Experimental serpentinization in alkaline systems

The aim of these studies was to determine fluid/serpentine partition coefficients of these elements as well as their effects on serpentine formation (reaction mechanism and kinetics, textural properties etc.).

We first develloped experimental protocols to synthesize serpentine under highly alkaline Experimental-products were characterized using X-ray powder diffraction (XRPD), Fourier Transform Infra-Red spectroscopy (FTIR), N2 sorption isotherms, ThermoGravimetric Analyses (TGA), Field Emission gun Scanning Electron Microscopy (FESEM), High Resolution Transmission Electron Microscopy (HRTEM), X-ray Absorption Spectroscopy (XAS) and Mössbauer Spectroscopy.

The first protocol consists in chrysotile synthesis from H2SiO3 and MgCl2 at 300 °C using batch and semi-continuous experiments. With this approach, we were able to chrarcterized chrysotile nanotubes nucleation and growth processes.graphical-abstractTransition from Proto-serpentine to individual serpentine depicted by SEM and TGA investigations

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First hours of “proto-serpentine” synthesis and conceptual models draw from TEM observations and High energy X-ray diffraction modelling

The second protocol, consist in olivine serpentinization reaction under high hydroxyl-alkalinity or high carbonate-alkalinity at 200 °C. We note the efficiency of serpentine formation under high alkaline conditions in the both protocols and the significant effect of the carbonate component on the serpentinization processes and crystal growth rates.

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Exemple of kinetic curves for olivine alteration in CO2 free systems and under carbonate-alkalinity conditions (fluids S1 and S2).

Olivine replacement is total after 1 month (<30 µm) and 3 month (30-56 µm). The presence of a carbonate component induces a lower reaction kinetic and is characterized by the co-precipitation of magnesite and lizardite.

We report new results concerning the sequestration and the distribution of the trace elements during olivine replacement by serpentine and brucite. We highlight that Li act as a catalyst during olivine serpentinisation. Moreover, from XAS measurements, we indicate that Sb and As sequestration is dominated by adsorption mechanism. The precipitation of secondary As- and Sb-bearing phases was also revealed by Electron Microprobe X-ray mapping. Finally, Sb-trapping within chrysotile tubes was also suspected by HRTEM measurements. The changes of redox conditions during serpentinisation induce a change of Sb sequestration mechanism and the precipitation of Sb-bearing phases.

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Cs, As and Sb behaviour during olivine alteration: solid/fluid Cs partitioning decrease throuhout reaction advancement, As is preferentially adsorbed by brucite and Sb is first trapped by serpentine as Sb+5 then by Sb-rich phases as Sb+3.

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