  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.
