Optimum experimental conditions and accuracy were investigated in quantitative X-ray microanalysis of serpentine-kaolin group minerals (caryopilite, antigorite, kaolinite) and some other sheet silicates (Mg-chlorite, margarite, sericite) by analytical transmission electron microscopy operated in the TEM mode.
Elemental analyses have been carried out at 200 kV on a JEOL JEM-2000FX with a Tracor Northern TN2000 EDX. Experimental conditions were dark current of 103μA, beam current of 4 to 14μA, spot size of 4L (partly 6L which is used at small electron beam of 50nm and 20nm in diameter), condenser aperture of 70μm, and X-ray take off angle of 70°.
The TEM mode gives with wider control over the beam current density on analyzed area, which is controlled by gun bias current (beam current) and 2nd condenser lens current (beam diameter).
The beam damage gives higher Si weight fraction in all studied minerals. The optimum beam current density, which means no change of element compositions, is different in mineral species.
The X-ray absorption effect is observed in serpentines with Mn and Fe. The simple absorption correction method is shown by the linear relationship in a plot of intensity ratio (I
Mn (Fe) /I
Si) as a function of I
Mn (I
Fe). The least-squares method was used to determine the straight line, (I
Mn (Fe) /I
Si)=a+b (I
Mn (Fe) ). Each measured intensity ratio, (I
Mn (Fe) /I
Si) m, is corrected using observed intensity, (I
Mn (Fe) ) m, by (I
Mn (Fe) /I
Si) c=(I
Mn (Fe) /I
Si) m-b (I
Mn) m The accuracy of the weight fraction using the ratio techniques is within the X-ray counting statistic under the optimum beam current density, which is unacceptable levels of beam damage.
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