Two Np5+ silicates, Li-6(NPO2)(4)(H2Si2O7)(HSiO4)(2)(H2O)(4) (LiNpSi1) and K-3(NPO2)(3)(SiO3OH)(2) (KNpSi1), were synthesized by hydrothermal methods. The crystal structures were determined using direct methods and refined on the basis of F-2 for all unique data collected with Mo K alpha radation and an APEX II CCD detector. LiNpSi1 crystallizes in orthorhombic space group Pnma with a = 13.189(6) angstrom, b 7.917(3) angstrom, c = 10.708(5) angstrom, V = 1118.1(8) angstrom(3), and Z = 2. KNpSi1 is hexagonal, P (6) over bar 2m, a = 9.734(1) angstrom, c 3.8817(7) angstrom, V = 318.50(8) angstrom(3), and Z = 1. LiNpSi1 contains chains of edge-sharing neptunyl pentagonal bipyramids linked into two-dimensional sheets through direct linkages between the neptunyl polyhedra and the vertex sharing of the silicate tetrahedra. The structure contains both sorosilicate and nesosilicate units, resulting in a new complex neptunyl silicate sheet. KNpSi1 contains edge-sharing neptunyl square bipyramids linked into a framework structure through the sharing of vertices with the silicate tetrahedra. The neptunyl silicate framework contains channels approximately 6.0 angstrom in diameter. These structures exhibit significant departures from other reported Np5+ and U6+ compounds and represent the first reported Np5+ silicate structures.