Reinvestigation of the crystal chemistry of phosgenite, Pb2(CO3)Cl2

The chemical composition of phosgenite (ideal formula Pb2(CO3)Cl2, sp. gr. P4/mbm with a8.15 and c8.87 Å) from Monteponi Mine (Iglesias, Sardinia, Italy), its crystal structure and its high-temperature behaviour up to the onset of decomposition were investigated by a series of chemical analytical and diffraction techniques, including single-crystal X-ray (data collected at 293 K) and neutron diffraction (at 293 and 20 K), and in-situ high-temperature powder X-ray diffraction and thermogravimetric analysis. The concentrations of more than 65 elements were measured. The empirical mineralogical formula of phosgenite, obtained by the multi-analytical approach used in this study, is almost identical to the ideal one, with only a few elements measured above the detection limit: Σ(Na2O+K2O+CaO+SiO2) = 0.11 wt%. The concentration of other industrially relevant elements is insignificant. X-ray and neutron refinements, based on data collected at room temperature, confirm the previously reported general structural model of phosgenite, while providing a full description of the displacement parameters of all the atomic sites. The building unit of the crystal structure of phosgenite is represented by a Pb-polyhedron, in which Pb is coordinated by 5Cl + 4O (coordination number CN= 9), forming a monocapped square antiprism. The combination of face-sharing Pb-polyhedra generates dense layers parallel to (001), which are connected by (edge-sharing) CO3-groups to form the crystalline edifice. Low-temperature neutron diffraction data show evidence of a T-mediated phase transition toward a lower symmetry (space group P̅), with a modest distortion of the building units of the structure. A tentative description of the low-T mechanisms, at the atomic scale, that can lead to the phase transition is provided.The high-T behaviour of phosgenite proves that this mineral is stable at least up to 523 K, at which the first evidence of transformation (with the coexistence of phosgenite + Pb2O2Cl, and likely an amorphous phase) takes place. The X-ray diffraction pattern at 548 K unveils a more complex scenario, with coexisting: phosgenite (dominant) + Pb2O2Cl (minor) + mendipite [Pb3O2Cl2] (minor) + Pb5O2Cl6 (subordinate) (+ amorphous phase). This phase composition is preserved up to 648 K, after which phosgenite is no longer preserved, and the stable compounds are: mendipite [Pb3O2Cl2] (dominant), Pb2O2Cl (subordinate) + Pb5O2Cl6 (subordinate) + kutnohorite-type [Ca(Mn,Mg,Fe2+)(CO3)2] (likely, very minor) (+ amorphous phase). The same assemblage is observed up to 698 and back to 298 K after T-decrease, showing an irreversible transformation of the pristine material. Therefore, the irreversible Tinduced degradation of phosgenite is substantially governed by a decarbonation process.