No.3 August 2004
  
 
  What 's inside?

- Cover Page
- Forward
- Editor and Editional Review Board		    
 
       
 
  BOX 4A EMPA ELECTRON MICROPROBE ANALYSIS (IMA Application)
 
Fig.P15 Example of an Electron Microprobe. University of New Orleans, USA (A.U. Falster).

Explanation of method. The method is used to analyze the chemical composition. For analytical purposes, the analyzed mineral must be coated with an electron-conducting layer and positioned in a highly evacuated vacuum chamber (1). The mineral is bombarded by electrons, which have previously been produced and accelerated in an electron gun (2).The different atoms in the mineral react to the impacting electrons by creating signals. These signals include X-rays, which can be detected by various kinds of detectors, such as WDS (3). From the responding signals, the chemical composition can be determined, providing standard materials (materials of known composition or "Probe Standards") are analyzed for comparison. Light elements such as beryllium or lithium are determined by other methods

ELECTRON MICROPROBE WDS 20 kV 10 nA beam : 5 Fm
NOTE: H2O was determined by LOI and a value of 1.72 wt % was obtained. However, the crystals are riddled with tubules that are occupied by a fluid: the paragenesis (granitic pegmatite) indicates that the fluld will be an aqueous fluid which will therefore result in an anomalously high value of H2O. The crystal-structure refinement gives the total scattering within the channel, and this value minus the scattering from the channel constituents determined by EMPA gives the amount of (H2O) in the channel: ~ 0.1 apfu, 0.28 wt.% H2O, Li by ICP.