Abstract An experimental study was carried out on the preparation of schoenite from carnallite and thenardite by the double-decomposition method. According to the metastable phase diagram for the quinary system, Na+, K+, Mg2+//Cl–, and SO42−-H2O at 25 °C, three theoretical models were established for the double-decomposition processes. The mineralogy and morphology of schoenite, sylvite, epsomite/hexahydrite, and halite were studied by X-ray powder diffractometry and scanning electron microscopy during the double-decomposition processes. It is confirmed the preparation of schoenite via carnallite and thenardite is feasible. The double-decomposition reactions processes were described by three model equations. First, sylvite was obtained by carnallite decomposition in aqueous solution; second, epsomite/hexahydrite was obtained by a conversion reaction via thenardite and an MgCl2-rich mother liquor; third, schoenite was obtained via sylvite and epsomite/hexahydrite. The results suggest that (1) sylvite exists in the primary reaction time, epsomite/hexahydrite form simultaneously, and their contents decrease as the reaction time increase. (2) The terminal product of the double-decomposition is schoenite, and its formation results from conversion via sylvite and epsomite/hexahydrite. (3) A tendency for the X-ray diffraction intensity of sylvite and epsomite/hexahydrite to weaken increases visibly with reaction time, and its change agrees well with the third model. (4) Scanning electron microscopy images of the halite, sylvite, epsomite/hexahydrite, and schoenite are clear and integrated, and their formation results from the precipitation of the three model conversion reactions. Results showed that a good agreement exists between the experimental and theoretical models.

中文翻译：

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X射线衍射和扫描电子显微镜在25°C光卤石和芒硝向软锰矿转变的研究中的应用

摘要 对光卤石和芒硝双分解法制备软锰矿进行了实验研究。根据五元体系Na+、K+、Mg2+//Cl-和SO42--H2O在25°C下的亚稳态相图，建立了双重分解过程的三个理论模型。在双重分解过程中，通过 X 射线粉末衍射和扫描电子显微镜研究了软锰矿、钾盐、泻盐/六水合物和石盐的矿物学和形态。证实了光卤石和芒硝制备软锰矿是可行的。双分解反应过程由三个模型方程描述。首先是光卤石在水溶液中分解得到钾盐；第二，通过芒硝和富MgCl2母液的转化反应得到epsomite/六水合物；第三，通过钾盐和泻石/六水合石获得软锰矿。结果表明(1)钾盐存在于初级反应时间，epsomite/hexahydrite同时形成，并且随着反应时间的增加其含量降低。(2)双分解的终产物为软锰矿，其形成是通过钾盐和泻石/六水合石转化而成的。(3)钾盐和泻盐/六水合物的X射线衍射强度随着反应时间的增加而减弱的趋势明显增加，其变化与第三种模型非常吻合。(4) 岩盐、钾盐、泻盐/六水合物、软锰矿的扫描电镜图像清晰完整，它们的形成是由三个模型转化反应的沉淀形成的。结果表明，实验模型和理论模型之间存在良好的一致性。