Compositional trends in tetrahedrite

Available mmpositional data for l27t samples of natural tetrahedrite and 295 of synthetic tetrahedrite were s1amingd. They show: compositional ranges of four to ten Cu, zero to six Ag, and a total of two (Fe, Zn, Hg) atoms, complete substitution (up to four atems) amongAs, Sb and Te, and a total of 13 atoms of S per formula unit. Data on contents of Pb, Bi and Cd in natural samples are insufficient to define their compositional ranges, and virtually no data exist on other substitutions, involving Co, Ni, Mn and Au. A generalized formula (Cu,Ag)6Cua@e,Zn,Cu,Hg, Cd)2(Sb,As,Bi,Te)a(S,Se)13 is proposed on the basis of these compositions. The cell rlimensisn is a linear function of chemical elements in the formula errcept inAg-rich tetrahedrite, where it increases with Ag content, up to four Ag atoms per formula unit, and then decreases. Ag and As have a low tolerance for each other in the structure, and plots of Fe versus Ag, Zn versus Ag, and Hg versus Cl suggest that incorporation of Ag'is controlled by several factors. For the purpose of explaining compositional variations in tetrahedrite, the simple Brillouin zone model of 6eading (Jotrnson & Jeanloz 1983) is superior to any ionic model.