ИСТИНА

Daubreelite is a common mineral in enstatite chondrites, but its thermodynamic properties have not been studied. This greatly complicates the study of the physico - chemical parameters of enstatite chondrites formation in their parent bodies. Analysis of the quaternary system Ag-Cr-Fe-S showed that at temperatures below 423 K can be stable phase association Ag2S + Cr2S3 + FeS2 + FeCr2S4, potential silver which can be defined in a completely solid state galvanic cell: (-) Pt | Ag | RbAg4I5 | Ag2S, Cr2S3, FeS2, FeCr2S4 | Pt (+), with a RbAg4I5 as a solid electrolyte with a specific conductivity of Ag+ ion. The overall potential forming process in the cell corresponds to a chemical reaction: 2Ag + Cr2S3 + FeS2 = Ag2S + FeCr2S4 Gibbs energy of this reaction is associated with the electromotive force of galvanic cells by fundamental equation of thermodynamics DeltarG =-nFE, where n = 2 - the number of electrons in the electrochemical process, F = 96485 C•mol-1 - Faraday constant, and E-electromotive force (emf) of galvanic cell in volts. Temperature dependence of the emf was determined in an electrochemical cell, a device which is described in detail in the works Osadchii and Chareev (2006), and Osadchii and Echmaeva (2007). The results were approximated by a linear dependence of E(T), which corresponds to the condition DeltarCp constant and equal to zero: E(mV)=76.32+0.2296•T, 339<T<400, R2=0.9936, k=38 (number of E/T points). The Gibbs energy, entropy and enthalpy changes of the reaction were calculated from E(T) dependence using base thermodynamic equations: DeltarG=-nFE•10-3, DeltarS=nF(dE/dT)•10-3, DeltarH=-nF•[E-(dE/dT)T]•10-3. Using auxiliary data Barin (1995) and Mills (1974) were determined standard thermodynamic properties formation of daubreelite from elements at 1 bar pressure: DeltafG°(298.15 K) = -483.8 kJ•mol-1, S°(298.15 K) = 187.53 J•mol-1•K-1, DeltafH°(298.15 K) = -488.4 kJ•mol-1. This work was supported by RFBR grants No.12-05-01005 and No.13-05-00405.