1 Revision 1 ( # 5603 )-Submitted to American Mineralogist 1 2 Constraints on the solid solubility of Hg , Tl and Cd in arsenian pyrite 3 4

13 Arsenic-rich (arsenian) pyrite can contain up to tens of thousands of parts per million (ppm) of toxic 14 heavy metals such as Hg, Tl and Cd, although few data are available on their solid solubility 15 behaviour. When a compilation of Hg, Tl and Cd analyses from different environments are plotted 16 along with As in a M(Hg, Tl, and Cd)-As log-log space, the resulting wedge-shaped distribution of 17 datapoints suggests that the solid solubility of the aforementioned metals is strongly dependent on 18 the As concentration of pyrite. The solid solubility limits of Hg in arsenian pyrite – i.e., the upper 19 limit of the wedge-shaped zone in compositional space – are similar to the one previously defined 20 for Au by Reich et al. (2005) (CHg,Au= 0.02CAs + 4 x10), whereas the solubility limit of Tl in 21 arsenian pyrite is approximated by a ratio of Tl/As = 1. In contrast, and despite a wedge-shaped 22 distribution of Cd-As datapoints for pyrite in Cd-As log-log space, the majority of Cd analyses 23 reflect the presence of mineral particles of Cd-rich sphalerite and/or CdS. Based on these data, we 24 show that arsenian pyrite with M/As ratios above the solubility limit of Hg and Tl contain 25 nanoparticles of HgS, and multimetallic Tl-Hg mineral nanoparticles. These results indicate that the 26