We present the first equation of state and structure refinements at high pressure of single-crystal phase egg, AlSiO3OH. Phase egg is a member of the Al2O3-SiO2-H2O system, which contains phases that may be stable along a typical mantle geotherm (Fukuyama et al. 2017) and are good candidates for water transport into Earth's deep mantle. Single-crystal synchrotron X-ray diffraction was performed up to 23 GPa. We observe the b axis to be the most compressible direction and the β angle to decrease up to 16 GPa and then to remain constant at a value of ~97.8° up to the maximum experimental pressure reached. Structure refinements performed at low pressures reveal a distorted octahedron around the silicon atom due to one of the six Si-O bond lengths being significantly larger than the other five. The length of this specific Si-O4 bond rapidly decreases with increasing pressure leading to a more regular octahedron at pressures above 16 GPa. We identified the shortening of the Si-O4 bond and the contraction of the vacant space between octahedral units where the hydrogen atoms are assumed to lie as the major components of the compression mechanism of AlSiO3OH phase egg.

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