Single crystals of an Fe-bearing high-pressure magnesium silicate “anhydrous phase B,” (Mg0.88Fe0.12)14Si5O24, have been synthesized at 15 GPa and 1800°C. A remarkable feature of this Fe-bearing variant is the significant partitioning of Fe into one of the six symmetrically distinct octahedral sites—a degree of Fe-Mg ordering not observed in any other rock-forming silicate quenched from very high temperature. The presence of Fe, as well as other octahedral cations such as Ca, Mn, or Al, may, therefore, expand the stability of anhydrous phase B into a pressure-temperature regime characteristic of the Earth's transition zone.

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