  Dramatic volume collapses under pressure are fundamental to geochemistry and
of increasing importance to fields as diverse as hydrogen storage and
high-temperature superconductivity. In transition metal materials, collapses
are usually driven by so-called spin-state transitions, the interplay between
the single-ion crystal field and the size of the magnetic moment. Here we show
that the classical S=5/2 mineral Hauerite undergoes an unprecedented 22 %
collapse driven by a conceptually different magnetic mechanism. Using
synchrotron x-ray diffraction we show that cold compression induces the
formation of a disordered intermediate. However, using an evolutionary
algorithm we predict a new structure with edge-sharing chains. This is
confirmed as the thermodynamic groundstate using in situ laser heating. We show
that magnetism is globally absent in the new phase, as low-spin quantum S=1/2
moments are quenched by dimerisation. Our results show how the emergence of
metal-metal bonding can stabilise giant spin-lattice coupling in Earth's
minerals.
