Direct quartz-coesite transformation in shocked porous sandstone from Kamil Crater (Egypt)

Coesite, a high-pressure silica polymorph (pressure 3–10 GPa, temperature <3000 K), is a diagnostic feature of shock metamorphism associated with impact cratering on quartz-bearing target rocks. It is preserved as a metastable phase in sedimentary target rocks that experienced peak pressures in excess of ~10 GPa, where it typically occurs as intergranular polycrystalline aggregates of microcrystals embedded in silica glass known as “symplectic regions.” The presence of coesite in the symplectic regions of rocks experiencing shock conditions beyond the limits of the coesite stability field is a controversial issue. Through a combined scanning and transmission electron microscopy and Raman spectroscopy study of shocked quartzarenites from the 45-m-diameter Kamil Crater (southwest Egypt), we show that coesite in symplectic regions forms through direct subsolidus transformation from quartz, in contrast with the prevailing hypothesis for crystalline targets. The quartz-to-coesite transformation takes place during localized shock-wave reverberation at the beginning of the pore collapse process. Complete pore collapse generates the high temperature regimes responsible for the subsequent production of the embedding silica melts, in part at the expense of the previously formed coesite. This work documents the role of pore collapse in producing localized pressure-temperature-time gradients in shocked porous targets, as predicted by numerical models in the literature

3D electron diffraction in nano-geology: present and perspectives

When working on advanced research topics in geosciences, one must often deal with small yields and cryptocrystalline polyphasic samples. Conventional optical and X-ray crystallographic tools may not be sufficient for the proper characterization of these samples. The development of efficient probes able to investigate the nanoworld becomes therefore crucial for pushing forward our understanding about the geochemical and mineralogical processes that regulate Earth and extraterrestrial environments. In the last ten years, electron diffraction (ED) evolved from a qualitative method restricted to few dedicated TEM users, to a robust protocol for phase identification and abinitio structure determination [1]. Such change has been mostly propelled by the development of routines for 3D data collection. This methodology is in principle equivalent to single-crystal X-ray diffraction, but allows sampling crystals of few tens on nanometers. We will show here some examples of recent applications of ED in geosciences, namely how to achieve an easy and relatively fast characterization of minor and cryptocrystalline phases in natural and experimental samples. We were able identify and characterize modulated phases able to carry hydrogen at upper-mantle conditions, to follow aragonite growth from the first nucleation seeds [2] and to identify mineralogical phases and polytypes in non-equilibrated extraterrestrial samples and in impact rocks [3]

Environmental impacts and risk assessment in the re-use of Cr-bearing pyrolyzed tannery wastes: A case study in a residential area

Increasing concern has been raised on the environmental impacts of chromium -tanning wastes recycling. In particular, the pyrolytic conversion of leather industry sludges into Cr(VI)-free carbonized residues is believed to represent a viable route for a sustainable re -use of this type of wastes. The aim of this study was to determine the impact of recycled Cr-bearing pyrolyzed char (named KEU) as backfilling material for road construction in an urban area in Tuscany (Italy). Geochemical and chromium -isotope data, together with microstructural analyses (HR-TEM and XAS), indicate that the presence of KEU results in a significant enhancement of the natural Cr background. The results support the hypothesis that, in environmental conditions, the Cr (III) hosted in KEU is converted into Cr(VI), which is leached out by rainwater. Indeed, Cr(VI) is dispersed in surface water, reaching concentrations up to 18 mg/L and it also occurs in coatings on the surface gravel of unpaved roads. The 53 Cr/ 52 Cr ratio measured in KEU, reported in delta 53 Cr notation, was in the restricted range delta 53 Cr = - 0.031 +/- 0.057 %o ; on the contrary, the delta 53 Cr in water varied between + 1.581 +/- 0.038 %o and + 3.261 +/- 0.191 %o , indicating the reduction of Cr(VI) after the pristine oxidative mobilization. The risk -based soil screening levels (SSLs) for total Cr are well above the concentration measured in soil for all the exposure pathways. On the contrary, the SSL for Cr(VI)indicates that soil contamination poses health hazards for residents

Wodegongjieite, ideally KCa3(Al7Si9)O-32, a new sheet silicate isostructural with the feldspar polymorph kokchetavite, KAlSi3O8

