Irregular Grains Research Articles

Synthesis of a bicontinuous structured SrTiO3 porous film with significant photocatalytic activity by controlling phase separation process

A series of hierarchically porous SrTiO3 films with different pore structure characteristics from bicontinuous macroporous–mesoporous structure to ordered and fragmented structures have been synthesized using sol–gel method, by controlling the competition between polyethylene glycol (PEG)-induced phase separation and sol–gel transition. Field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), Brunauer–Emmett–Teller (BET) analysis, X-ray diffractometry (XRD) and photoluminescence (PL) spectroscopy were used to characterize the as-prepared samples. The bicontinuous structured SrTiO3 porous film constructed by irregular grains possessed three-dimensional (3D) interconnected network with both macropores (80–400 nm) and mesopores (1–10 nm), which provides abundant active sites (the specific surface area up to 121.9 m2/g) and ensures fast mass transfer simultaneously. In addition, PL spectra illustrated that the film consisting of 3D bicontinuous macroporous– and mesopores has a relatively low photocarrier recombination rate. As a result, the SrTiO3 film with 3D bicontinuous porous structure exhibits more outstanding photocatalytic behavior with high degradation capability for Rhodamine B (RhB) (99.8% after 90 min irradiation by 500 W mercury lamp) than the others. Moreover, nearly 80% of the degradation efficiency can be achieved after five cycles.

Issues with integrating carbonate sand texture data generated by different analytical approaches: A comparison of standard sieve and laser-diffraction methods

Palaeoenvironmental reconstructive work relies on sediment-texture information as a proxy for particle-transport dynamics. As the integration of data from multiple sources is often sought for studies at the regional scale, a body of work exists that addresses compatibility levels of different analytical approaches. Past insights are derived predominantly from siliciclastic deposits. This study instead addresses standard sieve and laser-diffraction methods of texture analysis on a suite of carbonate beach sands from San Salvador Island, Bahamas. While the near-equant grain shapes of siliciclastic sands generate comparable results between methods, our findings show that laser diffraction tends to overestimate the coarser fractions in carbonate sands, given indiscriminate measurement of elongate particle axes. Peloidal grains that have width-to-length ratios on the order of 0.5 can pass through much smaller mesh sizes when sieved than diffraction would suggest. The resulting deviation in analytical outputs significantly influences measures of skewness, sorting, and other derivative metrics, which vary by up to several Wentworth classes between procedures. Meaningful integration of texture data obtained by different methods is ill-advised when dealing with carbonate beach sands. Similar problems are documented for muddy deposits, wherein platy clay minerals favor size-overestimation by laser diffraction and a size underestimation by settling tube technique. An improved understanding of methodological biases resulting from application of different particle-size methods is needed to help facilitate data-reconciliation efforts. This study provides a glimpse of issues facing the integration of carbonate sand-texture information, troublesome given prevalence of irregular grain shapes.

Microstructure Investigations of Powders and Additive Manufactured Parts

This paper presents an advanced microstructural analysis of the AlSiMg, Ti64, and N700 powders used for additive manufacturing. The internal microstructure of the regular and irregular powder grains were characterized down to atomic resolution by using scanning electron microscopy and high resolution scanning transmission electron microscopy.The accretionary forms on top of the irregular AlSiMg powder grains exhibit a slightly coarse microstructure with a network of eutectic Si consisting of nano-crystallites, suggesting a slower cooling than the grain itself that contain a predominately amorphous Si network.A nm thin amorphous C layer on the surface of some Ti64 plasma atomized powder grains promoted the attachment of satellites and growth of envelopes. In case of gas atomized N700 powder grains, we identified thin oxide and carbon amorphous layers as well as metal segregations at the interface between the grain body and the accretionary forms.

