Junitoite, CaZn₂Si₂O₇·H₂O, is orthorhombic, space group Area2, with a = 12.510, b = 6.318, c = 8.561 Å, Z = 4. The structure, determined on the basis of three-dimensional Mo-Kα intensity data for 745 reflections refined to R = 0.10. The structure consists of ZnO₄ tetrahedral chains along b joined by Si₂O₄ groups along a and c to form a three-dimensional network. Ca occurs in distorted CaO₅(H₂O) octahedra. The structure shows similarities to hemimorphite and Ca-Zn silicates but represents a unique tetrahedral framework.