The structure of phase F, a non-stoichiometric hydrous silicate synthesized in a uniaxial, split-sphere, multi-anvil apparatus at conditions of 17 GPa and 1000° C, has been solved and refined in space group P63cm, using synchrotron X-ray data for a single crystal of a size 18 × 24 × 30 μm. The composition and unit cell for phase F are Mg3.35Si5.51H7.26O18, a=5.073(3) Å, c=14.013(9) Å, 7= 312.3(5) Å3. The structure contains layers with many similarities to superhydrous phase B. The layers of oxygen atoms are stacked in the ABCBAC-type double cubic closest packing arrangement. The bulk modulus of phase F was estimated from the structural and compositional relationship to superhydrous phase B and periclase.