Abstract Carbonated hydroxy-apatite (Ca10-b(NH4,K,Na)2b(PO4)6-y[(CO3)2x+(3/2)y(OH)2–2x]; CHAP) was synthesized in batch-type experiments by mixing of calcium (Ca)- and phosphate-bearing aqueous solutions and the transformation of calcite powder in aqueous solution between 11° and 65 °C. Compositional changes of the mother solution and solid phase products were followed by powder XRD, scanning-electron microscopy (SEM-EDX), Raman spectroscopy, and elemental analysis. It was found that the transformation at pH values between 7.5 and 7.9 occurs via the dissolution of calcite followed by the precipitation of CHAP. In the mixing approach, crystallization of CHAP took place via precipitation of metastable brushite (CaH(PO4).2H2O) as precursor phase that was further transformed into apatite. Calcium isotope fractionation led to an enrichment of the light isotope by about −0.5 to −1.1‰ in the solid CHAP compared to the aqueous solution, consistently in all experimental approaches, essentially independent of temperature. In some experiments metastable brushite formed prior to transformation to CHAP and showed a reduced fractionation compared to the CHAP. The observed Ca isotope fractionation into the CHAP lattice resembles that of natural phosphorites and lies within the range of recently published experiments.

中文翻译：

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实验性磷灰石形成时的钙同位素分馏

摘要 碳酸羟基磷灰石 (Ca10-b(NH4,K,Na)2b(PO4)6-y[(CO3)2x+(3/2)y(OH)2-2x]; CHAP)通过混合含钙 (Ca) 和含磷酸盐的水溶液以及方解石粉末在 11°C 和 65°C 之间的水溶液中转化的实验。通过粉末 XRD、扫描电子显微镜 (SEM-EDX)、拉曼光谱和元素分析跟踪母液和固相产物的组成变化。发现在 7.5 和 7.9 之间的 pH 值下的转变是通过方解石的溶解和 CHAP 的沉淀发生的。在混合方法中，CHAP 的结晶是通过亚稳态透钙磷石 (CaH(PO4).2H2O) 作为前体相的沉淀发生的，该相进一步转化为磷灰石。与水溶液相比，钙同位素分馏导致固体 CHAP 中轻同位素的富集约 -0.5 至 -1.1‰，这在所有实验方法中都是一致的，基本上与温度无关。在一些实验中，亚稳透钙磷石在转化为 CHAP 之前形成，并且与 CHAP 相比显示出降低的分馏。观察到的 Ca 同位素分馏进入 CHAP 晶格类似于天然磷矿的分馏，并且在最近发表的实验范围内。