Na元素的EPMA定量分析:矿物晶体结构对Na行为的制约

Quantitative Analysis of Na by EPMA:Constraints for the Behavior of Na by the Crystal Structure

Na属于热扩散元素,具有不稳定性,准确测定有难度。前人对含Na玻璃和具有不同晶体结构的含Na矿物的电子探针定量分析发现,随着分析时间的延长,Na的计数明显下降,并且含Na玻璃计数的下降要比含Na矿物相比更为明显,表明Na计数与试样的结构有关,但对其内在控制因素还不太明确。文章在前人研究的基础上,选择具有代表性不同结构含Na硅酸盐矿物和非晶质含Na玻璃作为研究对象,通过对不同晶体结构中Na元素所处的位置、配位数、键性类型的特征进行总结,针对不同晶体结构中Na原子扩散条件和是否存在附加阴离子与之相伴等因素,对比分析不同晶体结构中Na计数下降的程度,深入探讨了导致Na计数下降的内在原因。结果显示,试样分析过程中Na计数下降顺序为:非晶质>架状>层状>环状>单链状>双链,且有附加阴离子的含Na矿物要大于无附加阴离子的矿物。通过对不同测试条件下Na计数的对比分析发现,加速电压、束流、束斑直径等改变,都会对Na计数产生一定的影响。最终总结得出了准确测定含Na硅酸盐矿物的最佳分析条件,为准确测定含Na矿物和玻璃中的Na含量提供参考。

In the quantitative analysis of electron probe microanalysis,Na belongs to the unstable element(heating diffusion element),so it is difficult to measure accurately.Many works have been done on the problem,for example,quantitative analysis of Na-bearing glasses and Na-bearing minerals with different crystal structures by electron microprobe shows that the Na count decreased significantly with the extension of analysis time,and it is recognized that the decrease in the number of Na-bearing glasses is more obvious than that of Na-bearing minerals,showing that the decrease of Na count is related to the crystal structure of the sample,but the internal control factors are not clear.Based on previous studies,the typical Na-bearing minerals are studied with different structures:frame type,ring type,chain type,layered type and amorphous type(Na glass).By analyzing the position,coordination number,bond type and characteristics of Na atom in different crystal structures,as well as diffusion conditions of Na atoms in different crystal structures and additional accompanying anions,the internal reasons for the decrease of Na count were discussed,and the decreasing degree of Na count in different crystal structures was compared and analyzed.The descending order of Na counts of Na-bearing minerals with different crystal structures is as follows:noncrystalline>fram layered﹥circular﹥single chain﹥double chain,with additional anion﹥no accessory anion,and the Na-bearing minerals with additional anions are larger than those without accessory anions.Through the comparative analysis of Na count under different test conditions,it is found that the change of accelerating voltage,beam current and beam spot diameter will have a certain influence on Na counting.Combining the principle of Na count decrease of Na-bearing minerals with different crystal structures,the optimum analytical conditions for accurate determination of Na bearing silicate minerals are summarized,that provides a reference for the accurate determination of Na content in Na bearing minerals.