  We report on high-resolution acoustic, specific-heat and thermal expansion
measurements in the vicinity of the antiferromagnetic phase transition at T_N =
1.88 K on a high-quality single crystal of the natural mineral azurite. A
detailed investigation of the critical contribution to the various quantities
at T_N is presented. The set of critical exponents and amplitude ratios of the
singular contributions above and below the transition indicate that the system
can be reasonably well described by a three-dimensional Heisenberg
antiferromagnet.
