Dark octahedral crystals of K1-x[Fe1-xSi x O2] with x ≈ 0.2 were synthesized under ultra-alkaline conditions in a KOH hydroflux at 200 °C. The compound is a member of the solid solution series between SiO2 and K[FeO2]. Due to its SiO2 content, K0.8[Fe0.8Si0.2O2] is much less sensitive to moisture than K[FeO2]. The crystal structure is a stuffed cristobalite with a charged framework of vertex-sharing [MO4/2] tetrahedra (M = Fe3+, Si4+) and potassium counter ions in the large voids of the framework. It has the pseudo-symmetry and metrics of the cubic space group Fd3m, but adopts the tetragonal space group I41md with four formula units in the cell. Unlike other tectosilicates of this type, the [MO4/2]- tetrahedra are not regular but distorted to disphenoids. The O atom can be modeled by a huge disk-shaped ellipsoid or, better, by split atom positions forming a six-membered ring with a diameter of 1.1 Å. The M-O distances range from 1.66(2) to 1.96(2) Å, the M-O-M angles are 135(2)°, 137(1)°, and 151(1)°.