Abstract

The equilibrium partitioning of Na and K between alkali feldspar and NaCl–KCl salt melt was determined at 800 \(^\circ\) C, 850 \(^\circ\) C, 900 \(^\circ\) C, 950 \(^\circ\) C and 1000 \(^\circ\) C and close to ambient pressure. Four different natural gem-quality alkali feldspars with low degree of Al–Si ordering covering the range from orthoclase to high sanidine and with slightly different minor element concentrations were used as starting materials. The partitioning curves obtained for the four feldspars are indistinguishable indicating that Na–K partitioning independent of the differences of Al–Si ordering state and minor element concentrations existing amongst these feldspars. A sub-regular two parameter Margules type solution model was fitted to the partitioning data, and the excess Gibbs energy describing the thermodynamic non-ideality of the alkali feldspar solid-solution and the respective Margules parameters \(W_{\text {g}\text {K}}\) and \(W_{\text {g}\text {Na}}\) including their temperature dependence expressed as \(W_g=W_h-TW_s\) were determined: $$\begin{aligned} W_{\text {g}\text {K}}&= 19754 \pm 3140 J\cdot \,\,{\hbox {mol}}\,\,^{-1} - T \cdot 2.33 \pm 2.67 J\cdot \,\,{\hbox {mol}}\,\,^{-1}\cdot K^{-1} \\ W_{\text {g}\text {Na}}&= 14916 \pm 4272 J\cdot \,\,{\hbox {mol}}\,\,^{-1} - T \cdot 3.55 \pm 3.64 J\cdot {\hbox {mol}}\,\,^{-1}\cdot K^{-1} \\ \end{aligned}$$ The corresponding solvus has a critical temperature slightly above 650 \(^\circ\) C and is well comparable with earlier direct experimental determinations of the low-sanidine-albite solvus curve. Comparison of the vibrational excess entropy determined from low-temperature heat capacity measurements with the total excess entropy derived from the temperature dependence of the excess Gibbs energy yields a negative configurational contribution to the excess entropy pointing towards short-range Na–K ordering on the alkali site.

中文翻译：

---

Na-K分配和低温量热法无序碱长石固溶体的热力学混合特性

摘要

在 800 \(^\circ\) C、850 \(^\circ\) C、900 \(^\circ\) C、950 C下测定了碱长石和 NaCl-KCl 盐熔体之间 Na 和 K 的平衡分配\(^\circ\) C 和 1000 \(^\circ\) C 且接近环境压力。使用四种不同的天然宝石级碱长石作为起始材料，这些碱长石具有低 Al-Si 有序度，涵盖从正长石到高透长石的范围，并且微量元素浓度略有不同。四种长石获得的分配曲线无法区分，表明 Na-K 分配与这些长石中存在的 Al-Si 有序状态和微量元素浓度的差异无关。对分区数据拟合了次正则二参数Margules型解模型，描述碱长石固溶体热力学非理想性的过剩吉布斯能量和各自的Margules参数\(W_{\text {g}\文本 {K}}\)和\(W_{\text {g}\text {Na}}\)包括其温度依赖性，表示为\(W_g=W_h-TW_s\)已确定：$$\begin{aligned} W_{\text {g}\text {K}}&= 19754 \pm 3140 J\cdot \,\,{\hbox {mol}}\,\,^{-1} - T \cdot 2.33 \pm 2.67 J\cdot \,\,{\hbox {mol}}\,\,^{-1}\cdot K^{-1} \\ W_{\text {g}\text {Na}}&= 14916 \ pm 4272 J\cdot \,\,{\hbox {mol}}\,\,^{-1} - T \cdot 3.55 \pm 3.64 J\cdot {\hbox {mol}}\,\,^{- 1}\cdot K^{-1} \\ \end{aligned}$$相应的溶线具有略高于 650 \(^\circ\) C 的临界温度，并且与早期直接实验测定的低-钠长石-钠长石固溶曲线。将低温热容测量确定的振动过剩熵与过剩吉布斯能量的温度依赖性得出的总过剩熵进行比较，得出对过剩熵的负构型贡献，指向碱金属上的短程 Na-K 有序化地点。