Cranswickite is a newly recognized mineral of composition MgSO4·4H2O from Calingasta, San Juan Province, Argentina (IMA2010-016). Cranswickite is monoclinic, space group C2/c, a = 11.9236(3), b = 5.1736(1), c = 12.1958(3) Å, β = 117.548(2)°, V = 667.0(1) Å3, Z = 4, dobs = 1.917 g/cm3, and dcalc = 1.918 g/cm3. The mineral occurs0 as a soft white vein filling in a metasedimentary rock. The atomic structure has been determined by direct methods and refined by Rietveld analysis of powder diffraction data. The atomic structure consists of chains of corner-sharing magnesium-containing octahedra and sulfate tetrahedra similar to the structure of pentahydrite. All the water molecules directly coordinate magnesium in the structure. The five strongest lines in the powder X-ray diffraction data are [dobs in angstroms (I) (hkl)]: 5.259 (100) (200), 3.927 (46) (112̅), 3.168 (45) (113̅), 4.603 (29) (111̅), 2.570 (23) (311). Infrared and Raman spectra are very similar to the spectra measured from starkeyite. The chemical composition of cranswickite is the same as starkeyite MgSO4·4H2O, but starkeyite has an atomic structure where two sulfate tetrahedra and two Mg(H2O)6 octahedra share corners to form a four-membered ring and not a chain as in cranswickite. The new mineral is named in honor of Lachlan M.D. Cranswick (1968–2010), an Australian crystallographer who helped to developed and maintain the Collaborative Computational Project No. 14 in Powder and Small Molecule Single Crystal Diffraction (CCP14).