The heat capacity of natural atacamite has been measured between T = 2 K and T = 305 K by cryogenic-adiabatic calorimetry. At T = 298.15 K, the standard molar heat capacity C(p,m)degrees and entropy S-m degrees for atacamite are (19.16 +/- 0.07).R and (20.18 +/- 0.08).R (where R = 8.3144 J.K-1.mol(-1)), respectively. The heat capacity exibits anomalous behaviour at temperatures lower than 10.5 K. Smoothed thermodynamic properties : molar heat capacity; molar entropy increment Delta(0)(T)S(m) degrees; and (Phi(m) degrees + Delta(0)(T)H(m) degrees/T), where T = 298.15 K, have been calculated. Between T = 290 K and T = 508 K the heat capacity of atacamite measured by d.s.c. is represented by the equation : C(p,m)degrees/R = 18.80 + 8.11.10(-3).(T/K) - 1.87.10(-5).(K/T)(2).