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

Isotopic and mineralogic bias introduced by pulverization of aragonite

RATIONALE

Stable carbon and oxygen isotope data of biogenic and abiogenic aragonite are of fundamental relevance in paleoclimate research. Wet-chemical analysis of such materials requires well-homogenized, fine-grained powder. In the present study, the effect of different grinding/milling methods on sample homogeneity and the potential risk of unintentional calcite formation and isotope shift were evaluated.

METHODS

Shells of Arctica islandica and aragonite sputnik crystals were pulverized using a set of commonly used methods, including a hand-held drill, a vibromill operated at various settings (with and without liquid nitrogen cooling, changes in ball diameters, frequencies, and processing durations), and an agate mortar and pestle. Stable isotope values were measured using an isotope ratio mass spectrometer operated in continuous flow mode. Identification of mineral phases was obtained by powder X-ray diffraction (PXRD), Raman spectroscopy, and attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy. Calcite content was quantified by PXRD Rietveld refinement.

RESULTS

Samples showed substantial homogeneity, in particular after vibromilling (duration 3-10 min). More vigorous grinding resulted in larger fractions of calcite (0.5-4.2 wt%) and a concomitant δ18O and δ13C decrease, specifically in bivalve shells. The only method for producing pure aragonite powder was by pounding the aragonite sputniks manually with an agate mortar and pestle.

CONCLUSIONS

None of the studied, commonly used machine-based pulverization methods produced pure aragonite powder from samples consisting originally of aragonite. These findings have significant implications for light-stable isotope-based paleoclimate reconstructions. Except for abiogenic aragonite powder produced by pounding in an agate mortar, paleotemperatures would be overestimated.

Intrafibrillar mineralization of type I collagen with calcium carbonate and strontium carbonate induced by polyelectrolyte-cation complexes

Calcium carbonate (CaCO3), possessing excellent biocompatibility, bioactivity, osteoconductivity and superior biodegradability, may serve as an alternative to hydroxyapatite (HAp), the natural inorganic component of bone and dentin. Intrafibrillar mineralization of collagen with CaCO3 was achieved through the polymer-induced liquid precursor (PILP) process for at least 2 days. This study aims to propose a novel pathway for rapid intrafibrillar mineralization with CaCO3 by sequential application of the carbonate-bicarbonate buffer and polyaspartic acid (pAsp)-Ca suspension. Fourier transform infrared (FTIR) spectroscopy, zeta potential measurements, atomic force microscopy/Kelvin probe force microscopy (AFM/KPFM), and three-dimensional stochastic optical reconstruction microscopy (3D STORM) demonstrated that the carbonate-bicarbonate buffer significantly decreased the surface potential of collagen and CO32-/HCO3- ions could attach to collagen fibrils via hydrogen bonds. The electropositive pAsp-Ca complexes and free Ca2+ ions are attracted to and interact with CO32-/HCO3- ions through electrostatic attractions to form amorphous calcium carbonate that crystallizes gradually. Moreover, like CaCO3, strontium carbonate (SrCO3) can deposit inside the collagen fibrils through this pathway. The CaCO3-mineralized collagen gels exhibited better biocompatibility and cell proliferation ability than SrCO3. This study provides a feasible strategy for rapid collagen mineralization with CaCO3 and SrCO3, as well as elucidating the tissue engineering of CaCO3-based biomineralized materials.

Micro/Nano Structural Investigation and Characterization of Mussel Shell Waste in Thailand as a Feasible Bioresource of CaO

Mussel shell waste, which is regularly disposed by households, restaurants, markets, or farms, causes environmental problems worldwide, including in Thailand, because of its long decomposing time. Owing to a large amount of calcium (Ca) content from calcium carbonate (CaCO₃) in mussel shell waste, many Thai local businesses grind the shell waste into powder and sell it as a source of Ca. Generally, these powdered waste shells are a mixture of various types of mussel shell waste. In this study, we investigated and characterized powdered mixed waste shells sold in a local Thai market (called mixed shell powder) and ground shells from waste green mussel shells (called green mussel shells) prepared in the laboratory after calcination at different temperatures (800 °C, 900 °C, and 1000 °C). Mixed shell powder containing five different types of mussel shells and green mussel shells were calcined for 2 h and 3 h, respectively. The time used for calcination of mixed shell powder and green mussel shells was different due to the different particle sizes of both shell wastes. We found that an optimal temperature of 1000 °C completely converted CaCO₃ to CaO in both samples. The nanoscale size of CaO was detected at the surface of calcined shells. These shell wastes can be used as a bioresource of CaO.