Abstract:

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.

Key words: Electron Probe Micro Analysis, Na-bearing minerals, crystal structures, decrease of Na counts, high voltage electron
beam, analysis conductions