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oai:docta.ucm.es:20.500.14352/882302023-10-10T01:55:29Zcom_20.500.14352_14col_20.500.14352_15

Mechanism and kinetics of the pseudomorphic replacement of anhydrite by calcium phosphate phases at hydrothermal conditions Roza Llera, Ana Jiménez, Amalia Fernández Díaz, María Lourdes 549.761 Anhydrite hydroxyapatite β-tricalcium phosphate mineral replacement pseudomorphism kinetics textures coupled dissolution-precipitation Mineralogía (Geología) 2506.11 Mineralogía Mineral replacement reactions mediated by fluids are common in sedimentary basins, where they influence geochemical cycles. Phosphorous (P) pollution of soils, sediments and water bodies is currently a widespread problem. Some apatite accumulations in sediments may have formed through the interaction of P-bearing aqueous solutions with mineral surfaces that result in mineral replacement reactions. Here, we investigate the pseudomorphic replacement of anhydrite single crystals by aggregates of β-tricalcium phosphate and hydroxyapatite upon interaction with a P-bearing solution at temperatures between 120 and 200 °C. SEM imaging is used to study the texture of the aggregates. Rietveld refinement of the X-ray diffraction patterns and Raman spectra analysis of the reacted samples provide information on the kinetics of the replacement. At all temperatures β-tricalcium phosphate forms alongside hydroxyapatite at early stages of the replacement reaction. At T ≥ 180 °C, hydroxyapatite/β-tricalcium phosphate ratio rapidly increases, and hydroxyapatite is the only phase in fully replaced samples. At T < 180 °C hydroxyapatite/β-tricalcium phosphate ratio increases slowly and fully replaced samples still contain significant amounts of β-tricalcium phosphate. The progress of the replacement is facilitated by the formation of porosity. The evolution of the hydroxyapatite/β-tricalcium phosphate ratio and the crystal habit of both phases strongly influence the arrangement of this porosity. The empirical activation energy Ea (kJ/mol) of the replacement reaction is determined by the Avrami and the iso-conversion methods. Both approaches yield an Ea of ~40 kJ/mol. Anhydrite dissolution appears as the rate-limiting process and the overall kinetics of the replacement reaction is controlled by the rate diffusion of dissolved species through the porosity network. The ripening of the metastable β-tricalcium phosphate into hydroxyapatite affects the characteristics of the porosity network and further modulates the kinetics of the replacement. These results may improve the understanding of the mechanisms of P-sequestration by mineral surfaces through coupled dissolution–precipitation reactions and shed light on the origin of apatite accumulations associated to evaporitic sedimentary rocks. Ministerio de Ciencia e Innovación y Universidades Depto. de Mineralogía y Petrología Fac. de Ciencias Geológicas TRUE pub 2023-10-09T17:08:43Z 2023-10-09T17:08:43Z 2023-09-01 journal article VoR https://hdl.handle.net/20.500.14352/88230 0003-004X 10.2138/am-2022-8592 1945-3027 eng CGL2016-77138-C2-1-P CGL2016-77138-C2-2-P PID2021-125467NB-I00 BES-2017-081759 open access application/pdf Mineralogical Society of America