Abstract:Five crystalline members of the hydroxyapatite (HAP; Ca 5 (PO 4 ) 3 OH)-johnbaumite (JBM; Ca 5 (AsO 4 ) 3 OH) series were crystallized at alkaline pH from aqueous solutions and used in dissolution experiments at 5, 25, 45, and 65 • C. Equilibrium was established within three months. Dissolution was slightly incongruent, particularly at the high-P end of the series. For the first time, the Gibbs free energy of formation ∆G

Five lead-arsenate apatites (mimetites)-Pb5(AsO4)3X—where X denotes fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and hydroxide (OH)—were synthesized via precipitation from aqueous solutions. The crystal structures were determined through Rietveld refinement of powder synchrotron X-ray data. All the compounds crystallized in the hexagonal class symmetry (space group P63/m). The Rietveld refinement indicated that mimetite-Cl, -Br, -I, and -OH had an anion deficiency at position X. Substitution of halogens in a mimetite structure brought about systematic changes in unit-cell parameters, interatomic distances, and metaprism twist angles φ, proportional to the substituted halogen’s ionic radius. Mimetite-OH did not follow the linear correlations determined within the series. Twist angle φ, a useful device for monitoring changes in apatite topology, ranged from 20.34° for mimetite-F to 11.42° for mimetite-I. The geometric method has been proposed for determining the diameter of hexagonal channels hosting halogens in apatites. A comparison of the results with halogenated pyromorphites showed similar systematic trends: the substitutions in mimetites have comparable effect on the interatomic distances as in their phosphorous analogues.

Thermodynamic parameters have been measured for synthetic analogs of the mimetite-group minerals Pb5(AsO4)3X (X = OH, Cl, Br, I) belonging to the apatite supergroup. Phases precipitated from aqueous solutions under ambient conditions with well characterized structures and compositions were studied. For each phase, dissolution enthalpy was experimentally determined by oxide melt drop solution calorimetry in a molten solvent of sodium molybdate (3Na2O·4MoO3) at 976 K. The enthalpy of formation from the elements ΔHf,elo was calculated using thermochemical cycles and was −3030.6 ± 11.5, −3026.6 ± 15.8, −2967.6 ± 25.0, and −2993.1 ± 12.2 kJ/mol for Pb5.00(AsO4)3.00OH0.86(CO3)0.07, Pb5.00(AsO4)3.00Cl0.80(CO3)0.10, Pb5.00(AsO4)3.00Br0.80(CO3)0.10, and Pb5.00(AsO4)3.00I0.45OH0.35(CO3)0.10, respectively. These ΔHf,elo values exhibit typical trends for apatites: they increased (were less negative) with the increasing molar mass and ionic radius of X and decreased with the electronegativity and ionization energy of X. The compilation and comparison of data for Ca-, Pb-, P-, and As-apatites revealed correlations indicating that thermodynamic enthalpic stability is largely influenced by chemical factors (e.g., differences in electronegativities of the elements, ionization energy, or ionic characteristics of the bonds) and to a lesser extent by physical and geometric parameters in the crystal structure related to the mass and size of the X anion. Using the correlations, it was possible to estimate the value of hitherto unknown ΔHf,elo for Pb5(AsO4)3F, −3144.3 ± 66.5 kJ/mol. The observed relationships apply to the entire apatite supergroup and can be used to predict the values of ΔHf,elo for phases that have not been studied experimentally. The new data on environmentally significant phases will contribute to the modeling of mineral-water interactions, particularly for potential use in the remediation of soils and wastes contaminated with Pb and As and in the immobilization of radioactive waste containing I-129.

As so ci ate Ed i tor: Jacek Szczepañski The pa per pres ents the re sults of ex per i men tal stud ies on the syn the sis and ther mo dy namic sta bil ity of se lected Pb-apatites in terms of cri te ria de ter min ing ter mi na tion of the ex per i ments. Based on the case study, we in di cate dif fi cul ties in ana lys ing the ob tained ex per i men tal data. Time-re solved sam pling of pre cip i tate formed dur ing a dropwise syn the sis of pyromorphite was per formed and the re sults were com pared to the lit er a ture data. It has been con cluded that the Ostwald rip en ing time for syn the sized sol ids de pends pri mar ily on the chem i cal com po si tion of the in tended Pb-ap a tite phase. We pre sented that heter o ge ne ity of pre cip i tate af fects its dis so lu tion in terms of re peat abil ity of the re sults and equilibrating time. A unique 9-year-long ex per i ment on vanadinite sta bil ity at a pH range from 2.0-6.0 re vealed that among all tested dis so lu tion con ditions only the re ac tions at the pH = 3.5 can per form as the ba sis for some ther mo dy namic cal cu la tions. It has been con cluded that the rate of phase tran si tions in the Pb-apatites group can be mis lead ing in terms of de ter min ing the equi lib rium of the sys tem, and the ex per i men tal setup de signed par tic u larly to pro vide re li able con trols in this as pect should be in volved. Means in this re spect have been pro posed. Key words: pyromorphite, vanadinite, ex per i men tal min er al ogy, dis so lu tion, sol u bil ity syn the sis.

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