Effect of grain structure on the mechanical properties and in vitro corrosion behavior of additively manufactured pure Zn

Laser powder bed fusion (LPBF) has been previously used to produce customized medical implants from biodegradable Zn and its alloys. In this study, we investigated the effect of the grain structure on the mechanical properties and in vitro corrosion behavior of pure Zn samples, by varying the scanning speed and building direction during the LPBF process. Increasing the scanning speed from 300 to 700 mm/s resulted in finer grains, irregular grain morphology, and a weaker grain texture, which enhanced the strength and ductility. Vertically built LPBF Zn tensile samples had higher strength and ductility compared with horizontally built samples, indicating strong anisotropy of the mechanical properties. Electrochemical tests revealed that the in vitro corrosion behavior was not strongly correlated with the scanning speed. This was attributable to the random distribution of tiny pores on the surface of the LPBF samples, although immersion tests showed that the sample prepared with the highest scanning speed exhibited the highest corrosion rate. With increasing immersion time in Hank’s solution, the Zn2+ concentrations of the samples produced with different scanning speeds increased, their pH stabilized, and the differences between the corrosion rates narrowed. The effects of the processing parameters on the final performance of the samples could be well explained by the grain structures. The findings of this study afford bases for selecting the processing parameters for optimizing the properties of LPBF-produced Zn parts for biodegradable applications.

Investigating the Origin of Enhanced C2+ Selectivity in Oxide-/Hydroxide-Derived Copper Electrodes during CO2 Electroreduction.

Oxide-/hydroxide-derived copper electrodes exhibit excellent selectivity toward C2+ products during the electrocatalytic CO2 reduction reaction (CO2RR). However, the origin of such enhanced selectivity remains controversial. Here, we prepared two Cu-based electrodes with mixed oxidation states, namely, HQ-Cu (containing Cu, Cu2O, CuO) and AN-Cu (containing Cu, Cu(OH)2). We extracted an ultrathin specimen from the electrodes using a focused ion beam to investigate the distribution and evolution of various Cu species by electron microscopy and electron energy loss spectroscopy. We found that at the steady stage of the CO2RR, the electrodes have all been reduced to Cu0, regardless of the initial states, suggesting that the high C2+ selectivities are not associated with specific oxidation states of Cu. We verified this conclusion by control experiments in which HQ-Cu and AN-Cu were pretreated to fully reduce oxides/hydroxides to Cu0, and the pretreated electrodes showed even higher C2+ selectivity compared with their unpretreated counterparts. We observed that the oxide/hydroxide crystals in HQ-Cu and AN-Cu were fragmented into nanosized irregular Cu grains under the applied negative potentials. Such a fragmentation process, which is the consequence of an oxidation-reduction cycle and does not occur in electropolished Cu, not only built an intricate network of grain boundaries but also exposed a variety of high-index facets. These two features greatly facilitated the C-C coupling, thus accounting for the enhanced C2+ selectivity. Our work demonstrates that the use of advanced characterization techniques enables investigating the structural and chemical states of electrodes in unprecedented detail to gain new insights into a widely studied system.

Effect of SiC on the dielectric and microwave absorption performance of F-doped Si3N4 ceramics in X-band

In this study, silicon carbide reinforced silicon nitride (SiC/Si3N4) ceramics were fabricated by hot press (HP) sintering with the addition of fluorides (F=AlF3, MgF2). In order to investigate the dielectric and microwave (MW) absorption properties of SiC/Si3N4 ceramics in X-band, various concentrations of silicon carbide (SiC) have been used. The results demonstrate a complete phase transformation and the intergranular behavior of the SiC/Si3N4 ceramics with large and irregular grains of β−Si3N4. The relative complex permittivity and dielectric loss tangent increase significantly with the increasing fraction of SiC. The Cole-Cole semicircle analysis for SiC/Si3N4 ceramics shows a ternary dielectric behavior with the addition of SiC. Meanwhile, the reflection loss (RL) decreases with the increase of SiC content and the lowest value of RL was achieved −8.2 dB at 12.4 GHz, which reveal a favorable prospect as MW absorbers.

