Critical Phenomena at the Antiferromagnetic Phase Transition of Azurite
  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. 
P. T. Cong, B. Wolf, R. S. Manna, A. Br\"uhl, S. K\"ohler and M. Lang
cond-mat.str-el