Evidence for Antiferromagnetism in ${\mathrm{Cu}}_3$ ${\left(\mathrm{C}{\mathrm{O}}_3\right)}_2$ ${\mathrm{}\left(\mathrm{OH}\right)\mathrm{}}_2$

We have used the proton resonance to study the magnetic behavior of the mineral azurite —${\mathrm{Cu}}_3$ ${\left(\mathrm{C}{\mathrm{O}}_3\right)}_{2^{-}}$ ${\mathrm{}\left(\mathrm{OH}\right)\mathrm{}}_2$. Below 30°K the temperature dependence of the splitting of the proton resonance lines by the copper ions shows strong exchange effects. At 1.86°K a transition takes place and the resonance pattern below the transition indicates an antiferromagnetic state.