Influence of pulsed laser ablation temperature on structure, morphology and electrocatalytic properties of cobalt-based films deposited on carbon cloth

Cobalt-based electrocatalytic films on electrochemically activated carbon cloth (EACC) were prepared by pulsed laser deposition (PLD) technique at different substrate temperatures. The composition, nanostructure and morphology of the films were thoroughly characterized and their electrocatalytic activities towards hydrogen and oxygen evolution in alkaline solutions were assessed. The electrochemical performances were correlated with material characteristics. The cobalt-based film morphology and structure changed significantly with increasing growth temperature, from nanocrystalline dense film, through uniform monocrystalline nanopillar structure, up to coarser irregular grains. Also, the film composition has clearly changed in the series of electrodes. The films prepared at lower temperatures were predominantly composed of CoCO3, while at higher temperatures their primary component was Co3O4. For prepared electrodes, the influence of material morphology on hydrogen evolution activity is shown to be pivotal, while no effect of material composition on performance is confirmed. Careful control over deposition parameters enabled formation of the film with optimal electrocatalytic performance towards both HER and OER, characterized with nanopillar morphology topped with pyramid-shape caps, providing high electrochemically active surface area for electrochemical reactions. The optimized electrode requires the overpotentials of 270 and 400 mV to deliver the current density of 10 mA cm−2 for HER and OER respectively.

Structural, magnetic and Mössbauer analysis of lanthanum and nickel doped Co2Y-type hexaferrite nanomaterial matrix synthesized by sol-gel auto-combustion technique

Sol-gel auto-combustion technique was used to prepare the lanthanum (La3+) and nickel (Ni2+) doped Co2Y-type Sr2-yLayCo2NixFe12-xO22; x = 0.0, 0.1, 0.2, 0.3; y = 0.0, 0.01, 0.02, 0.03 nanohexaferrites materials. Synthesized nanohexaferrites were characterized by XRD, FTIR and EDS analysis. The collective XRD analysis reveals the single-phase patterns corresponding to various dopants concentration in the powder system matrix. The calculated structural - auxiliary parameters viz. Lattice constants (a and c), cell volume (Vcell), X-ray density (dx), bulk density (db) and porosity (P) have been found in range; a = 5.8798–5.8321 Å, c = 43.7316–44.7214 Å, Vcell = 1309.303–1317.304 Å3, dx = 5.014–4.987 gm/cm3, db = 3.774–3.128 gm/cm3 and P = 0.247–0.37. The XRD results were supported by FESEM and TEM images, which showed the plate-like shape of particles. Unlike irregular grains, regular platelet-like grains were composed of several crystallites with almost parallel c-axis and this is the most suitable shape for microwave absorption. Fourier-transform infrared spectroscopy (FTIR) showed the location of the ions including their respective bonds within the structure of the lattice matrix and variation of ion concentration was depicted by EDS analysis of the powder matrix. The investigation of magnetic properties was done by vibrating sample magnetometer (VSM). The saturation magnetization (Ms) and coercivity (Hc) were found to be in the range of 34.29–43.711 emu/g and 16.27–299.441 Oe, respectively. The improved values of Ms and Hc mean that these powders can be used as pre-eminent contestant materials for perpendicular recording media (PMR) applications. In addition, the synthesized Co2Y-type nanohexaferrites were characterized for the Mössbauer analysis at room temperature. The normal Zeeman (six-line patterns) splitting confirms the hexagonal phase of synthesized nanohexaferrites.

Doping mechanisms and dielectric properties of Ca-doped bismuth magnesium niobate pyrochlores

Here we report the doping mechanisms and dielectric properties of Ca-doped bismuth magnesium niobate (BMCN) pyrochlores synthesised using solid-state reaction at 1025 °C over 24–48 h. The extensive solid solution of (Bi3.36Mg0.64-xCax)(Mg1.28Nb2.72)O13.76 (0 ≤ x ≤ 0.7) pyrochlores formed with a fully indexed cubic structure, space group Fd3m and lattice constants, a = b = c in the range 10.5621 (17)−10.5332 (14) Å. The SEM analysis confirmed that the surface morphologies of BMCN pyrochlores were of irregular polyhedral grains and their crystallite sizes, as calculated by Scherrer and Williamson-Hall methods, were found to be in the range 46–70 nm and 38–75 nm, respectively. These thermally stable BMCN pyrochlores were highly insulating materials that showed a reproducible dielectric behaviour over heat-cool cycles and their activation energies were estimated to be in the range 1.17–1.47 eV. High bulk dielectric constants, ε’, 69–171 and low dielectric losses, tan δ, in the order of ~10−3 were recorded at 30 °C and 1 MHz; negative temperature coefficient of capacitance (TCC) in the range, (−319)−(-933) ppm/°C was measured over ~30–300 °C at 1 MHz.

