Incorporation of Ti into the crystal structures of the high-pressure dense silicates anhydrous phase B and superhydrous phase B

Abstract

The structures of Ti-bearing anhydrous phase B and superhydrous phase B synthesized at 12 GPa/1600 °C and 21 GPa/1600 °C, respectively, have been determined by single-crystal X-ray diffraction. Anhydrous phase B has space group Pmcb. The structure of superhydrous phase B was refined in space groups Pnnm and Pnn2; crystallographic arguments unambiguously indicated that the centrosymmetric space group Pnnm is the correct choice for the crystal studied here. Ti orders at the octahedrally coordinated Si(1) site in both phases. The refined site occupancies at the Si(1) sites of anhydrous phase B and superhydrous phase B are 0.848(3) Si\u2009+\u20090.152 Ti and 0.92(1)\u2009+\u20090.08 Ti, respectively. These Ti occupancy levels correlate with a 4% expansion of the Si(1)O6 octahedron relative to values typically observed for end-member anhydrous phase B, Fe2+-bearing anhydrous phase B, and end-member superhydrous phase B. Ordering of Ti at the Si(1) is explained in terms of a homovalent substitution VISi\u2009→\u2009VITi4+, being the only option for these structures. There is no evidence for a deprotonation substitution of the kind Mg\u2009+\u20092OH−\u2009→\u2009Ti4+\u2009+\u20092O2−, as occurs in humites. Synthesis of Ti-bearing superhydrous phase B indicates that it is stable at typical P–T conditions of the mantle transition zone and the lower mantle. Hence, water can be transported by Ti-bearing superhydrous B into the lower mantle even at temperatures of the normal mantle geotherm.