flows: A way to boost understanding of emplacement dynamics

During emplacement, lavas modify the pre‐existing topography and release a large amount of heat. In spite of the relevance of both heat and mass release, combined morphological and thermal analyses have been seldom carried out at a flow‐field scale. Here, we consider a channelised lava flow unit formed at Mt Etna during the 2001 flank eruption, and we show that, by combining a morphological analysis of the pre‐ and post‐emplacement topography with the analysis of the syn‐eruptive thermal signature, critical insights about the processes driving mass and heat dissipation can be derived. Our results suggest that, in the considered lava flow, the pre‐emplacement slope controls heat dissipation and can influence the thickness of the final lava deposit, with possible implications for hazard assessment. The width of the lava channel, instead, appears less sensitive to the pre‐emplacement slope, and tends to regularly increase with increasing distance from the vent.     

High-pressure X-ray and Raman study of a ferrian magnesian spodumene

The high-pressure behaviour of a synthetic P2₁/c ferrian magnesian spodumene, ^M2 (Li_0.85Mg_0.09Fe²⁺ _0.06)^M1(Fe³⁺ _0.85Mg_0.15)Si₂O₆, has been investigated using in situ single-crystal X-ray diffraction and Raman spectroscopy. No phase transition has been observed within the pressure range investigated. The isothermal equation of state up to 7 GPa was determined. V₀, K_T0 and K, simultaneously refined with a Murnaghan equation of state, are: V₀= 415.66(7) ų, K_T0=83(1) GPa and K=9.6(6). The magnitudes of the principal unit-strain coefficients were calculated and their ratios 1:2:3=1.00:1.85:2.81 at P=6.83 GPa indicate a very strong anisotropy. Monitoring of the intensity of b-type reflections (h+k= 2n+1) confirms that from room conditions up to 7 GPa the primitive lattice is maintained. Raman spectra have been collected up to 7.4 GPa. No change in the number of observed vibrational modes occurs in the pressure range investigated. At high frequency, the Raman doublet relative to the Si–O–Si vibrations of the two distinct tetrahedral chains is a broad band at room pressure, however, the frequency difference between the two modes increases with increasing pressure.Operating system: Windows NT     

Middle Pleistocene large‐mammal faunas from North Iberia: palaeobiogeographical and palaeoecological implications

The rich Quaternary fossil record from the Cantabrian region (NW Iberia) is virtually restricted to the Late Pleistocene, with Middle Pleistocene findings very rare. This work presents the study of two Middle Pleistocene palaeontological sites in the Cantabrian region: Llantrales quarry and Mestas de Con mine, both discovered and first described in the 1950s but never studied in detail thereafter. The material was here re‐analysed and re‐evaluated. Taxonomic attributions have been changed in most cases and, consequently, chronological assignments of the fossils have also been changed. This first detailed study of the assemblage from Llantrales quarry revealed the occurrence of the large‐sized deer Praemegaceros solilhacus, which has never been reported previously in North Iberia. Cervus cf. elaphus and Stephanorhinus cf. hundsheimensis were also identified. The age of these fossils was originally regarded as Pontian (late Miocene), but new palaeontological results indicate a much younger age, most likely from around 0.8 to 0.5 Ma. A taxonomic re‐evaluation of the fossils from Mestas de Con provided different results with respect to the 1950s works. The faunal association is composed of a large‐sized deer (probably corresponding to Praemegaceros), Cervus elaphus, Capreolus cf. capreolus, Bison sp. (small sized), Equus sp. (large sized), Stephanorhinus hemitoechus, Ursus sp. and Homotherim latidens. This faunal association was evaluated within a western European context and proves to be highly analogous to other contemporary assemblages there. The chronology was initially regarded as late Villafranchian (in a broader sense). New results, documenting the co‐occurrence of Stephanorhinus hemitoechus and Homotherium latidens, indicate a more precise and younger age, most likely between 0.6 and 0.4 Ma. These two fossil assemblages provide new and relevant information on the Middle Pleistocene faunas from a geographical region where this information, compared with other western European areas, is rare and poorly known.