From Quantum Spin Chains to Classical Helimagnets in Natural Magnetic Minerals

Low-dimensional and frustrated spin systems, especially large single crystals that are difficult to prepare synthetically, are often found in nature. This review compares three mineral families: green and black dioptase, Cu 6 [Si 6 O[Formula: see text]]⋅6H 2 O and Cu 6 Si 6 O[Formula: see text]; brochantite, Cu 4 SO 4 (OH) 6 ; and aegirine, NaFeSi 2 O 6 . These compounds span spin-[Formula: see text] chain physics and large-spin helical magnetism. The comparison focuses on how the hierarchy of intrachain, interchain, and anisotropic interactions determines magnetic dimensionality, ordered states, and spin dynamics. The dehydrated black dioptase shows how the removal of water from the structure changes the exchange hierarchy of Cu[Formula: see text] spin-[Formula: see text] spiral chains. Compared with green dioptase, it has stronger intrachain exchange, much weaker interchain coupling, reduced ordered moment, and continuum-like spinon excitations. Another spin-1/2 system, brochantite, shows a commensurate noncollinear magnetic order, while its excitations are incommensurate because of a uniform Dzyaloshinskii-Moriya interaction. High-field measurements reveal anisotropic transitions and short-range correlations beyond the minimal XXZ-chain model. In aegirine, a Fe[Formula: see text] spin-5/2 pyroxene, spin-wave measurements reveal a three-dimensional exchange network within a chain-like crystal structure. Magnetic field drives a helix-to-fan-to-collinear evolution controlled by anisotropic exchange. These minerals show how neutron scattering distinguishes structural from magnetic dimensionality and identifies the interactions that select ordered and field-induced states.