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abundance [Mg(HCO3,OH) 2H2O] is dominant, possibly to
the exclusion of the tri-hydrate. It seems unlikely that the
temperature of synthesis plays a pivotal role in determining
whether basic or hydrate nesquehonite forms, as both variants
have been synthesised at low temperatures, and both
occur in natural near-surface ambient temperature settings
(e.g., Kazakov et al., 1959; Coleyshaw et al., 2003; Hales
et al., 2008). Accordingly, it is plausible that the pH influences
the extent to which HCO 3 is incorporated into
nesquehonite or which isomer of nesquehonite is produced.
In this respect, it is interesting to note that the synthesis of
nesquehonite [MgCO3 XH2O] phases by Zhang et al. (2006)
occurred at pH values of 8.5–12.5. In contrast, in the
experiment documented here, nesquehonite synthesis was
achieved at pH < 8, yet the temperatures of synthesis and
timeframes for mineral formation in the two studies are
comparable.
The solubility of dypingite-type phases is not known
with any degree of certainty. Nevertheless, XRD results
indicate that the system was supersaturated with dypingite-type
phase(s) 20 min after the beginning of the heating
stage, coincident with the onset of very low levels of undersaturation
in nesquehonite (Fig. 7). The progressive emergence
of dypingite-type phases is associated with particles
heterogeneously nucleating on decomposing nesquehonite,
giving rise to house of cards textures. Heterogeneous nucleation
may be responsible for Ostwald step rule behaviour,
as the next most stable phase is often more structurally similar
to the precursor phase than a thermodynamically more
stable phase (Morse and Casey, 1988).
Nesquehonite dissolution and dypingite-type mineral
formation are more extensive in the agitated environment,
relative to the static environment. Strong hydrodynamic
shear forces generated by sonication can increase the rate
of dissolution of suspended solids by de-agglomeration,
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Phase transitions in the system MgO–CO2–H2O 11
Fig. 7. Agitated experiment saturation indices. Note, there are no available thermodynamic data for dypingite-type phases. See text for
details.
simultaneously accelerating the formation of viable nuclei
to increase the rate of crystallization of carbonate mineral
phases (e.g., Kim et al., 2011). No protohydromagnesite
was identified in this study, either because it was rapidly
superseded by [Mg5(CO3)4(OH)2 XH2O] phases, or because
its formation was prohibited by the nature of the nesquehonite
precursor. This may be due to conditions of synthesis
or incongruent water loss prior to complete dissolution.
Samples [AS0] (hydromagnesite), the static environment
powders, and samples [A80], [A120] and [A240] (dypingitetype
and hydromagnesite bearing) all show pronounced
broad band infrared absorption in the ca 1000 cm 1 region,
assigned in large measure to Mg(OH) deformation modes.
The hydromagnesite spectra [AS0] and static environment
spectra are devoid of the corresponding Raman active
band(s), although the bands are clearly resolved in the
FT-Raman spectra of the three precipitates formed in the
agitated environment. These attributes are consistent with
reduced symmetry and therefore greater disorder in the
dypingite-type phases relative to hydromagnesite and, thus,
the greater ease of crystallization of these phases relative to
hydromagnesite. The greater numbers of waters of crystallization
of [Mg5(CO3) 4(OH) 2 8H2O] relative to hydromagnesite
evidently imparts distinct unit cell parameters,
simultaneously affecting Mg(OH) deformation modes, yet
retaining essentially uniform short-range order of the
[CO 2
3 ] anion with respect to hydromagnesite. The reduction
in FT-Raman intensity of scattering in the ca
1000 cm 1 region in [A240] relative to [A80] and [A120] is
in keeping with decreasing disorder of dypingite-type
phases with increasing heating (reaction) time, whereas
the smaller d-spacing of [A240] relative to [A120] is attributed
to cell shrinkage with decreasing waters of crystallization.
Experimental results indicate that, with increasing reaction
time, small amounts of dypingite in association with