From synthetic to biogenic Mg-containing calcites: a comparative study using FTIR microspectroscopy

Area of Science:

• Biomineralization
• Geochemistry
• Spectroscopy

Background:

• The formation of thermodynamically unstable high Mg calcite in biological organisms is a long-standing enigma.
• Conventional methods like FTIR and XRD have limitations in detecting local disorder, such as amorphous phases or Mg ion occlusion in calcite.
• FTIR microspectroscopy (FTIRM) offers enhanced capabilities for characterizing local structural variations in calcium carbonate.

Purpose of the Study:

• To systematically characterize synthetic and biogenic Mg-containing calcium carbonates, particularly sea urchin teeth (SUT).
• To compare FTIRM data with conventional KBr pellet FTIR and X-ray diffraction (XRD) measurements.
• To investigate the presence and distribution of amorphous calcium carbonate (ACC) in biominerals.

Main Methods:

• Utilized two FTIRM instruments for detailed characterization of Mg-containing calcium carbonate samples.
• Compared FTIRM spectra with KBr pellet FTIR spectra from geogenic calcite and dolomite minerals.
• Applied multi-peak curve fitting analysis on the in-plane-bending (ν(4)) and out-of-plane (ν(2)) bands.

Main Results:

• Identified two singlet bands (∼860-865 cm⁻¹ and ∼695-704 cm⁻¹) in SUT FTIRM spectra, attributed to amorphous calcium carbonate (ACC).
• Reported the first evidence of ACC at the mature end of sea urchin teeth (SUT).
• Observed no ACC in three other studied biominerals.
• Demonstrated that both ν(4) and ν(2) bands shift to higher wavenumbers with increasing Mg substitution for Ca in calcite lattices, as evidenced by FTIRM.

Conclusions:

• FTIRM is a powerful tool for detecting amorphous calcium carbonate (ACC) and local disorder in Mg-calcite biominerals.
• The presence of ACC in sea urchin teeth suggests a distinct biomineralization pathway.
• Mg content in calcite lattices influences vibrational modes (ν(4) and ν(2)), providing insights into Mg-calcite structure.