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Isothermal compression of staurolite: A single-crystal study

¹ Dipartimento di Scienze della Terra, Università di Perugia, 06100 Perugia, Italy
² Bayerisches Geoinstitute, Universität Bayreuth, D-95440 Bayreuth, Germany

Correspondence: ^* E-mail: zanazzi{at}unipg.it

The response of staurolite to pressure was studied by single crystal X-ray diffraction (XRD) in a diamond-anvil cell, using crystals with composition: (Fe_3.365Zn_0.025Li_0.114Co_0.009Mn_0.034)[T2, M4] (Al₂Mg_0.307)[M3] (Al_15.491Fe³⁺_0.104Mg_0.394Cr_0.004Ti_0.07)[M1, M2] (Si_7.534 Al_0.466)[T1] O₄₈H₃.

Lattice parameters, measured at various pressure up to 7.264(6) GPa, were fitted using a third-order Birch-Murnaghan equation of state (EoS). The resulting EoS parameters are: V₀ = 740.85(7) ų, K₀ = 180(2) GPa and K' = 4.7(6), a₀ = 7.8723 (2) Å, K₀ = 189(2) GPa, and K'_a = 4.1 (6), b₀ = 16.62453(1) Å, K₀ = 179(2) GPa, K'_b = 6.1(6) and c₀ = 5.6604 (4) Å, K₀ = 179(5) GPa, K'_c = 2(1); whereas the angle ß remained almost constant with increasing pressure. These data suggest an almost isotropic compressibility.

Structural evolution was studied by comparison of structural refinements carried out with data collected at 0.0001, 2.48, 4.15, 5.43, 6.84, and 8.74 GPa. All refinements were made in the Ccmm space group. Polyhedral evolution with P is a function of occupancy: whereas the T1 tetrahedron and the M1 and M2 octahedra, occupied by Si and Al, are practically incompressible, the T2 tetrahedron, and the M4 and M3 octahedra, only partially occupied principally by Fe (the first two) and by Al (the last), show larger changes as a function of pressure. As a consequence, the two kyanite and Fe-Al hydroxide layers, which can be used to describe the staurolite structure, have different compressibilities.

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