Tooeleite, nominally Fe 6 3+ (As 3+ O 3 ) 4 (SO 4 )(OH) 4 ·4H 2 O, is a relatively uncommon mineral of some acid-mine drainage systems. Yet, if it does occur, it does so in large quantities, indicating that some specific conditions favor the formation of this mineral in the system Fe-As-S-O-H. In this contribution, we report the thermodynamic properties of synthetic tooeleite. The sample was characterized by powder X-ray diffraction, scanning electron microscopy, extended X-ray absorption fine-structure spectroscopy, and Mössbauer spectroscopy. These methods confirmed that the sample is pure, devoid of amorphous impurities of iron oxides, and that the oxidation state of arsenic is 3+. Using acid-solution calorimetry, the enthalpy of formation of this mineral from the elements at the standard conditions was determined as −6196.6 ± 8.6 kJ mol −1 . The entropy of tooeleite, calculated from low-temperature heat capacity data measured by relaxation calorimetry, is 899.0 ± 10.8 J mol −1 K −1 . The calculated standard Gibbs free energy of formation is −5396.3 ± 9.3 kJ mol −1 . The log K sp value, calculated for the reaction Fe 6 (AsO 3 ) 4 (SO 4 )(OH) 4 ·4H 2 O + 16H + = 6Fe 3+ + 4H 3 AsO 3 + SO 4 2− + 8H 2 O, is −17.25 ± 1.80. Tooeleite has stability field only at very high activities of aqueous sulfate and arsenate. As such, it does not appear to be a good candidate for arsenic immobilization at polluted sites. An inspection of speciation diagrams shows that the predominance field of Fe 3+ and As 3+ overlap only at strongly basic conditions. The formation of tooeleite, therefore, requires strictly selective oxidation of Fe 2+ to Fe 3+ and, at the same time, firm conservation of the trivalent oxidation state of arsenic. Such conditions can be realized only by biological systems (microorganisms) which can selectively oxidize one redox-active element but leave the other ones untouched. Hence, tooeleite is the first example of an “obligatory” biomineral under the conditions prevailing at or near the Earth's surface because its formation under these conditions necessitates the action of microorganisms.