Thermodynamic data for binary tellurides are compiled and combined with available data for sulfides and oxides to calculate phase relations among tellurides, sulfides, and oxides at low temperatures (100 degrees -300 degrees C). Limits on the natural range in f (sub Te 2 ) , f (sub S 2 ) , and f (sub O 2 ) values during telluride deposition and/or reequilibration are estimated from natural assemblages relative to reactions, such as 2Fe 3 O 4 + 3S 2 + 3Te 2 = 3FeS 2 + 3FeTe 2 + 4O 2 and 2HgS + Te 2 = 2HgTe + S 2 . Calculated equilibria show that tellurides of Mo, Sn, Mn, and Zn are unstable with respect to corresponding sulfides and/or oxides in natural systems and that some tellurides, such as As 2 Te 3 , are likely to occur in nature, although they have not yet been found as minerals. The composition of electrum in equilibrium with hessitc is useful as a sliding-scale indicator of the fugacity of Te 2 at relatively low values of f (sub Te 2 ) . The calculated phase relations explain particular associations or antipathies among ore minerals and may be used to predict other minerals which should be present at equilibrium.

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