A New Mineral Ferrisanidine, K[Fe3+Si3O8], the First Natural Feldspar with Species-Defining Iron

Ferrisanidine, K[Fe3+Si3O8], the first natural feldspar with species-defining iron, is an analogue of sanidine bearing Fe3+ instead of Al. It was found in exhalations of the active Arsenatnaya fumarole at the Second scoria cone of the Northern Breakthrough of the Great Fissure Tolbachik Eruption, Tolbachik volcano, Kamchatka Peninsula, Russia. The associated minerals are aegirine, cassiterite, hematite, sylvite, halite, johillerite, arsmirandite, axelite, aphthitalite. Ferrisanidine forms porous crusts composed by cavernous short prismatic crystals or irregular grains up to 10 μm × 20 μm. Ferrisanidine is transparent, colorless to white, the lustre is vitreous. Dcalc is 2.722 g·cm−3. The chemical composition of ferrisanidine (wt. %, electron microprobe) is: Na2O 0.25, K2O 15.15, Al2O3 0.27, Fe2O3 24.92, SiO2 60.50, in total 101.09. The empirical formula calculated based on 8 O apfu is (K0.97Na0.03)Ʃ1.00(Si3.03Fe3+0.94Al0.02)Ʃ3.99O8. The crystal structure of ferrisanidine was studied using the Rietveld method, the final R indices are: Rp = 0.0053, Rwp = 0.0075, R1 = 0.0536. Parameters of the monoclinic unit cell are: a = 8.678(4), b = 13.144(8), c = 7.337(5) Å, β = 116.39(8)°, V = 749.6(9) Å3. Space group is C2/m. The crystal structure of ferrisanidine is based on the sanidine-type “ferrisilicate” framework formed by disordered [SiO4] and [Fe3+O4] tetrahedra.

Phase Field Modelling of Abnormal Grain Growth

Heterogeneous grain structures may develop due to abnormal grain growth during processing of polycrystalline materials ranging from metals and alloys to ceramics. The phenomenon must be controlled in practical applications where typically homogeneous grain structures are desired. Recent advances in experimental and computational techniques have, thus, stimulated the need to revisit the underlying growth mechanisms. Here, phase field modelling is used to systematically evaluate conditions for initiation of abnormal grain growth. Grain boundaries are classified into two classes, i.e., high- and low-mobility boundaries. Three different approaches are considered for having high- and low-mobility boundaries: (i) critical threshold angle of grain boundary disorientation above which boundaries are highly mobile, (ii) two grain types A and B with the A–B boundaries being highly mobile, and (iii) three grain types, A, B and C with the A–B boundaries being fast. For these different scenarios, 2D simulations have been performed to quantify the effect of variations in the mobility ratio, threshold angle and fractions of grain types, respectively, on the potential onset of abnormal grain growth and the degree of heterogeneity in the resulting grain structures. The required mobility ratios to observe abnormal grain growth are quantified as a function of the fraction of high-mobility boundaries. The scenario with three grain types (A, B, C) has been identified as one that promotes strongly irregular abnormal grains including island grains, as observed experimentally.

Textural and chemical variations of magnetite from porphyry Cu-Au and Cu skarn deposits in the Zhongdian region, northwestern Yunnan, SW China

