The unique cation-disordered crystal structures of two samples of phase E, a non-stoichiometric, hydrous silicate synthesized in a uniaxial, split-sphere, multi-anvil apparatus at conditions above 13 GPa and 1000 ~ C, have been solved and refined in space group R3m. The compositions and unit cells for the two materials, assuming six oxygens per cell, are MgzosSi-116H32o0#, a=2.9701(1) ~, c=13.882(1) ~,V= 106.05i4) A 3 for sample 1, and Mg/178il.olH3.6206, a:2.9853(6) A, c=13.9482(7) ~, V=i07.65(4) ~3 for sample 2. The structure contains layers with many features of brucite-type units, with the layers stacked in a rhombohedral arrangement. The layers are cross linked by silicon in tetrahedral coordination and magnesium in octahedral coordination, as well as hydrogen bonds. Interlayer octahedra share edges with intralayer octahedra. Interlayer tetrahedra would share faces with intralayer octahedra. To avoid this situation, there are vacancies within the layers. There is, however, no longrange order in the occupation of these sites, as indicated by the lack of a superstructure. Selected-area electron diffraction patterns show walls of diffuse intensity similar in geometry and magnitude to those observed in short-range-ordered alloys and Hgtgg phases. Phase E thus appears to represent a new class of disordered silicates, which may be thermodynamically metastable.