Abstract
In nature, calcium carbonate (CaCO3) in the form of calcite and aragonite nucleates through different pathways including geogenic and biogenic processes. It may also occur as pyrogenic lime plaster and laboratory-precipitated crystals. All of these formation processes are conducive to different degrees of local structural order in CaCO3 crystals, with the pyrogenic and precipitated forms being the least ordered. These variations affect the manner in which crystals interact with electromagnetic radiation, and thus formation processes may be tracked using methods such as X-ray diffraction and infrared spectroscopy. Here we show that defects in the crystal structure of CaCO3 may be detected by looking at the luminescence of crystals. Using cathodoluminescence by scanning electron microscopy (SEM-CL) and laser-induced fluorescence (LIF), it is possible to discern different polymorphs and their mechanism of formation. We were thus able to determine that pyrogenic calcite and aragonite exhibit blue luminescence due to the incorporation of distortions in the crystal lattice caused by heat and rapid precipitation, in agreement with infrared spectroscopy assessments of local structural order. These results provide the first detailed reference database of SEM-CL and LIF spectra of CaCO3 standards, and find application in the characterization of optical, archaeological and construction materials.
Highlights
In nature, calcium carbonate (CaCO3) in the form of calcite and aragonite nucleates through different pathways including geogenic and biogenic processes
Several biogenic crystals nucleate from an amorphous calcium carbonate (ACC) precursor crystallized by organisms[11], and may show different degrees of order within the same individual based on their anatomical location
Using high-resolution cathodoluminescence performed via field-emission scanning electron microscopy (SEM-CL), laser-induced fluorescence (LIF) and inductively coupled plasma mass spectrometry (ICP-MS), we show that pyrogenic and precipitated specimens are characterized by a strong blue luminescence, as opposed to the dominant orange luminescence of the starting material
Summary
Calcium carbonate (CaCO3) in the form of calcite and aragonite nucleates through different pathways including geogenic and biogenic processes. We were able to determine that pyrogenic calcite and aragonite exhibit blue luminescence due to the incorporation of distortions in the crystal lattice caused by heat and rapid precipitation, in agreement with infrared spectroscopy assessments of local structural order These results provide the first detailed reference database of SEM-CL and LIF spectra of CaCO3 standards, and find application in the characterization of optical, archaeological and construction materials. We demonstrate a correlation between luminescence wavelength and local structural order of CaCO3 crystals determined with FTIR These results provide the first reference database for both SEM-CL and LIF of calcite and aragonite, and find application in the structural characterization of modern binders used in bioarchitecture and cement research, in the preservation of cultural heritage, and in industrial chemistry
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