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

• Published in: The American mineralogist Vol. 108; no. 4; pp. 731 - 737
• Main Authors: Rucks, Melinda J, Finkelstein, Gregory J, Zhang, Dongzhou, Dera, Przemyslaw K, Duffy, Thomas S
• Format: Journal Article
• Language: English
• Published: Washington Mineralogical Society of America 01.04.2023; Walter de Gruyter GmbH
• Subjects: anvil cells; Apatite; bonding; Bulk modulus; Compression; compressive strength; Constants; coordination; crystal structure; diamond anvil cell; Diamonds; Elastic constants; Elastic properties; Equations of state; Fixing; Fluorapatite; Geophysics; GEOSCIENCES; high pressure; lattice; lattice parameters; phosphates; Polyhedra; Pressure; related to mantle; Single crystals; single-crystal method; single-crystal X-ray diffraction; space groups; Structural response; Synchrotrons; ultrahigh pressure; unit cell; X rays; X-ray diffraction; X-ray diffraction data
• ISSN: 0003-004X; 1945-3027
• DOI: 10.2138/am-2022-8410

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, K0T', 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 K0T' 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.