The zeolite mineral goosecreekite (CaAl2Si6O16.5H2O) is monoclinic, P21, with a = 7.401(3), b = 17.439(6), c = 7.293(3) Å, β = 105.44(4)°, and Z = 2. Its structure has been solved by Fourier and direct methods and refined to an unweighted residual of 0.061 using 2017 reflections. Three sets of intersecting channels parallel to the a, b, and c* axes, respectively, contain Ca atoms coordinated by two framework oxygens and five water molecules arranged in a distorted monocapped octahedron. The framework consists of Si04 and A104 tetrahedra grouped into 4-, 6-, and 8-membered rings, which share edges to form distorted layers parallel to (010). Si-Al ordering is almost complete, with two out of six tetrahedral sites showing minor solid solution. The (010) layers resemble those in brewsterite [(Sr,Ba)Al2Si6O16.5H2O] but are cross-linked in a different way to form the 3-dimensional framework. The structure can also be described in terms of T9O18 units, which form vertex-sharing chains in the b direction, adjacent chains being linked in the a and c directions by 4-rings. Goosecreekite cannot readily be assigned to any of the known zeolite families.