Thermally Induced Aragonite–Calcite Transformation in Freshwater Pearl: A Mutual Relation with the Thermal Dehydration of Included Water

Abstract

This study focuses on the relationship between the aragonite–calcite (A–C) transformation and the thermal dehydration of included water in the biomineralized aragonite construction using freshwater pearl. These thermally induced processes occur in the same temperature region. The thermal dehydration of included water was characterized through thermoanalytical investigations as an overlapping of three dehydration steps. Each dehydration step was separated through kinetic deconvolution analysis, and the kinetic parameters were determined. A single-step behavior of the A–C transformation was evidenced using high-temperature X-ray diffractometry and Fourier transform infrared spectrometry for the heat-treated samples. The kinetics of the A–C transformation was analyzed using the conversion curves under isothermal and linear nonisothermal conditions. The A–C transformation occurred in the corresponding temperature region of the thermal dehydration, ranging from the second half of the second dehydration step to the first half of the third dehydration step. Because the thermal dehydration process is constrained by the contracting geometry kinetics, the movement of the thermal dehydration reaction interface can be a trigger for the A–C transformation. In this scheme, the overall kinetics of the A–C transformation in the biomineralized aragonite construction is regulated by a contracting geometry.

Acceleration Effect of Atmospheric Water Vapor on the Thermal Decomposition of Calcium Carbonate: A Comparison of Various Resources and Kinetic Parameterizations

“…In all samples, the major mass loss attributed to the thermal decomposition of CaCO 3 were observed at higher than 850 K. However, a detectable mass loss before the thermal decomposition of CaCO 3 were observed in the oyster shell and FW-pearl powders, which was attributed to the thermal dehydration of the included water. 83,87 No distinguishable response for the aragonite−calcite transformation was detected for the samples with aragonite phase in DTA curves. The mass loss value during the major mass loss process was approximately agreed with the calculated value according to eq 1 irrespective of the samples.…”

“…The difference in the crystal phases was also confirmed by the FTIR spectra of the original CaCO 3 samples (Figure S3). 83,87 The samples with the calcite phase exhibited a single absorption peak at 714 cm −1 , which was attributed to the v 4 mode of CO 3 2− , while the samples with the aragonite phase exhibited split double peaks of the v 4 mode at 714 and 700 cm −1 . It is also noted from the FTIR spectra that the biomineralized CaCO 3 (i.e., oyster shell and FW-pearl powders) showed absorption peaks attributed to the O−H stretching and H−O−H bending vibrations at ∼3300 and ∼ 1680 cm −1 , respectively, indicating the presence of the included water.…”

“…(3) Pulverized oyster shell powder was purchased from Kure Brand Co. and used without further sieving. (4) A freshwater pearl (FW-pearl, produced in China) 83 and (5) mineral aragonite (produced in China) (Figure S1) were pulverized using a stainless-steel mortar and pestle and sieved to different particle size fractions. A 90−150 μm sieve fraction of FW-pearl powder and a <45 μm fraction of mineral aragonite powder were used as the samples.…”

“…It is also noted from the FTIR spectra that the biomineralized CaCO 3 (i.e., oyster shell and FW-pearl powders) showed absorption peaks attributed to the O−H stretching and H−O−H bending vibrations at ∼3300 and ∼ 1680 cm −1 , respectively, indicating the presence of the included water. 83,87 The crystallite sizes of each sample with the calcite and aragonite phases were calculated from the XRD pattern using the Rietveld method. The crystallite sizes of the samples with the calcite phase were different: ∼20 nm for the CaCO 3 reagent and the carbonation product of Ca(OH) 2 and ∼ 15 nm for the oyster shell powder.…”

“…Although the thermal dehydration of the included water occurs at a lower temperature before the thermal decomposition of CaCO 3 , the evolution of a small but detectable amount of water vapor was observed during the thermal decomposition of CaCO 3 in FWpearl in our previous study. 83,87 Therefore, in addition to the specific structural characteristics produced by the biomineralization process, the effect of self-generated water vapor can be one of the causes of the observed thermal behaviors in the thermal decomposition of the biomineralized CaCO 3 samples.…”