Wodegongjieite occurs in the Cr-11 chromitite orebody of the Luobusa ophiolite in the Kangjinla district, Tibet, China. It is found in two inclusions in corundum: (1) as a partial overgrowth (holotype) up to 1.5 mu m thick around a spheroid 20 mu m across of wenjiite (Ti-10(Si,P,square)(7)), kangjinlaite (Ti-11(Si,P)(10)), zhiqinite (TiSi2) and badengzhuite (TiP), and (2) as pools up to 0.25 mu m wide filling interstices between wenjiite, jingsuiite (TiB2), osbornite-khamrabaevite (Ti[N,C]) and corundum. Energy dispersive analyses gave Al2O3 34.09, SiO2 49.11, K2O 2.56, CaO 11.71, SrO 2.53, total 100.0 wt.%, corresponding to K0.58Sr0.26Ca2.25Al7.20Si8.80O31.20, ideally KCa3(Al7Si9)O-32, for Si + Al = 16 cations.Single-crystal studies were carried out with three-dimensional electron diffraction providing data for an ab initio structure solution in the hexagonal space group P6/mcc (#192) with a = 10.2(2) angstrom, c = 14.9(3) angstrom, V = 1340(50) angstrom(3) and Z = 2. Density (calc.) = 2.694 g.cm(-3). The refinement, which assumes complete Si-Al disorder, gives average T1-O and T2-O bond lengths both as 1.65 angstrom. It was not practical to use unconstrained refinement for the occupancies of the large cation sites 6f and 2a. The ab initio model shows clearly that the two cation sites have different sizes and coordination. Consequently, we imposed the condition (1) that all the K occupies the 2a site as the average K-O bond length of 3.07 angstrom is close to the average K-O bond lengths reported in kokchetavite and (2) that all the Ca occupies the 6f site as the average Ca-O bond length of 2.60 angstrom (2.36 angstrom and 2.84 angstrom for Ca-O1 and Ca-O3, respectively) is reasonable for Ca-O. Assuming that all K and all Ca are located at the 2a site and 6f site, respectively, Sr occupancies of these sites could be refined. Thermal parameters are positive and in a reasonable range. The structure is a sheet silicate isostructural with the K-feldspar polymorph kokchetavite, with two crystallographically distinct sites for K, but not with the topologically identical anorthite polymorph dmisteinbergite (CaAl2Si2O8) with only a single site for Ca. Substitution of K by Ca at the 6f site is associated with marked rotation of the Si,Al tetrahedra and a collapse of the structure to accommodate the smaller Ca ion.The spheroid of intermetallic phases is believed to have formed from the interaction of mantle-derived CH4 + H-2 fluids with basaltic magmas at depths of similar to 30-100 km, resulting in precipitation of corundum that entrapped intermetallic melts. Associated immiscible silicate melt of granodioritic composition crystallised metastably to wodegongjieite instead of a mixture of anorthite and K-feldspar

Pseudomonas aeruginosa infection in cystic fibrosis caused by an epidemic metallo-β-lactamase-producing clone with a heterogeneous carbapenem resistance phenotype

AbstractAn epidemic IMP-13 metallo-β-lactamase (MBL)-producing Pseudomonas aeruginosa clone, causing infections and even large outbreaks in Italian critical care settings, was detected in a young cystic fibrosis patient. In this patient, the chronic infection was sustained by distinct clonal sub-populations of the MBL-producing P. aeruginosa clone, either susceptible or resistant to carbapenems. These findings underscore the importance of infection prevention practices in cystic fibrosis settings and pose an important diagnostic and therapeutic challenge

Two New Organic Co-Crystals Based on Acetamidophenol Molecules

Herein we present two new organic co-crystals obtained through a simple solution growth process based on an acetamidophenol molecule, either paracetamol or metacetamol, and on 7,7,8,8-tetracyanoquinodimethane (TCNQ). These co-crystals are part of a family of potential organic charge transfer complexes, where the acetamidophenol molecule behaves as an electron donor and TCNQ behaves as an electron acceptor. Due to the sub-micron size of the crystalline domains, 3D electron diffraction was employed for the structure characterization of both systems. Paracetamol-TCNQ structure was solved by standard direct methods, while the analysis of metacetamol-TCNQ was complicated by the low resolution of the available diffraction data and by the low symmetry of the system. The structure determination of metacetamol-TCNQ was eventually achieved after merging two data sets and combining direct methods with simulated annealing. Our study reveals that both paracetamol-TCNQ and metacetamol-TCNQ systems crystallize in a 1:1 stoichiometry, assembling in a mixed-stack configuration and adopting a non-centrosymmetric P1 symmetry. It appears that paracetamol and metacetamol do not form a strong structural scaffold based on hydrogen bonding, as previously observed for orthocetamol-TCNQ and orthocetamol-TCNB (1,2,4,5-tetracyanobenzene) co-crystals

