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High-pressure single-crystal X-ray and powder neutron study of F,OH/OD-chondrodite: Compressibility, structure, and hydrogen bonding

Alexandra Friedrich^1,*, George A. Lager², Peter Ulmer³, Martin Kunz^1, and William G. Marshall⁴

¹ Laboratorium für Kristallographie, ETH, Sonneggstrasse 5, CH-8092 Zürich, Switzerland
² Department of Geography and Geosciences, University of Louisville, Louisville, Kentucky 40292-0001, U.S.A.
³ Institut für Mineralogie und Petrographie, ETH, Sonneggstrasse 5, CH-8092 Zürich, Switzerland
⁴ ISIS Facility, CLRC Rutherford Appleton Laboratory, Chilton, Didcot OX11 0QX, U.K.

Correspondence: ^* E-mail: a.friedrich{at}kristall.erdw.ethz.ch

The crystal structure of a natural F-bearing chondrodite [Mg_4.64Fe_0.28Mn_0.014Ti_0.023(Si_1.01O₄)₂F_1.16OH_0.84; a = 4.7328(1), b = 10.2749(2), c = 7.8756(1) Å, = 109.065(1)°; space group P2₁/b (a unique)] from the Tilly Foster mine (Brewster, New York) and the structure of a deuterated synthetic analogue [Mg₅(SiO₄)₂F_1.10OD_0.90] were refined at high pressure using single-crystal X-ray and time-of-flight neutron powder diffraction data, respectively. The isothermal equation of state was determined from single-crystal high-precision X-ray diffraction measurements of unit-cell parameters as a function of pressure up to 9.63(1) GPa. A third-order Birch-Murnaghan equation-of-state fit to pressure-volume data results in V₀ = 362.00(2) ų, K_0,T = 117.0(4) GPa and KT' = 5.6(1). The larger value of K_0,T relative to that determined for the OH end-member confirms that F makes the chondrodite structure less compressible. The axial compressibilities behave anisotropically with fitted parameters a₀ = 4.73282(9) Å, K(a)₀ = 458(3) GPa, and K(a)' = 28.9(9); b₀ = 10.2759(3) Å, K(b)₀ = 298(1) GPa, and K(b)' = 13.4(4); c₀ = 7.8759(2) Å, K(c)₀ = 316(2) GPa, and K(c)' = 16.7(5). X-ray structural investigations at 0.0001, 4.952(7), and 9.550(7) GPa in a diamond-anvil cell reveal a complex compression mechanism. The greatest changes with pressure occur in the M2 octahedra, which are located at the flexion points of the octahedral chain. These octahedra have the longest bonds at ambient conditions due to cation-cation repulsion, which displaces the cation from the centroid of the octahedron. The application of pressure overcomes the effect of cation-cation repulsion inducing a strong bond-length compression. The neutron powder data were collected at 0.0001, 1.26(1), 2.89(2), 3.87(1), 5.27(2), and 7.04(2) GPa in the Paris-Edinburgh cell using a new gasket design and a fluid pressure medium. Changes in the O-D···O/F hydrogen bond geometry with pressure can be interpreted in terms of a rotation of the OD vector and a reduction in the degree of static disorder associated with the hydrogen bond acceptor/donor site. These new crystallographic data are discussed in terms of corresponding spectroscopic measurements reported recently in the literature.

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