The atacamite family of minerals – a testbed for quantum spin liquids

Polymorphism of Cu₂(OH)₃Cl coupled with partial substitution of Jahn–Teller active Cu²⁺ by other divalent metal cations gives rise to the complex mineralogy of the atacamite family of secondary basic copper chlorides. Herbertsmithite, Cu₃Zn(OH)₆Cl₂, in which Zn substitutes for one quarter of the Cu atoms, provides a lattice of corner-sharing triangles of paramagnetic Cu²⁺ (spin ½) cations, rendering the mineral a perfect realization of a kagome antiferromagnet. Geometric frustration of conventional antiferromagnetism is expected to give rise to exotic ground states, with dynamic magnetic structures that might turn out to be physical realizations of quantum spin liquids. In this paper, a synopsis of the key topological, compositional and behavioural features of minerals in the atacamite family is given, with emphasis on the kagome character of the resulting lattice of Cu²⁺ cations.