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Kimberlites carry a dense cargo of olivine. Although normally referred to as macrocrysts, much of this olivine is present as multigrain xenoliths of dunite. Because of their well-rounded outlines, we refer to these objects as 'nodules' and apply the term to both multigranular and monogranular varieties. Although compositions within each nodule are very uniform, the entire population displays an enormous variation in Fo content (100*Mg/(Mg + Fe), cation proportions), from close to Fo95 to about Fo82. The change is continuous, and there is no indication of separate Fo-rich and Fo-poor populations. Likewise, the sizes, internal textures and habits of Fo-rich and Fo-poor olivines are indistinguishable. Fo-poor nodules have compositions like those of olivines of the megacryst suite, and they share similar sizes and internal structures. We propose that the dunite nodules in kimberlites and olivines of the megacryst suite have a common origin. In our interpretation, almost all olivines in kimberlites, except for the marginal rims and rinds, are fragments of dunite that was produced during interaction between mantle peridotite and CO2-rich fluid. Analogues of this process can be found in ophiolites where dunite is the product of reaction between basaltic magma and peridotite. In situ reactive crystallization eliminated pyroxene, leaving only olivine with Fo contents ranging from higher to lower Fo than in the host peridotite. This interaction occurred near the base of the lithosphere and not during transit of kimberlite towards the surface. [ABSTRACT FROM AUTHOR]

Heterosite FePO 4 is synthesized for the first time by direct thermal oxidation of sarcopside Fe 3 (PO 4 ) 2 . Both FePO 4 and Fe 3 (PO 4 ) 2 have a pseudo olivine structure. Complete isostructural conversion of sarcopside into FePO 4 is achieved at a temperature of 450 °C within 3 days according to the reaction Fe 3 (PO 4 ) 2 + ¾ O 2 → 2 FePO 4 + ½ Fe 2 O 3 which leads to the extraction of iron from the sarcopside structure. Appropriate heating ramp must be applied in order to avoid the crystallization of Fe 7 (PO 4 ) 6 . Electrochemical performances of the oxidation product are consistent with those of olivine FePO 4 . [ABSTRACT FROM AUTHOR]

Abstract: The 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) was 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: 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 (2Fe2+ +2H2O→2Fe3+ +H2 +2OH−). In this way, the released iron was incorporated in serpentine (Fe(II) and Fe(III)) and in magnesite (Fe(II). The 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 20days 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. [Copyright &y& Elsevier]

Geological storage of CO2 in mafic and ultramafic rocks relies on the dissolution of their silicate components, followed by the precipitation of carbonates, the overall process being commonly referred to as carbonation. To gain a better understanding of the rate- and yield-controlling factors of mineral carbonation, three batch experiments were conducted in CO2-saturated water, at (a) 90°C, (b) 120°C and (c) 170°C, respectively (with citrate ligands added to the 90°C experiment) in a Ti-reactor at pCO2 =280bar, using San Carlos olivine (Fo88) as a model mineral of (ultra)mafic environments. Those physicochemical conditions were purposely chosen so as to promote the dissolution rate of olivine. (a) At 90°C, ~40wt.% of olivine were dissolved within 3weeks, a result which suggests that the passivation barrier evidenced in previous studies carried out in citrate-free media had been overcome. However, while saturation with respect to magnesite was overstepped in this experiment, the carbonation yield remained below ~1wt.%, which we attributed to the slow kinetics of magnesite precipitation. (b) At 120°C, the concentration of Mg and Si monitored by aqueous fluid sampling reached an apparent plateau for over 4weeks, at conditions close to saturation with respect to amorphous silica, and with Fe concentration being below the detection limit. A resumption of the dissolution process was subsequently observed while the concentration of Fe suddenly increased in solution. The concentration plateau was attributed either to the formation of a protective Si–Fe(III) layer, or to the occurrence of Fe(III) phases clogging the porosity of a secondary silica layer. The resumption of olivine dissolution was ascribed to the increasingly reducing conditions that led to the breakdown of the protective phases. This late resumption led to a carbonation yield slightly above 2wt.%. (c) At 170°C, Fe was incorporated into a presumably permeable Fe-rich phyllosilicate layer, so that the dissolution process was likely less affected by such an interfacial layer. Nevertheless, carbonate minerals still formed in low quantity (~6wt.%), while other amorphous phases represented the main sink for Mg cations. Overall, this study emphasizes that achieving carbonation reactions in reducing environments and circum-neutral aqueous solutions may represent a necessary requirement for making olivine carbonation a viable process in these temperature ranges. It also insists on the importance of transitory phases, which form both at the fluid-silicate interface and from the bulk solution. While they may not appear in overall carbonation equations or in thermodynamic databases, arguably they can drive the global kinetics of olivine dissolution and its transformation towards magnesite. [ABSTRACT FROM AUTHOR]

Abstract: Olivine 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 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 (30μm–56μm). For the largest fraction (56–150μm), 55% of serpentinization extent was reached after 90 days. Based on the fitting of the serpentinization extent (ξ t ) versus time (t) using a kinetic pseudo-second-order model, the serpentinization rates vary from 3.6×10−6 s−1 to 1.4×10−7 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. [Copyright &y& Elsevier]