Thermodynamic properties of tellurite (β-TeO2), paratellurite (α-TeO2), TeO2 glass, and Te(IV) phases with stoichiometry M2Te3O8, MTe6O13, MTe2O5 (M2+ = Co, Cu, Mg, Mn, Ni, Zn)

Thermodynamic properties of several TeO2 polymorphs and metal tellurites were measured by a combination of calorimetric techniques. The most stable TeO2 polymorph is α-TeO2, with its enthalpy of formation (ΔfHo) selected from literature data as −322.0 ± 1.3 kJ·mol−1. β-TeO2 is metastable (in enthalpy) with respect to α-TeO2 by +1.40 ± 0.07 kJ·mol−1, TeO2 glass by a larger amount of +14.09 ± 0.11 kJ·mol−1. >200 experimental runs and post-synthesis treatments were performed in order to produce phase-pure samples of Co, Cu, Mg, Mn, Ni, Zn tellurites. The results of the hydrothermal and solid-state syntheses are described in detail and the products were characterized by powder X-ray diffraction. The standard thermodynamic data for the Te(IV) phases are (standard enthalpy of formation from the elements, ΔfHo in kJ·mol−1, standard third-law entropy So in J·mol−1·K−1): Co2Te3O8: ΔfHo = −1514.2 ± 6.0, So = 319.2 ± 2.2; CoTe6O13: ΔfHo = −2212.5 ± 8.1, So = 471.7 ± 3.3; MgTe6O13: ΔfHo = −2525.8 ± 7.9, So = 509.2 ± 3.6; Ni2Te3O8: ΔfHo not measured, So = 293.3 ± 2.1; NiTe6O13: ΔfHo = −2198.7 ± 8.2, So = 466.5 (estimated); CuTe2O5: ΔfHo = −820.2 ± 3.3, So = 187.2 ± 1.3; Zn2Te3O8: ΔfHo = −1722.5 ± 4.0, So = 299.3 ± 2.1. The solubility calculations show that the Te(IV) concentration in an aqueous phase, needed to produce such phases, must be at least 3–5 orders of magnitude higher than the natural Te background concentrations. The occurrence of these minerals, as expected, are restricted to hotspots of Te concentrations. In order to produce more reliable phase diagrams, more work needs to be done on the thermodynamics of potential competing phases in these systems, including Te(VI) phases.