The crystal structure of clinoatacamite, Cu 2 (super 2+) (OH),Cl, has been determined and refined to indices R = 5.2 and wR = 4.9%. The structure is monoclinic, space group P2 1 /n (14), with cell parameters a 6.144(1), b 6.805(1), c 9.112(1) Aa, beta 99.55(3) degrees , V 375.7(2) Aa 3 , and with Z = 4. The structure analysis was essential in determining the valence state of copper (Cu (super 2+) ) and the number of (OH) (super -) anions in the formula. The structure consists of layers of partly occupied, edge-sharing octahedra of Jahn-Teller-distorted [Cu(OH) 4 Cl 2 ] octahedra parallel to (101). This layer is topologically the same as that in mica. Adjacent layers of octahedra are offset, such that vacant sites in one sheet align with occupied sites in the neighboring sheet. Layers are linked by individual, slightly Jahn-Teller-distorted octahedra of composition [Cu(OH) 6 ]. The structures of paratacamite and the other polymorphs of Cu 2 (OH) 3 Cl, atacamite and botallakite, also are layered, but they differ from the layering of clinoatacamite in topology, composition and cross-linkage.