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Licensed Unlicensed Requires Authentication Published by De Gruyter March 30, 2023

Single-crystal X-ray diffraction of fluorapatite to 61 GPa

  • Melinda J. Rucks ORCID logo , Gregory J. Finkelstein , Dongzhou Zhang ORCID logo , Przemyslaw K. Dera and Thomas S. Duffy
From the journal American Mineralogist


Apatite is a mineral of widespread importance in Earth and planetary science. Here we examine the behavior of a natural fluorapatite (FAp) crystal from Durango (Mexico) under compression to 61 GPa. Single-crystal X-ray diffraction experiments were carried out in a diamond-anvil cell using a synchrotron source. The apatite structure persists up to 32.4 GPa. Birch-Murnaghan equation of state parameters were fit to the pressure-volume data for fluorapatite for two cases: fixing V0 at its measured ambient value resulted in a bulk modulus, K0T, of 97.0(8) GPa and a pressure derivative of the bulk modulus, K′0T, of 3.3(1), while fixing V0 and K0T at its ambient value 90.5 GPa (derived from ultrasonically measured elastic constants) resulted in a K′0T value of 4.1(1). At 35.6 GPa, fluorapatite transforms to a triclinic phase (P1, Z = 4), designated here as fluorapatite II (FAp-II). This phase persists up to at least 61 GPa. The major structural differences between FAp and FAp-II involve the buckling of the Ca polyhedra along the c-axis and changes in the number and coordination of the Ca sites. Our study extends the pressure range over which fluorapatite has been examined by more than a factor of three, providing new insights into its structural response to high-pressure conditions.

Acknowledgments and funding

We thank Sergey Tkachev for assistance with gas loading, and Celine Martin for assistance with microprobe data collection at the American Museum of Natural History. This work was supported by the Department of Energy/National Nuclear Security Agency under Cooperative Agreement DE-NA0003957. GeoSoilEnviro-CARS is supported by the NSF and the Department of Energy (DOE). Use of the gas loading system was supported by COMPRES and GSECARS. This research used resources of the Advanced Photon Source, a DOE User Facility operated by Argonne National Laboratory.

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Received: 2021-12-09
Accepted: 2022-06-16
Published Online: 2023-03-30
Published in Print: 2023-04-25

© 2023 by Mineralogical Society of America

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