Apjohnite, MnAl₂ (SO₄)₁₄. 22H₂O,is monoclinic, space group P21/c, a 6–198 (2), b 24·347 (4), c 21–66 (4) Å, β 100·28 (3)° and Z = 4. The crystal structure was determined by means of direct methods applied to X-ray data collected with a single-crystal diffractometer. At the end of the refinement, performed with least-squares method, the R index was 0·039.

The SO₄ tetrahedra, AI(H₂O)₆ octahedra, and MnO(H₂O)₅ octahedra are connected by a hydrogen bonding system; the only direct connection between polyhedra is by sharing of an oxygen between S(4) and Mn. In the asymmetric unit there are twenty-two water molecules, five of which lie in channels of the structure and are not linked to the cations but only to ligand water oxygens by means of hydrogen bonds.

Powder data indicate a close structural relationship between apjohnite, halotrichite, and pickeringite.