Amorphous Calcium Carbonate: Synthesis and Transformation - Bioinspiration: From Nano to Micro Scales - page 208

Interestingly, Xu and co-workers reported two different transformations via
thermal and solution pathways [ 107 ]. They found that water was released from
the ACC samples when the temperature was above 373 K and the CaCO 3 began
to crystallize at around 543-673 K, which resulted in the formation of the calcite
crystals. When ACC is heated, the water molecules involved in the ACC can be
released and ACC will transform into the crystal phases. On the other hand, the
other metastable polymorphs of CaCO 3 , i.e., aragonite and vaterite, were readily
to be induced during the solution transformation of ACC when the low water
amounts were used. Nevertheless, water could still make them turn into the stable
calcite phase as the experimental time period goes on. Based on the experimental
results, the roles of water and anionic polyelectrolytes in the phase transformation of
ACC were confirmed. Water could promote the transformation and crystallization
of ACC. Anionic polyelectrolytes, poly sodium styrenesulfonate (PSS) and PAA,
inhibited the crystallization process and the crystallization temperature increased to
the higher values.
Mann and co-workers [ 108 ] also found that the transformation pathways are
determined by extent of water penetration into the ACC cores and electrostatic
interactions at the mineral-surfactant interface. It indicated that the complex hybrid
nanostructures can be assembled in situ when these processes are coupled synergis-
tically at mesoscopic level.

6.3.6 The Role of Additive during Crystallization

of Amorphous Calcium Carbonate

Magnesium can certainly be expected to influence the biogenic CaCO 3 ,suchas
stabilizing the ACC, incorporating into the calcite lattice [ 105 , 109 - 113 ], inducing
the formation of the aragonite [ 114 , 115 ], and so on. Interestingly, almost all
known biogenic ACC contains magnesium, which is presented in large quantities
in seawater (around 50-60 mM Mg 2C , relative to the 12 mM Ca 2C )[ 116 ].
More recently, it has been known that the organism magnesium calcite is often
produced through the magnesium amorphous precursor pathway. Yu et al. reported
a facile confined crystallization method for incorporation of high content Mg in
calcite by crystallization of compact tablet of Mg-ACC in water without using
any organic additives [ 117 ]. In a typical experimental procedure, calcium chloride
(0.15 mol dm 3 , 50 mL) and magnesium chloride hexahydrate (1.5225 g) were
mixed with mechanical stirring to obtain a mixed solution. Then, an anhydrous
sodium carbonate solution (0.15 mol dm 3 , 50 mL) was added rapidly to the
mixed solution with mechanical stirring at an ambient temperature of 25 ı C. The
precipitated colloidal phase was filtered immediately and washed with ethanol.
The precipitates were dried in a vacuum desiccator for 1 day. Then, the flake of
Mg-ACC powder or Mg-ACC powder (about 0.25 g) was put into 500 mL double-
distilled water at 25 ı C for 2 days (Fig. 6.18 ). Around 20.90 mol% Mg was