Two hexagonal aluminous phases, which could serve as potential Na- and K-host minerals in the lower mantle, with compositions K1.00Mg2.00Al4.80Si1.15O12 and Na1.04Mg1.88Al4.64Si1.32O12 were synthesized at 22–25 GPa and 1500 °C. The K-rich hexagonal aluminous phase was synthesized for the first time. Crystal structures of both hexagonal aluminous phases were refined using the Rietveld method. Obtained interatomic distances and bond angles were compared to published data on the hexagonal aluminous phase CaMg2Al6O12. The general chemical formula of the hexagonal aluminous phase is represented as [M3][M2]2[M1]6O12, where the small-, middle-, and large-sized cations occupy the M1, M2, and M3 sites, respectively. Changes of size and shape of M1O6 octahedra by the substitution of Si4+ for Al3+ in the M1 site make it possible to adjust the size of the M2 and the M3 sites to accommodate Na+ and Mg2+ in the M2 sites and Na+ and K+ in the M3 sites, respectively. The stability of hexagonal aluminous phases in a relatively wide compositional range of 30–50 mol% in NaAlSiO4 component along the NaAlSiO4-MgAl2O4 join can be explained by possible replacement of Mg2+ by Na+ in the M2 site and by shrinkage and deformation of M1O6 octahedra with the coupled substitution: M2Mg2+ + M1Al3+ → M2Na+ + M1Si4+.