The new orthorhombic mineral vuagnatiteCaAl(OH)SiO₄ (P2₁2₁2₁, a = 7.055(6), b = 8.542(7), c = 5.683(5)Å, Z = 4) is isotypic with conichalcite, CaCu(OH)AsO₄. From counter diffractometer data corrected for absorption [μ(MoKα) = 22.5cm⁻¹] the structure was refined anisotropically to a final R value of 0.043 for 974 reflections (F > 3σ). Vuagnatite is characterized by AlO₄(OH)₂ octahedra sharing edges to form chains linked by isolated SiO₄ tetrahedra and CaO₇OH polyhedra. The Donnay-Allmann empirical method of recognizing O²⁻, (OH)⁻, and H₂O in crystal structures indicates the presence of only one hydroxyl group, creating a hydrogen bond between the tetrahedral oxygen O(2) and the octahedral oxygen O(5). The position of the hydrogen atom was confirmed from a difference Fourier synthesis and successfully refined. The bond distance O(5)-H (the hydroxyl group) is 0.94(5)Å and the O(2)-O(5) distance is 2.530(3)Å with a value of 178(6)° for the angle O(2)...H-O(5). Two oxygen atoms of each AlO₆ octahedron are hydroxyl groups. The crystal chemical formula of vuagnatite is therefore Ca⁽⁸⁾Al⁽⁶⁾(OH)Si⁽⁴⁾O₄.