  The mineral malachite, Cu2(OD)2CO3, has a quantum spin-liquid ground state
and no long-range magnetic order down to at least T=0.4 K. Inelastic neutron
scattering measurements show that the excitation spectrum consists of
dispersive gapped singlet-triplet excitations, characteristic of spin-1/2
dimer-forming Heisenberg antiferromagnets. We identify a new two-dimensional
dimerized coupling scheme with strong interdimer coupling J'/J1~0.3 that places
malachite between strongly coupled alternating chains, square lattice
antiferromagnets, and infinite-legged ladders. The geometry of the interaction
scheme resembles the staggered dimer lattice, which may allow unconventional
quantum criticality.
