Work

Reinvestigation of the crystal chemistry of phosgenite, Pb 2 (CO 3 )Cl 2

Year: 2025

Type: article

Abstract: The composition of phosgenite (ideal formula Pb 2 (CO 3 )Cl 2 , sp. gr. P 4/ mbm with a ≈ 8.15 and c ≈ 8.87 Å) from Monteponi Mine, Iglesias, Sardinia, Italy, its crystal structure and its high- T 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), in situ high- T powder X-ray diffraction (XRD) and thermogravimetric analysis. Concentrations of >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(Na 2 O+K 2 O+CaO+SiO 2 ) = 0.11 wt.%. The concentration of other industrially relevant elements is insignificant. X-ray and neutron refinements, based on data collected at room T , 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) CO 3 -groups to form the crystalline edifice. Low- T neutron diffraction data show evidence of a temperature-mediated phase transition towards a lower symmetry (space group P $\bar 4$ ), 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 this mineral is stable at least up to 523 K, at which the first evidence of transformation (with the coexistence of phosgenite + Pb 2 O 2 Cl, and probably an amorphous phase) takes place. The XRD pattern at 548 K unveils a more complex scenario, with coexisting: phosgenite (dominant) + Pb 2 O 2 Cl (minor) + mendipite [Pb 3 O 2 Cl 2 ] (minor) + Pb 5 O 2 Cl 6 (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 [Pb 3 O 2 Cl 2 ] (dominant), Pb 2 O 2 Cl (subordinate) + Pb 5 O 2 Cl 6 (subordinate) + kutnohorite-type [Ca(Mn,Mg,Fe 2+ )(CO 3 ) 2 ] (likely, very minor) (+ amorphous phase). The same assemblage is observed up to 698 and back to 298 K after a decrease in T , showing an irreversible transformation of the pristine material. Therefore, the irreversible temperature-induced degradation of phosgenite is substantially governed by a decarbonation process.

Source: Mineralogical Magazine

DOI: 10.1180/mgm.2025.10131

Authors: G. Diego Gatta, Giorgio Guastella, Candida Pipitone, Silvia C. Capelli, Andrea Ienco, Ferdinando Costantino