I was not born cubic, said low-temperature metamorphic garnet

Garnet is the paradigmatic cubic mineral of metamorphic and igneous rocks, and is generally regarded as optically isotropic. Nonetheless, evident birefringence is observed, particularly in the rare Ca-Fe3+ hydrogarnets, which is attributed to the coexistence of two or more cubic phases. A weak birefringence, with rare examples of optical sector zoning, has also been documented in much more common Fe2+-Mg-Mn garnets, but an adequate explanation for its cause is, so far, lacking. Here we show that optically anisotropic garnets are much more widespread than previously thought, both in blueschists and blueschist-facies rocks, as well as in lower greenschist-facies phyllites, but they are frequently overlooked when working with conventional, 30-µm-thick thin sections. Utilizing a multi-technique approach including optical microstructural analysis, BSEM, EMPA, EBSD, FTIR, TEM, EDT and single-crystal XRD, we demonstrate here that the birefringence in these garnets is related to their tetragonal symmetry, that it is not due to strain, and that crystals are twinned according to a merohedral law. We also show that the birefringent garnets from blueschists and phyllites are anhydrous, lacking any hydrogarnet component, and have compositions dominated by almandine (58-79%) and grossular (19-30%) with variable spessartine (0-21%) and very low pyrope (1-7%). Considering the widespread occurrence of optically anisotropic OH-free garnets in blueschists and phyllites, their common low-grade metamorphic origin, and the occurrence of optically isotropic garnets with similar Ca-rich almandine composition in higher-grade rocks, we conclude that garnet does not grow with cubic symmetry in low-temperature rocks (< 400 ◦C). The tetragonal structure appears to be typical of Fe-Ca-rich compositions, with very low Mg contents. Cubic but optically sector-zoned garnet in a lower amphibolite-facies metapelite from the eastern Alps suggests that preservation of tetragonal garnet is favored in rocks which did not progress to T> ≈500 ◦C, where transition to the cubic form, accompanied by change of stable chemical composition, would take place. Our data show that the crystal-chemistry of garnet, its thermodynamics and, in turn, its use in unravelling petrogenetic processes in cold metamorphic environments need to be re-assessed

Garnet, the archetypal cubic mineral, grows tetragonal

Garnet is the archetypal cubic mineral, occurring in a wide variety of rock types in Earth’s crust and upper mantle. Owing to its prevalence, durability and compositional diversity, garnet is used to investigate a broad range of geological processes. Although birefringence is a characteristic feature of rare Ca–Fe3+ garnet and Ca-rich hydrous garnet, the optical anisotropy that has occasionally been documented in common (that is, anhydrous Ca–Fe2+–Mg–Mn) garnet is generally attributed to internal strain of the cubic structure. Here we show that common garnet with a non-cubic (tetragonal) crystal structure is much more widespread than previously thought, occurring in low-temperature, high-pressure metamorphosed basalts (blueschists) from subduction zones and in low-grade metamorphosed mudstones (phyllites and schists) from orogenic belts. Indeed, a non-cubic symmetry appears to be typical of common garnet that forms at low temperatures (<450 °C), where it has a characteristic Fe–Ca-rich composition with very low Mg contents. We propose that, in most cases, garnet does not initially grow cubic. Our discovery indicates that the crystal chemistry and thermodynamic properties of garnet at low-temperature need to be re-assessed, with potential consequences for the application of garnet as an investigative tool in a broad range of geological environments

Hexavalent chromium release over time from a pyrolyzed Cr-bearing tannery sludge

Pyrolysis in an inert atmosphere is a widely applied route to convert tannery wastes into reusable materials. In the present study, the Cr(III) conversion into the toxic hexavalent form in the pyrolyzed tannery waste referred to as KEU was investigated. Ageing experiments and leaching tests demonstrated that the Cr(III)–Cr(VI) inter-conversion occurs in the presence of air at ambient temperature, enhanced by wet environmental conditions. Microstructural analysis revealed that the Cr-primary mineral assemblage formed during pyrolysis (Cr-bearing srebrodolskite and Cr-magnetite spinel) destabilized upon spray water cooling in the last stage of the process. In the evolution from the higher to the lower temperature mineralogy, Cr is incorporated into newly formed CrOOH flakes which likely react in air forming extractable Cr(VI) species. This property transforms KEU from an inert waste to a hazardous material when exposed to ordinary ambient conditions