Ferromagnesian jeffbenite synthesized at 15 GPa and 1200 °c

Single crystals of Al-free, ferromagnesian jefbenite up to 200 μm in size have been synthesized at 15 GPa and 1200 °C in a 1200 tonne multi-anvil press from a starting composition in the forsteritefayalite-magnetite-water system. This phase has the approximate formula Mg2.62Fe20.87+ Fe31.63+ Si2.88O12 and is observed to coexist with a Ca-free clinopyroxene plus what appears to be quenched melt. The crystal structure has been refined from single-crystal X-ray difraction data and is similar to that determined for natural Al-bearing jeffbenite, Mg3Al2Si3O12, reported from inclusions in superdeep diamonds. The structure is a tetragonal orthosilicate in space group I4¯ 2d with a = 6.6449(4) Å, c = 18.4823(14) Å, and is structurally more closely related to zircon than to garnet. The T2 site is larger than T1, shares an edge with the M2 octahedron, and incorporates significant Fe3+. Because of the tetrahedral incorporation of trivalent cations, jeffbenite appears to be compositionally distinct from garnet. Previous speculations that the phase may only occur as a retrograde decompression product from bridgmanite are not supported by its direct synthesis under transition zone conditions. The phase has a calculated density of 3.93 g/cm3, which is indistinguishable from a garnet of comparable composition, and is a possible component in the mantle transition zone under oxidizing conditions or with Al-rich compositions.