High-Pressure Phases of Cordierite from Single-Crystal X-Ray Diffraction to 15 GPa

High-pressure single-crystal X-ray diffraction experiments were conducted on natural cordierite crystals with composition Mg1.92Fe0.13Al4.02Si4.98Na0.03O18.18 using a synchrotron X-ray source. The samples were compressed at 300 K in a diamond anvil cell to a maximum pressure of 15.2(1) GPa with a neon pressure-transmitting medium and a gold pressure calibrant. We observed a recently described orthorhombic to triclinic transition, as well as a further transition to a second triclinic phase. We solved and refined both new triclinic phases in space group P1, and designate them cordierite II and III. The structures of cordierite II and III were refined at 7.52(2) GPa at 15.2(1) GPa, respectively. The lattice parameters at these pressures are a = 15.567(2) Å, b = 9.6235(4) Å, c = 9.0659(5) Å, α = 89.963(5)°, β = 86.25(1)°, and γ = 90.974(8)° for cordierite II, and a = 8.519(2) Å, b = 8.2448(3) Å, c = 9.1627(4) Å, α = 85.672(4)°, β = 85.986(7)°, and γ = 70.84(1)° for cordierite III. Across the phase transitions there is a significant reduction in the length of the a­-axis (~2 Å per phase transition), whereas both the b­- and c-axis remain largely unchanged. Cordierite II has four- and five-coordinated Si and Al, while cordierite III has four-, five-, and six-coordinated Si, four- and five-coordinated Al, and five- and six-coordinated Mg. The sequence of high-pressure phases shows increasing polymerization of coordination polyhedra. These results, together with other recent studies, suggest that mixed 4-, 5-, and 6-fold coordination states may occur more commonly in silicate structures compressed at 300 K than previously recognized.