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International audienceOlivine mineral replacement by serpentine is one major alteration reaction of oceanic hydrothermalism. In the present experimental study, olivine grains were replaced by chrysotile and brucite under high alkaline conditions. In our study, olivine replacement implied a spatial and temporal coupling of dissolution and precipitation reactions at the interface between olivine and chrysotile-brucite minerals. Coupled dissolution-precipitation led to the alteration of starting olivine grains (so-called primary or parent mineral) to a porous mineral assemblage of chrysotile and brucite with preservation of the initial olivine morphology. This mineral replacement reaction of olivine (serpentinization) has been characterized using XRD, FESEM and FTIR measurements. Moreover, a simple and novel method is here proposed to quantify the mineral replacement rate (or serpentinization rate) of olivine by using thermogravimetric (TG) and differential TG (DTG) analyses. Serpentinization extent depends on the grain size: it is complete after 30 days of reaction for the smallest olivine grains (<30µm), after 90 days of reaction for the intermediate olivine grains (30 µm-56 µm) and reaches 55% of olivine replacement after 90 days for the largest fraction (56-150 µm). Based on the fitting of the serpentinization extent (t) versus time (t) by using a kinetic pseudo-second-order model, the serpentinization rates vary from 3.6x10-6 s-1 to 1.4x10-7 s-1 depending on the olivine grain size. An additional correlation between FTIR spectra analysis and TG measurements is proposed. The mineral replacement reactions frequently observed in natural alteration processes could be a powerful synthesis route to design new porous and/or nanostructured materials

The first structure of a luminescent gold thiolate coordination polymer, [Au(SPh)]n.

13 pagesInternational audienceThe present study reports original experiments in order to investigate the simultaneous serpentinization and carbonation of olivine with relevance in Earth systems (e.g. functioning of hydrothermal fields) or in engineered systems (e.g. ex-situ and in-situ mineral sequestration of CO2). For this case, specific experimental conditions were examined (200°C, saturated vapor pressure ≈ 16bar, solution/solid weight ratio = 15, olivine grain size < 30µm and high-carbonate alkalinity ≈ 1M NaHCO3). Under these conditions, competitive precipitation of magnesite and serpentine (preferentially lizardite type) were clearly determined by using conventional analytic tools (XRD, FESEM, FTIR and TGA); excluding the fate of the iron initially contained in olivine, the alteration reaction for olivine under high-carbonate alkalinity can be expressed as follows: 2〖Mg〗_2 SiO_4+2H_2 O+H〖CO〗_3^-→Mg〖CO〗_3+〖Mg〗_3 〖Si〗_2 O_5 〖(OH)〗_4+〖OH〗^- This reaction mechanism implied a dissolution process, releasing Mg and Si ions into solution until supersaturation of solution with respect to magnesite and/or serpentine. The released iron contained in the olivine has not implied any precipitation of iron oxides or (oxy)hydroxides; in fact, the released iron was partially oxidized (about 50%) via a simple reduction of water (2〖Fe〗^(2+)+〖2H〗_2 O→2〖Fe〗^(3+)+H_2+2〖OH〗^-). In this way, the released iron was incorporated in serpentine (Fe(II) and Fe(III)) and in magnesite (Fe(II). This latter was clearly determined by FESEM/EDS chemical analysis on the single magnesite crystals. The nucleation and epitaxial growth processes at the olivine-fluid interfaces cannot be excluded in our investigated system. The experimental kinetic data fitted by using a kinetic pseudo-second-order model have revealed a retarding process of serpentine formation with respect to magnesite (about three times slower); in fact, the magnesite seems to reach an apparent stabilization after about 20 days of reaction while the serpentine follows a progressive slower evolution. We assumed that the magnesite has reached a fast apparent equilibrium with solution because the available carbonate species are not renewed from fluid phase as typically constrained in aqueous carbonation experiments where a given CO2 pressure is imposed in the system. On the other hand, the reactivity of serpentinized olivine (chrysotile+brucite+small amount of residual olivine) and high-purity chrysotile at the same above investigated conditions; and the olivine serpentinization in initial acid pH ≈ 0.66 are also reported as complementary information in this study. These novel experimental results concerning simultaneous serpentinization and aqueous carbonation of olivine expand the thermodynamic conditions where serpentine and magnesite can simultaneously precipitate; this could contribute to a better understanding of fluid-rock interactions in natural active hydrothermal fields on Earth

Low mechanical pressure on amorphous gold thiolate coordination polymers allows the formation of transparent and red emissive glasses.

Herein, we present the successful synthesis and full characterization (by (11) B magic-angle-spinning nuclear magnetic resonance spectroscopy, infrared spectroscopy, powder X-ray diffraction) of sodium hydrazinidoborane (NaN2 H3 BH3 , with a hydrogen content of 8.85 wt %), a new material for chemical hydrogen storage. Using lab-prepared pure hydrazine borane (N2 H4 BH3 ) and commercial sodium hydride as precursors, sodium hydrazinidoborane was synthesized by ball-milling at low temperature (-30 °C) under an argon atmosphere. Its thermal stability was assessed by thermogravimetric analysis and differential scanning calorimetry. It was found that under heating sodium hydrazinidoborane starts to liberate hydrogen below 60 °C. Within the range of 60-150 °C, the overall mass loss is as high as 7.6 wt %. Relative to the parent N2 H4 BH3 , sodium hydrazinidoborane shows improved dehydrogenation properties, further confirmed by dehydrogenation experiments under prolonged heating at constant temperatures of 80, 90, 95, 100, and 110 °C. Hence, sodium hydrazinidoborane appears to be more suitable for chemical hydrogen storage than N2 H4 BH3 .

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