AS-SB EXCHANGE ENERGIES IN TETRAHEDRITE-TENNANTITE FAHLORES AND BOURNONITE-SELIGMANNITE SOLID-SOLUTIONS

Reversed brackets on As and Sb partitioning between tetrahedrite-tenna ntite fahlores {similar to Cu-10(Fe,Zn)(2)(Sb,As)(4)S-13} and bournoni te-seligmannite solid solutions {CuPb(Sb,As)S-3} (400 degrees C, evacu ated silica tubes, NH4Cl flux) indicate that the maximum nonidealities associated with the As-Sb substitution are about 165 (bournonite) and 250 (fahlore) cal/gfw on a per atom exchange basis. The experimental constraints are consistent with the following calibration of the As-Sb exchange reaction between these phases: RTIn (1-X(3)(FAH))X(As)(BRN)/ X(3)(FAH)X(Sb)(BRN)]=Delta ($) over bar G(As(Sb)-1)(0FB)+1/4(Delta ($) over bar )+W-AsSb(FAH)(1-2X(3)))-W-AsSb(BRN)(1-2X(As)(BRN)) where Del ta ($) over bar G(As(Sb)-1)(0FB) {similar to Delta ($) over bar H-As(S b-1)}=-1.39+/-0.10, W-AsSb(FAH)=4.00+/-0.80, and W-AsSB(BRN)=0.66+/-0. 18 kcal/gfw, X(2)=Zn/(Zn+Fe) (or one-half the number of Zn atoms/formu la unit) and X(3)=As/(As+Sb) refer to atomic ratios in fahlore, and De lta ($) over bar G(23)(0)=Delta ($) over bar H-23(0)=2.59+/-0.14 kcal/ gfw (Raabe and Sack, 1984; Sack and Loucks, 1975; O'Leary and Sack, 19 87; Sack, 1992).