Home

Search Abstracts

Author Index

Symposia Programmes

Sponsors

Help

MRD-09 Au-Ag telluride-selenide deposits

Experimental determination of phase relations and thermodynamic properties of Ag-Au-Te system minerals using EMF method

Ekaterina Echmaeva, Institute of Experimental Mineralogy (Russian Federation)
Evgeniy Osadchii, Institute of Experimental Mineralogy (Russian Federation)
 

The ternary Ag-Au-Te system was investigated experimentally by many scientists (Markham, 1960; Honea, 1964; Cabri, 1965; Petzow and Effenberg, 1988). According to literature data there is hessite-petzite equilibrium on the diagram at the temperatures below 393 K. But this fact was denied by the electrochemical experimental results. Using literature and experimental data the new ternary diagram of phase relations was constructed for the low-temperature area. Results of the electrochemical experiment show the presence of stuetzite-petzite equilibrium on the diagram instead of hessit-petzite one.
Also thermodynamic functions of mineral formation (?¢_fG^o, S^o, ?¢_fH^o) were calculated from experimental data and compared to literature values. These functions were determined for minerals Ag₅Te₃ (stuetzite), Ag₂Te (hessite), Ag₃AuTe₂ (petzite) and AuTe₂ (calaverite). The next reactions were investigated experimentally in corresponding electrochemical cells:

5Ag(cr) + 3Te(cr) = Ag₅Te₃(cr);T/K(330-620); (-) Pt | Ag | Ag₄RbI₅(AgI)| Ag₅?Se₃, ?S?u | Pt (+); Ag(cr) + Ag₅Te₃(cr) = 3Ag₂Te(cr);T/K(310-510); (-) Pt | Ag | Ag₄RbI₅ | Ag₅Te₃, Ag₂Te | Pt (+); Ag(cr) + 3Ag₃AuTe₂(cr) = 2Ag₅Te₃(cr)+3Au(cr);T/K(330-480); (-) Pt | Ag | Ag₄RbI₅ | Ag₃AuTe₂, Ag₅Te₃, Au | Pt (+); 3Ag(cr) + AuTe₂(cr)=Ag₃AuTe₂(cr);T/K(310-520); (-) Pt | Ag | Ag₄RbI₅ | Ag₃AuTe₂, AuTe₂, Au | Pt (+);

The E(T) dependences were obtained from the solid-state galvanic cells measurements. According to appearance of E(T) curves the conclusions about phase relations were drawn. Then, using these conclusions and basic thermodynamic equations: ?¢_rG= -nFE•10^-3, ?¢_rS = nF(dE/dT)•10^-3, ?¢_rH = -nF•10^-3•[E - (dE/dT)T] the formation thermodynamic properties of the minerals were calculated:

?¢_fG^o (Ag₅Te₃, cr) =-101859 ±135 J•mol^-1; S^o(Ag₅Te₃, cr) =416.39 ±4.19 J•K^-1•mol^-1; ?¢_fH^o(Ag₅Te₃, cr) =-85605 ±572 J•mol^-1;

?¢_fG^o (Ag₂Te, cr) =-40199 ±46 J•mol^-1; S^o(Ag₂Te, cr) =154.18 ±1.42 J•K^-1•mol^-1; ?¢_fH^o(Ag₂Te, cr) =-34425 ±563 J•mol^-1;

?¢_fG^o(Ag₃AuTe₂, cr) =-63967 ±139 J•mol^-1; S^o(Ag₃AuTe₂, cr) =304.67 ±2.98 J•K^-1•mol^-1; ?¢_fH^o(Ag₃AuTe₂, cr) =-55078 ±1150 J•mol^-1;

?¢_fG^o (AuTe₂, cr) =-10726 ±147 J•mol^-1; S^o(AuTe₂, cr) =120.75 ±3.03 J•K^-1•mol^-1; ?¢_fH^o(AuTe₂, cr) =-18522 ±1160 J•mol^-1;

An electromotive force measurement is the unique direct method of Gibbs energy determination. Thermodynamic parameters, obtained from this investigation, allow to determinate physical-chemical parameters of mineralization, deposit?fs forms and noble-metal transport processes and might became the fundamental part of genetic model of epithermal gold mineralization that is developed at present time.