Ralphcannonite, AgZn (sub 2) TlAs (sub 2) S (sub 6) , a new mineral of the routhierite isotypic series from Lengenbach, Binn Valley, Switzerland

The new mineral species ralphcannonite, AgZn (sub 2) TlAs (sub 2) S (sub 6) , was discovered in the Lengenbach quarry, Binn Valley, Wallis, Switzerland. It occurs as metallic black equant, isometric to prismatic crystals, up to 50 mu m, associated with dufrenoysite, hatchite, realgar and baryte. Minimum and maximum reflectance data for COM wavelengths in air are [lambda (nm): R(%)]: 471.1: 25.8/27.1; 548.3: 25.2/26.6; 586.6: 24.6/25.8; 652.3: 23.9/24.8. Electron microprobe analyses give (wt.%): Cu 2.01(6), Ag 8.50(16), Zn 10.94(20), Fe 3.25(8), Hg 7.92(12), Tl 24.58(26), As 18.36(19), Sb 0.17(4), S 24.03(21), total 99.76(71). On the basis of 12 atoms per formula unit, the chemical formula of ralphcannonite is Ag (sub 0.63(2)) Cu (sub 0.25(2)) Zn (sub 1.35(5)) Fe (sub 0.47(1)) Hg (sub 0.32(2)) Tl (sub 0.97(3)) [As (sub 1.97(6)) Sb (sub 0.01(1)) ] (sub Sigma 1.98(5)) S (sub 6.03(8)) . The new mineral is tetragonal, space group I4I2m, with a=9.861 (2), c=11.125(3) Aa, V=1081.8(4) Aa (super 3) , Z=4. The main diffraction lines of the calculated powder diagram are [d(in Aa), intensity, hkl]: 4.100, 85, 211; 3.471, 40, 103; 2.954, 100, 222; 2.465, 24, 400; 2.460, 39, 303. The crystal structure of ralphcannonite has been refined by X-ray single-crystal data to a final R (sub 1) =0.030, on the basis of 140 observed reflections [F (sub o) >4sigma (F (sub o) )]. It shows a three dimensional framework of (Ag,Zn)-centred tetrahedra (1 M1+2 M2), with channels parallel to [001] hosting TlS (sub 6) and (As,Sb)S (sub 3) disymmetric polyhedra. Ralphcannonite is derived from its isotype routhierite (super M1) Cu (super M2) Hg (sub 2) TlAs (sub 2) S (sub 6) through the double heterovalent substitution (super M1) Cu (super +) + (super M2) Hg (super 2+) --> (super M1) Zn (super 2+) + (super M2) Ag (super +) . This substitution obeys a steric constraint, with Ag (super +) , the largest cation relative to Zn (super 2+) and Cu (super +) , entering the largest M2 site, as observed in arsiccioite. The ideal crystal-chemical formula of ralphcannonite is (super M1) Zn (super M2) (Zn (sub 0.5) Ag (sub 0.5) ) (sub 2) TlAs (sub 2) S (sub 6) .