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Soft x-ray scanning transmission spectromicroscopy
Published in Elaine DiMasi, Laurie B. Gower, Biomineralization Sourcebook, 2014
Julie Cosmidis, Karim Benzerara
Gloeomargarita lithophora. Until recently, calcium-carbonate formation by cyanobacteria was thought to occur exclusively outside of the cells, leading to their encrustation in extracellular precipitates. However, this general scheme was questioned recently by the discovery of a new cyanobacterium species, originating from a lacustrine microbialite in Mexico, which is able to form intracellular carbonate precipitates providing a new case of controlled biomineralization (Couradeau et al., 2012). A combination of several microscopies, including STXM, TEM, and confocal laser scanning microscopy (CLSM), has been used to characterize these intracellular precipitates. CLSM revealed the presence of chlorophyll-a and phycobiliproteins in the bacteria, con rming that they are cyanobacteria. The major element composition of the precipitates was measured by TEMand SEM-coupled x-ray energy dispersive spectroscopy (XEDS) and showed that they contain calcium, magnesium, strontium, and barium. Electron diffraction in TEM demonstrated that the precipitates are poorly crystallized, that is, providing an example of amorphous calcium carbonate (ACC). By comparing the XANES signatures at the Ca L2,3-edge of these poorly crystallized precipitates with those of various carbonate phases, Couradeau et al. (2012) suggested that intracellular precipitates display some local ordering consistent with the structure of benstonite, a Mg-, Ca-, Sr-, and Ba- bearing carbonate. Poorly crystallized materials are common in biomineralization, notably as transient precursors to more stable crystalline phases. As XANES spectroscopy provides a very local atomic information, it is a powerful tool to characterize these lowcrystallinity phases. For example, Figure 8.10 summarizes spectral features at the Ca L2,3-edges of various Ca-containing
Flame spray coating of α-tricalcium phosphate on AISI 316L alloy
Published in Cogent Engineering, 2022
R. B. Taqriban, D. F. Fitriyana, R. Ismail, J. Jamari, A. P. Bayuseno
The weight percentages of O, P, and Ca atoms suggested that Ca-orthophosphate ceramics could form on the specimen surface. Furthermore, the presence of carbon atoms (C) suggests that the calcium carbonate phase may exist as amorphous calcium carbonate (ACC) in the layer coating (Jeon et al., 2020). However, the presence of other metal atoms corresponded to the chemical composition of the substrate as well as the presence of porosity (Wang et al., 1993). Furthermore, using the percentage values of Ca and P atoms from the EDX data, the predicted Ca/P molar ratio was 1.4. This ratio may correspond to coexisting amorphous calcium phosphates (ACP) with molar ratios ranging from 1.2 to 2.2 (Dorozhkin, 2015). Furthermore, the low Ca/P value could be attributed to the thermal transformation of a powder precursor of calcium-deficient hydroxyapatite (CDHA) with a Ca/P molar ratio of 1.5. Indeed, the standard Ca/P molar ratio of 1.67 may be suitable for the hydroxyapatite phase for good biocompatibility (Kang et al., 2013; Łatka et al., 2020). As a result, increasing the Ca/P molar ratio on the sample after coating necessitates heating or hydrolyzing (Rocha et al., 2018).