Ferrihydrite is a fine-grained and variable, in terms of structure and water content, iron oxyhydroxide found in many geologic environments, in living organisms, and in environmental pollution. The thermodynamic study of ferrihydrite is important for understanding geochemical transformations involving iron as well as in the study of iron in chemical synthesis and biological systems. In this study, the heat capacity of a synthetic two-line ferrihydrite, which showed typically broad X-ray diffraction peaks due to small particle size and/or disorder but had a rather low excess water content, FeOOH center dot 0.027H(2)O, was measured over the temperature range of (2 to 300) K. Using these data, the standard molar entropy, S-m(o), of this two-line ferrihydrite was calculated to be (71.7 +/- 0.2) J . K-1 . mol(-1), while that of the stoichiometric (no excess water) material, FeOOH was estimated to be (69.8 +/- 0.2) J . K-1 . mol(-1). These measured entropies are in moderate agreement with, though slightly higher, than those estimated much more indirectly by Majzlan et al. (2004) [15] The heat capacity data below T = 12 K were fit to theoretical models to study the underlying contributions to the heat capacity. The fit required an anisotropic ferrimagnetic spin wave component, which agrees with studies that characterize two-line ferrihydrite as a ferrimagnet. The fit also required a linear term, which most likely stems from defects in the ferrihydrite particles. Using enthalpy of formation from the elements -(542.3 +/- 0.8 kJ . mol(-1) for FeOOH center dot 0.027H(2)O or -(534.6 +/- 1.0) kJ . mol(-1) for FeOOH obtained from acid solution calorimetry of the same sample, the standard Gibbs energy of formation from the elements for this ferrihydrite sample was calculated to be -(473.0 +/- 1.0) kJ . mol(-1) for FeOOH center dot 0.027H(2)O and -(465.3 +/- 1.0) kJ . mol(-1) for FeOOH. (C) 2012 Elsevier Ltd. All rights reserved.