The Zhongdian region in northwestern Yunnan (SW China) hosts a number of important porphyry and skarn deposits. In this study, we integrate petrographic observations, SEM-EDS elemental mapping, EPMA and LA-ICP-MS geochemical analyses on magnetite from several key deposits in the region. Magnetite grains from the Pulang porphyry Cu-Au, Lannitang porphyry Au-Cu, Langdu Cu skarn, Disuga porphyry, and andesite of Tumugou Formation were analyzed to unravel the magnetite formation mechanism, and the relative oxygen fugacity of the porphyry/skarn ore-related hydrothermal fluids in this region.Two types of magnetites (Magnetite I and II) have been identified. Magnetite I is commonly distributed in the chlorite and/or sericite alteration zone(s). Magnetite I grains are characterized by being textually inhomogeneous. The Magnetite I core (with local ilmenite exsolution lamellae) is Ti-Cr rich, indicative of a dominantly igneous origin. The Magnetite I core and its surrounding primary igneous minerals (biotite, amphibole and plagioclase) underwent late alteration. Magnetite I grains are characterized by core-rim texture, sharp core-rim contact, and irregular grain boundaries. These textural features indicate that Magnetite I may have undergone dissolution-reprecipitation processes, during which the Magnetite I core was chemically purified via losing Ti and gaining Fe. Magnetite II (in potassic and sericite alteration zones) is textually homogeneous with relatively low contents of Ti, Mg, Al, V, Ni, and Mn. We suggest that Magnetite II was formed by hydrothermal alteration. Compared to Magnetite II grains from the Pulang and Langdu deposits, those from the Lannitang deposit have lower Mn and V but higher Sn contents, indicative of higher oxygen fugacity of the hydrothermal fluids. We propose that binary plots such as Ti vs. Sc, V vs. Sn and Ga vs. Sn can distinguish the different origins of magnetites.From an exploration perspective, Magnetite II from the altered Disuga porphyries has similar texture and composition to its counterpart from the potassic-altered Pulang porphyries. This, together with the presence of Cu-sulfide inclusions in the Disuga Magnetite I, suggests significant mineralization potential in these porphyries.

Lifting grains by the transient low pressure in a martian dust devil

Lifting dust and sand into the thin Martian atmosphere is a challenging problem. Atmospheric pressure excursions within dust devils have been proposed to support lifting. We verify this idea in laboratory experiments. Pressure differences up to a few Pa are applied along particle layers of 50 to 400 μm. As samples we used glass beads of ∼50 μm diameter and irregular basalt grains of ∼20 μm in size. The total ambient pressure of air was set to 600 Pa. Particles are ejected at pressure differences as low as 2.0 ± 0.8 Pa. In the case of glass beads, the ejected grains returning to the particle bed can trigger new particle ejections as they reduce cohesion and release the tension from other grains. Therefore, few impacting grains might be sufficient to sustain dust lifting in a dust devil at even lower pressure differences. Particle lift requires a very thin, ∼100 μm, low permeability particle layer on top of supporting ground with larger pore space. Assuming this, our experiments support the idea that pressure excursions in Martian dust devils release grains from the ground.

Effect of sintering temperature on the mechanical properties and microstructures of pressureless-sintered B4C/SiC ceramic composite with carbon additive

B4C/SiC (15 wt%) ceramic composite with 2 wt% carbon additive was fabricated via solid-phase pressureless sintering under an argon atmosphere for 60 min between 2100 °C and 2200 °C. The effects of the sintering temperature on the microstructure and mechanical properties of the B4C/SiC ceramic composites were studied in detail. The optimum values of the relative density, Vickers hardness, flexural strength, and fracture toughness of the B4C/SiC ceramic composite sintered at 2150 °C were 95.3%, 25.5 GPa, 296 MPa, and 2.81 MPa m1/2, respectively. The microstructural investigations revealed that when the sintering temperature increases from 2100 °C to 2150 °C, sintered necks begin to grow and the densification zone increases. With sintering temperatures above 2150 °C, the grains grow rapidly such that the pores are not eliminated, causing the formation of large closed pores and resulting in a decrease in the relative density and mechanical properties of the B4C/SiC ceramic composite. The plate-like and irregular SiC grains are uniformly dispersed in the B4C matrix to inhibit B4C grain growth. The interface between SiC and B4C is clean and narrow, and the clean boundary suggests strong binding, which contributes to the mechanical properties of the B4C/SiC ceramic composite. At 2150 °C, the C additive undergoes significant crystallisation, forming lamellar structures with high aspect ratios, thereby promoting the pores in the ceramic composite to be discharged to the outside through volume diffusion to obtain a denser sintered body. The interface between B4C and graphite is a semi-coherent interface that increases the flexural strength with a low defect concentration. The main toughening mechanisms of the B4C/SiC ceramic composite are crack deflection and crack branching around the SiC grains and crack bridging by SiC grains in the wake of the crack tip.

