A lithium aluminum phosphate mineral of basic formula LiAl(PO₄)(F). The structure of amblygonite consists of phosphate (PO₄) groups of tetrahedra and AlO₆ groups of octahedra. Each PO₄ tetrahedron is connected to an AlO₆ octahedron. Corner-sharing octahedra form zig-zag chains along the b axis. Lithium (Li) is in fivefold coordination, and lies between the PO₄ tetrahedra and nearest AlO₆ octahedra. The dominant substitution in this mineral structure is hydroxyl (OH) for fluorine (F). This substitution gives rise to the amblygonite-montebrasite (LiAlPO₄(OH)) solid solution series. When OH is greater than F, the mineral is known as montebrasite. Appreciable amounts of sodium substitute for lithium in the five-coordinated polyhedra. The sodium-rich varieties such as natromontebrasite and hedronite are rare.