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The OH-stretching region in infrared spectra of the apatite OH-Cl binary system

  • Robert C. Tacker , John Rakovan EMAIL logo , Daniel Harlov ORCID logo , John M. Hughes and Sarah B. Cichy
Published/Copyright: November 29, 2024
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American Mineralogist
From the journal American Mineralogist Volume 109 Issue 12

Abstract

Polarized Fourier transform infrared (FTIR) microspectroscopy of the OH-stretching region of hydroxylapatite-chlorapatite solid solutions presents novel problems for the assignment of peaks to specific OH-Cl pairs. Crystal structure refinements of Hughes et al. (2016) identified new positions for column anions in synthetic mixed Cl-OH apatites, with three different column anion arrangements depending on composition. These structural refinements, combined with bond-valence calculations, allow for interpretation of the OH-stretching region.

A peak at 3574 cm–1 is identified as that from end-member hydroxylapatite. A second major peak at 3548 cm–1 is only found in mixed chlorapatite-hydroxylapatite solid solutions, as is a third peak at 3592 cm–1. Both represent perturbations of the OH-stretching vibration as compared to hydroxylapatite, to lower and higher frequency, respectively. Both of the new peaks are the result of a Clb-OH sequence, with adjacent anions in crystallographically similar positions, both above or both below adjacent mirror planes. One configuration has the hydrogen atom pointed toward the chlorine atom. The second has the hydrogen of the OH group pointed away from the chlorine atom.

Both configurations present novel problems. The shift to lower wavenumber at 3548 cm–1 is characteristic of hydrogen bonding in fluorapatite-hydroxylapatite mixtures, yet the distance between O(H) and Clb is too great to allow it. The shift of OH-stretching vibrations to lower wavenumber is produced through changes in polarization of intervening Cl-Ca2′ (or Ca2) and Ca2(′)-O3 bonds, which are affected by the presence of the large chlorine atom. Lowering the OH-stretching vibration mimics the expected effect of chlorine on a neighboring OH group in the apatite c-axis column, though without hydrogen bonding. The shift to higher wavenumbers, i.e., higher frequency at 3592 cm–1, is the opposite of that expected for hydrogen bonding between column anions in the apatite mineral group. It is ascribed to the interaction between an adjacent Clb and the oxygen end of an adjacent OH dipole. This pairing places an oxygen and a chlorine atom in close proximity. Possible means of accommodation are discussed.

A ubiquitous peak at 3498 cm–1 represents hydrogen bonding between an OH and the OHa site, with an interoxygen distance of about 2.9 Å. Published modeling supports the hypothesis that the OHa site is occupied by an O rather than an OH. However, no clear counterpart to this pairing is observed in crystal structure refinements for specimens lacking OHa, although the infrared absorbance is present. The existence of oxyapatite is inferred from studies of plasma-sprayed biomaterials, but the crystallographic details of the substitution have remained elusive.

A minor shoulder at 3517 cm–1 does not have a clear counterpart in the structural refinements. Sequences of three columnar anions (e.g., OH-Cl-OH or Cl-OH-OH) can be ruled out, but an unequivocal assignment awaits further research.

Keywords: Apatite; solid solution; FTIR; structure refinements; hydrogen bonding; oxyapatite

Acknowledgments and Funding

Our thanks to Eugen Libowitzky and an anonymous reviewer for their thorough reviews. This research was supported by the National Science Foundation in the form of grants EAR-0910902 and EAR-0929898, both to R.C.T. Additional grant support was supplied by the Friends of the North Carolina Museum of Natural Sciences. R.C.T. gratefully acknowledges the patience of his co-authors as he went through multiple rounds of surgery and recovery.

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Received: 2023-05-09
Accepted: 2024-02-27
Published Online: 2024-11-29
Published in Print: 2024-12-15

© 2024 by Mineralogical Society of America

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https://doi.org/10.2138/am-2023-9059
Keywords for this article
Apatite; solid solution; FTIR; structure refinements; hydrogen bonding; oxyapatite

Articles in the same Issue

  1. Fluids in the shallow mantle of southeastern Australia: Insights from phase equilibria
  2. Compositional effects on the etching of fossil confined fission tracks in apatite
  3. Evaluation of the Rietveld method for determining content and chemical composition of inorganic X-ray amorphous materials in soils
  4. High-pressure phase transition of olivine-type Mg2GeO4 to a metastable forsterite-III type structure and their equations of state
  5. The application of “transfer learning” in optical microscopy: The petrographic classification of opaque minerals
  6. Mechanisms of fluid degassing in shallow magma chambers control the formation of porphyry deposits
  7. The OH-stretching region in infrared spectra of the apatite OH-Cl binary system
  8. Thermal equation of state of Li-rich schorl up to 15.5 GPa and 673 K: Implications for lithium and boron transport in slab subduction
  9. Raman scattering of omphacite at high pressure: Toward its possible application to elastic geothermobarometry
  10. Interpreting mineral deposit genesis classification with decision maps: A case study using pyrite trace elements
  11. Geochemical characteristics of mineral inclusions in the Luobusa chromitite (Southern Tibet): Implications for an intricate geological setting
  12. Three new iron-phosphate minerals from the El Ali iron meteorite, Somalia: Elaliite Fe82+ Fe3+(PO4)O8, elkinstantonite Fe4(PO4)2O, and olsenite KFe4(PO4)3
  13. Intervalence charge transfer in aluminum oxide and aluminosilicate minerals at elevated temperatures
  14. Electron probe microanalysis of trace sulfur in experimental basaltic glasses and silicate minerals
  15. New Mineral Names
  16. Book Review
  17. Book Review: Celebrating the International Year of Mineralogy: Progress and Landmark Discoveries of the Last Decades
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