Abstract
Abstract. X-ray absorption near edge structure (XANES) studies on calcium phosphate species (Ca-P) deal with marginal differences among subtle spectral features despite a hitherto missing systematic breakdown of these differences. Related fingerprinting approaches depend, therefore, on spectral libraries that are not validated against each other, incomplete and scattered among publications. This study compiled a comprehensive spectral library from published reference compound libraries in order to establish more clear-cut criteria for Ca-P determination by distinctive phosphorus K-edge XANES features. A specifically developed normalization method identified diagnostic spectral features in the compiled library, e.g. by uniform calculation of ratios between white-line and secondary peak heights. Post-processing of the spectra (n = 81) verified distinguishability among most but not all phases, which included hydroxylapatite (HAP), poorly crystalline HAP, amorphous HAP, fluorapatite, carbonate fluorapatite (CFAP), carbonate hydroxylapatite, β-tricalcium phosphate, octacalcium phosphate (OCP), brushite, monetite, monocalcium phosphate, amorphous calcium phosphate (ACP), anapaite, herderite, scholzite, messelite, whiteite and P on CaCO3. Particularly, peak-height ratios significantly improved analyte specificity, e.g. by supplementary breakdown into OCP and ACP. The spectral analysis also revealed Ca-P standards that were rarely investigated or inappropriately synthesized, and thus provides a basis for standard selection and synthesis. The method developed and resulting breakdown by species were subsequently tested on Ca-P spectra from studies on bone and sediment. The test indicated that bone material likely comprises only poorly crystalline apatite, which confirms direct nucleation of apatite in bone. This biological apatite formation is likely opposed to that of sedimentary apatite, which apparently forms by both direct nucleation and successive crystallization. Application of the method to μXANES spectra of sediment particles indicated authigenic apatite formation by an OCP precursor.
Highlights
Calcium phosphates (Ca-P) are a group of several mineral species that are present in various biological and environmental matrices, such as soil, sediment, ores, fertilizers, agroindustrial by-products, milk, bone and tooth enamel
Among the 76 spectra of the compiled library (Table 1), HAP had the highest number of available spectra (n = 17), followed by DCPA (n = 13), DCPD (n = 9), FAP (n = 8), octacalcium phosphate (OCP) (n = 8), carbonated apatites (n = 6), β-TCP (n = 4), amorphous calcium phosphate (ACP) (n = 4), monocalcium phosphate (MCP) (n = 3), P on CaCO3 (n = 2), amorphous HAP (n = 1) and poorly crystalline HAP (n = 1)
A novel normalization method was developed, which, for the first time, allowed for a systematic comparison of P K-edge X-ray absorption near edge structure (XANES) spectra of a compiled library, and which facilitated the identification of important diagnostic spectral Ca-P features in this library
Summary
Calcium phosphates (Ca-P) are a group of several mineral species that are present in various biological and environmental matrices, such as soil, sediment, ores, fertilizers, agroindustrial by-products, milk, bone and tooth enamel. Some Ca-P species form naturally occurring polymorphs, which further increases the number of target phases. Authigenically formed carbonate fluorapatite (CFAP) is suggested to be a primary sink of reactive phosphorus (P) in the ocean (Ruttenberg, 1992; Ruttenberg and Berner, 1993) and a key compound during the ultimate stage of a complex P-removal mechanism in the global P cycle. It is distinguished from detrital fluorapatite (FAP), which is delivered to marine environments by continental weathering. Due to their occurrence in P-bearing rocks, apatites mark the initial stage of P transformations
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
