Determination of trace element profiles in coarse-grained minerals by external millibeam PIXE - A schorl tourmaline study

Abstract

The thick-target PIXE technique was applied to direct analysis of trace elements in tourmaline minerals. Analyses of these minerals with conventional wet-chemical methods are difficult and time consuming. The PIXE analyses were carried out in air with a collimated proton beam. As the technique was used in combination with a polarizing microscope it provided excellent control over the spots analyzed. A scanning device was developed for remote-controlled spot analyses across the surface of the samples. Several scans were run across polished sections of multicrystal schorl-tourmaline aggregates. Pellets of crushed and ground tourmaline samples were also analyzed. The method was calibrated against USGS standard reference materials. The precision of the method varied between 1 and 10 % and the detection limits were 1-10 ppm for most of the elements analyzed. Different sets of X-ray absorbers were used to improve the sensitivity and reliability of the method for the elements of interest. The concentrations of fourteen elements were calculated from their X-ray yields and the fluorine content was simultaneously determined using the 109.8 keV gamma line. The tourmaline samples were taken from a complex granite pegmatite on the Kimito island in SW Finland. They were found to contain mainly elements with Z < 33 (lighter than As). Relatively high concentrations of Fe, Zn, Ga and F were observed. Zinc was found to be a geochemically important minor element in schorl with a concentration ranging from 0.10 to 0.28 %. The pellets were found to contain inclusions of other minerals.

Keywords

• granites
• pegmatite
• tourmaline
• schorl
• trace elements
• zinc
• particle induced X-ray emission
• Proterozoic
• Kemiö
• Finland