Mercury-arsenic sulfosalts from the Apuan Alps (Tuscany, Italy). II. Arsiccioite, AgHg2TlAs2S6, a new mineral from the Monte Arsiccio mine: occurrence, crystal structure and crystal chemistry of the routhierite isotypic series

Abstract

The new mineral species arsiccioite, AgHg₂TlAs₂S₆, was discovered in the baryte-pyrite-iron oxide ore deposit exploited at the Monte Arsiccio mine, near Sant’Anna di Stazzema (Apuan Alps, Tuscany, Italy). It occurs as anhedral grains scattered in microcrystalline baryte, associated with cinnabar, laffittite, protochabournéite, pyrite, realgar, Hg-bearing sphalerite and stibnite. Arsiccioite is red, with a metallic to sub-metallic lustre. Minimum and maximum reflectance data for COM wavelengths in air are [λ (nm):R(%)]: 471.1: 29.0/29.4; 548.3: 27.6/28.3; 586.6: 26.1/26.5; 652.3: 24.2/24.6. Electron microprobe analyses give (wt.%): Cu 0.78(6), Ag 8.68(21), Zn 0.47(27), Fe 0.04(1), Hg 35.36(87), Cd 0.20(5), Tl 18.79(33), As 10.77(19), Sb 4.75(10), S 18.08(21), Se 0.07(5), total 97.99(44). On the basis of ΣMe= 6 a.p.f.u., the chemical formula is Ag_0.87(2)Cu_0.13(1)Zn_0.08(4)Fe_0.01(1)Hg_1.91(5)Cd_0.02(1)Tl_1.00(2)(As_1.56(2)Sb_0.42(1))S_1.98S_6.12(6)Se_0.01(1). Arsiccioite is tetragonal,I2m, witha10.1386(6),c11.3441(5) Å,V1166.1(2) Å3,Z= 4. The main diffraction lines of the powder diagram are [d(in Å), visually estimated intensity,hkl]: 4.195, m, 211; 3.542, m, 103; 3.025, vs, 222; 2.636, m, 114; 2.518, s, 400 and 303. The crystal structure of arsiccioite has been refined by single-crystal X-ray data to a finalR₁= 0.030, on the basis of 893 observed reflections. It shows a three dimensional framework of (Hg,Ag)- centred tetrahedra (1M1 + 2M2), with channels parallel to [001] hosting TlS₆and (As,Sb)S3 disymmetric polyhedra. Arsiccioite is derived from its isotype routhierite^M1Cu^M2Hg₂TlAs₂S₆through the double heterovalent substitution^M1Cu⁺+^M2Hg²⁺→^M1Hg²⁺+^M2Ag⁺. This substitution obeys a steric constraint, with Ag⁺, the largest cation relative to Hg²⁺and Cu⁺, entering the largestM2 site. The ideal crystal chemical formula of arsiccioite is^M1Hg^M2(Hg_0.5Ag_0.5)₂TlAs₂S₆. The crystal chemistry of the routhierite isotypic series is discussed. Finally, the distribution of Hg ore minerals in the Apuan Alps is reviewed.