Innovative technological circuits for regular aggregates production

The aim of the article is to discuss possibilities of production of aggregates with increased content of regular particles, on some well-known examples, as well as presentation of methods of aggregate production, which was not previously present in the industry. Traditional aggregate production circuits require application of three or four grinding stages (depending on the particle size of the feed), but for fine particle fractions of aggregates irregular grains occur on average approximate content of 10%. The innovative technological circuit can produce the aggregates with the content below 3% irregular grains, even in a single- or two-stage crushing circuit.

Blue Luminescent Phosphor Sr3Y1−X(BO3)3:XBi3+ for WLED Applications

AbstractThe polycrystalline powder sample of Bi3+‐activated strontium yttrium borate phosphor Sr3Y1−x(BO3)3:xBi3+ (x = 3, 4, 5 mole%) is prepared by solution combustion technique. Formation of phosphor in the desired crystalline phase is confirmed by powder X‐ray diffraction characterization and FTIR, SEM images of the synthesized phosphor show the irregular grains with average particle size 2.5 µm. Luminescence properties of the synthesized phosphor are investigated at room temperature. The excitation spectrum consists of a single broad absorption band from 200 to 370 nm with the prominent excitation peak at 336 nm [1S0 to 3P1 of Bi3+ ions] and weak excitation peak at 254 nm [1S0 to 1P1 of Bi3+ ions]. Strongest emission peak of 492 nm wavelength that is of blue light is observed at 336 nm UV light excitation. Sr3Y1−x(BO3)3:xBi3+ phosphor emits blue light. As Bi3+ is sensitive to the surrounding crystal field environment it shows weak excitation peak at 254 nm, which gives emission at 413 nm. Hence, Sr3Y1−x(BO3)3:xBi3+ is new UV excited blue emitting phosphor useful for UV/blue chip WLEDs.

VO2 film with small hysteresis width and low transition temperature

In this paper, we investigate the vanadium dioxide (VO2) film as optical switching deposited on Al doped zinc oxide (AZO) glass substrates by DC reactive magnetron sputtering. Field-emission scanning electron microscopy reveal that the deposited VO2 film consists of irregular nanostructured grains. The optical tests by means of UV–visible spectrophotometer show that the deposited VO2 film exhibits excellent optical switching properties with a very small phase-transition hysteresis width of 2.9 °C as well as a low phase-transition temperature of about 48 °C. More interesting, the semiconductor-to-metal transition is readily induced by Joule heat produced by electrical current through the AZO conductive layer, which is more efficient and convenient compared with the traditional heating of the sample by heating plate. This work is of crucial importance for potential applications of VO2 film in compact optical switching devices.

Study on in-air electro-contact discharge (ECD) truncating of coarse diamond grinding wheel for the dry smooth grinding of hardened steel

Abstract Dressing is used to protrude the micron-scale grains from wheel bond, but the grain protrusion height is not identical due to the irregular grain shape and distribution. In order to assure the grain uniformity with identical protrusion height, the electro-contact discharge (ECD) truncating is proposed after dressing. In the ECD truncating, an in-air impulse discharge thermal between wheel and electrode is transferred to cutting interface of diamond grain through the cutting chip. Accordingly, the outstanding thermal conductivity of diamond is utilized to perform a mechanical and thermochemical removal of grain edge layer without any damage to its hardness. The objective is to realize an efficient truncating of coarse diamond wheel for dry smooth grinding of hardened steel. First, a constant-voltage/constant-current transform was employed to suppress the short-circuiting current between wheel and electrode and to adjust the discharge thermal to diamond grain surface. Then the grain temperature distribution was modeled with reference to the transfer of impulse discharge thermal to cutting interface for diamond graphitization. Finally, the truncated diamond wheel was used to perform a grinding in contrast to general CBN wheel. It is shown that the spark-discharge thermal transferred from the rolled-up cutting chip to the grain cutting interface contributes to truncating. In in-air ECD truncating, the spark-discharge voltage ranges 19–23 V, which increases the grain truncating rate by 95% against the general arc-discharge. The truncating temperature may be maintained as about 626 °C to induce the layer graphitization of diamond grains with the mechanical and thermochemical removal up to about 6435 μm3/min. In contrast to wet CBN grinding, the dry grinding greatly improves the ground surface with the truncated diamond wheel rather than the dressed one.

