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 degrees C, saturated vapor pressure approximate to 16 bar,
solution/solid weight ratio = 15, olivine grain size < 30 mu m and
high-carbonate alkalinity approximate to 1 M 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:
2Mg(2)SiO(4) + 2H(2)O + HCO3- -> MgCO3 + Mg3Si2O5(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 (2Fe(2+) + 2H(2)O -> 2Fe(3+) + H-2 + 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 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 approximate to 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. (C) 2014 Elsevier B. V. All rights reserved.