Carbonates are excellent carriers for divalent cations such as Ca, Mg, and Sr, and knowledge about their stability and solid solutions is important to understanding the global strontium cycle. To shed light on the topology of the two-phase field between calcite-type and aragonite-type (Ca,Sr)CO3 solid solutions as a function of temperature and pressure, and to learn more about the distribution of Sr and Ca between carbonates and fluid, we studied the system CaCO3-SrCO3-H2O at 600 °C in the pressure range 0.5–5 kbar. Conventional and rapid-quench hydrothermal syntheses were performed using a range of different starting materials. All bulk compositions were within the assumed/postulated two-phase field of calcite-type and aragonite-type (Ca,Sr)CO3 solid solutions. The run products were analyzed by scanning electron microscopy, electron microprobe analysis, and powder X-ray diffraction with Rietveld refinement. The results show that the heterotype solid solution is more extensive than previously assumed, with calcite incorporating up to 20 mol% SrCO3, which is twice as much as previously predicted. The compositional range of the aragonite-type solid solution was identical to that found in the literature. Using the data from this study, an updated version of the phase diagram P-X (Sr) at 600 °C for the CaCO3-SrCO3 system was calculated. The phase diagram does not support a phase transition within the trigonal (Ca,Sr)CO3 solid solution associated with rotational disorder of the CO3-groups. This order-disorder phase transition was previously postulated to explain some observed compositional trends in this system. Our new data are in line with other more recent studies. The distribution of Sr and Ca between the fluid and solid phases D = XSrsolid/XSrfluid is near to 1.0 for calcite-type and on average around 2.0 for aragonite-type solid solutions. This contrasts with silicate-fluid systems in which Sr typically shows a strong preference for the fluid phase compared with Ca.

中文翻译：

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方解石和菱锰矿异型固溶体和 Ca-Sr 流体-碳酸盐分布的重新检验：CaCO3-SrCO3-H2O 系统在 0.5-5 kbar 和 600 °C 下的实验研究

碳酸盐是 Ca、Mg 和 Sr 等二价阳离子的优良载体，了解它们的稳定性和固溶体对于理解全球锶循环非常重要。阐明方解石型和文石型 (Ca,Sr)CO3 固溶体之间作为温度和压力函数的两相场的拓扑结构，并了解更多关于 Sr 和 Ca 在碳酸盐和流体，我们在 600 °C 和 0.5-5 kbar 的压力范围内研究了 CaCO3-SrCO3-H2O 系统。使用一系列不同的起始材料进行常规和快速淬火水热合成。所有主体成分均在方解石型和文石型 (Ca,Sr)CO3 固溶体的假定/假定两相场内。通过扫描电子显微镜分析运行产物，电子显微探针分析和采用 Rietveld 精修的粉末 X 射线衍射。结果表明，异型固溶体比之前假设的更广泛，方解石包含高达 20 mol% 的 SrCO3，是之前预测的两倍。文石型固溶体的组成范围与文献中发现的相同。使用该研究的数据，计算了 CaCO3-SrCO3 系统在 600 °C 时的更新版本的相图 PX (Sr)。相图不支持与 CO3 基团旋转无序相关的三角 (Ca,Sr)CO3 固溶体中的相变。这种有序-无序相变先前被假设可以解释该系统中观察到的一些组成趋势。我们的新数据与其他最近的研究一致。Sr 和 Ca 在流体和固相之间的分布 D = XSrsolid/XSrfluid 方解石型接近 1.0，文石型固溶体平均约为 2.0。这与硅酸盐流体系统形成对比，在硅酸盐流体系统中，与 Ca 相比，Sr 通常表现出对流体相的强烈偏好。