Supergene gold characterization by geochemistry, grain morphology and Au-Ag-Cu-Te classification

Primary characteristics and transformations during laterization processes were investigated in lateritic profiles integrating geochemical analysis and gold grain morphology provided by ICP-ES, electron microprobe and electron microscope in nine sites in three distinct gold provinces in Midwest Brazil. The acquired chemical data indicate a supergene concentration of Al, Fe, Au, As, Co, Cr, Cu and V allowing to detect higher leaching rates of the lateritic profiles in Area I where lateritic duricrust is well developed, relative the lower leaching related to truncated lateritic profiles in Area II. The totally free Au grains of veins, colluvium and alluvium deposits in area II and III show sub-rounded to rounded, irregular, elongated, delicate and polygonal-shaped grains with corrosion cavities, indicating primary vein source, feeble dissolution related to subtler lateritization effects and little mechanical reworking in the colluvium and alluvium deposits. These results allowed to identify subtler weathering effects and small distance transport for the Au grains collected in colluvium and alluvium samples. The quantification of Au-Ag-Cu-Te chemical composition of the free Au grains offered important preliminary discrimination, proving to be a fast, easy and useful approach. It allows classifying the studied deposits in five groups, strongly related to their primary sources (gold-rich porphyry deposits and epithermal deposits).

In situ evidence of earthquakes near the crust mantle boundary initiated by mantle CO2 fluxing and reaction-driven strain softening

This study aims to understand the process behind the worldwide connection between deep crustal/upper mantle earthquakes and CO2 emissions along faults in rift zones. We do this by studying CO2-induced mineral reactions that facilitate strain localization in peridotites from an ancient rift zone in the Seiland Igneous Province (SIP), North Norway.Strain localization in association with hydration processes is well documented in all types of tectonic settings and has major implications for rheological behavior in active plate margin processes. The implications of CO2-bearing fluids are less studied, though experiments have shown how CO2 can influence the flow laws of olivine by imposing a brittle and more localized type of deformation.This study documents narrow shear zones observed within ultramafic rocks from the Seiland Igneous Province (SIP) comprising large volumes (>20,000 km3) of mafic, ultramafic, silicic and alkaline melts that were emplaced into the lower continental crust (25–30 km) between 570 and 560 Ma under an extensional regime. The extensional shear zones are mm cm-scale and contain extremely fine-grained material with a distinct shape preferred orientation (SPO), but weak to absent crystallographic preferred orientation. The shear zones offset dykes across numerous micro-faults that are documented in areas close to a major fault zone cutting through the area. Within the shear zones, olivine and clinopyroxene react to form orthopyroxene and dolomite at approximately 11 kb and 850°C according to the reaction:2 Olivine + Clinopyroxene + 2 CO2 = Dolomite + 2 OrthopyroxeneThis reaction formed coronas of orthopyroxene and dolomite between olivine and clinopyroxene in the shear zones. In addition, large olivine grains proximal to the shear zones show a microfabric with subgrain walls decorated by rounded grains of dolomite and more irregular and elongated grains of orthopyroxene. Clinopyroxene grains are separated from the enstatite and dolomite by at least hundreds of microns, suggesting material transport within the shear zone. The shear zones thus provide a unique insight into the interplay between CO2-metasomatism and reaction accommodated strain softening. Carbonation-driven cracking and mineral reaction also serves to reduce grain size, making grain boundary sliding an efficient process, further enhancing the rheological contrast between the shear zone and the host rock. The sudden decrease in rock strength could lead to rapid deformation and triggered pseudotachylite formation during earthquake events in the near proximity of the micro-shear zones. Our observations match the relations between CO2 emissions and earthquakes observed in present rift environments such as the East African rift and in New Zealand, and underline the importance of active shear zones as fluid conduits in the lower crust and upper mantle.