172-9 Thermodynamic properties of lead apatites with substitution on the X (OH,F,Cl,Br,I) site

Session: Environmental Geochemistry and Health (Part II)

Presenting Author:

Juraj Majzlan

Authors:

Majzlan, Juraj¹, Puzio, Bartosz², Dachs, Edgar³, Benisek, Artur⁴, Lempart-Drozd, Malgorzata⁵, Maniecki, Maciej⁶

(1) Institute of Geosciences, Friedrich Schiller University, Jena, Germany, (2) Department of Mineralogy, Petrography and Geochemistry, AGH University, Krakow, Poland, (3) Chemistry and Physics of Materials, Paris Lodron University, Salzburg, Austria, (4) Chemistry and Physics of Materials, Paris Lodron University, Salzburg, Austria, (5) Institute of Geological Sciences, Polish Academy of Sciences, Krakow, Poland, (6) Department of Mineralogy, Petrography and Geochemistry, AGH University, Krakow, Poland,

Abstract:

Lead apatites include the common end members pyromorphite [Pb₅(PO₄)₃X], mimetite [Pb₅(AsO₄)₃X], and vanadinite [Pb₅(VO₄)₃X], where X is a monovalent anion (OH, F, Cl, Br, I). Their crystal chemistry can be additionally complicated by the presence of the carbonate (CO3) group in their structure. In this work, we focused on thermodynamic properties of mimetite and synthesized phases with all X anions. Chemical analyses of the samples indicated compositions Pb₅(AsO₄)₃Cl_0.8(CO₃)_0.1, Pb₅(AsO₄)₃F, Pb₅(AsO₄)₃Br_0.8(CO₃)_0.1, Pb₅(AsO₄)₃OH_0.86(CO₃)_0.07, and Pb₅(AsO₄)₃I_0.45OH_0.35(CO₃)_0.1. Thermodynamic properties were measured by a combination of acid-solution calorimetry (in 5 M HCl) and relaxation calorimetry. The resulting standard entropies are 638.9 ± 4.5, 627.8 ± 4.4, 639.7 ± 4.5, 630.5 ± 4.5 and 637.7 ± 4.5 J mol⁻¹ K⁻¹, respectively. The resulting heats of formation are − 3068.7 ± 9.4, − 3001.4 ± 3.5, − 2963.4 ± 9.2, − 2919.1 ± 9.1, and − 2936.3 ± 8.7 kJ mol⁻¹, respectively. These data can be used to calculate log K_sp values for the mimetite phases and predict their solubility under a range of conditions. The data show that the inclusion of the CO₃ group diminishes the stability and increases the solubility of these phases. Hence, for modeling of natural systems, it is imperative to consider the CO₃ impurity in the mimetite phases. Phase diagrams (Puzio et al. 2026) show the degree of contraction of the stability field upon introduction of CO₃ in the structure. The minerals of the mimetite group can be used efficiently to store toxic Pb and As in and around contaminated sites. In addition, the iodine members, not known from nature, can be considered as a sink of radioactive ¹²⁹I from radioactive waste.

Puzio, B., Majzlan, J., Dachs, E., Benisek, A., Lempart, M., Manecki, M., 2026: Energetics and IR spectroscopy of Pb5(AsO4)3X (X = F, OH, Cl, Br, I) synthetic apatite phases. Physics and Chemistry of Minerals 53, 1, 10.1007/s00269-025-01334-8

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