Experimental evidence for a spin gap in the s=1/2 quantum   antiferromagnet Cu$_2$(OH)$_2$CO$_3$
  Magnetic properties of the natural mineral Cu$_2$(OH)$_2$CO$_3$ (malachite) were investigated through DC and AC susceptibility measurements. The analysis of the low-temperature part reveals a quantum spin-gap behavior with $\Delta \approx 130 K$. Consistently with the crystal structure, the magnetic susceptibility can be accurately described by a model of alternating chain. The non-frustrating residual inter-chain magnetic couplings, describing a sort of dimerized square planar lattice, are not strong enough to push the system towards a long-range ordered ground state, in good agreement with recent theoretical studies. 
E. Janod, L. Leonyuk, V. Maltsev