Do you see PPM level differences in mineral chemistry

By Paul Mainwaring, Gatan

INTRODUCTION

Cathodoluminescence (CL) occurs when an electron beam strikes a material and causes it to luminesce. During the process of electron bombardment, electron-hole pairs are created and their radiative recombinations give rise to the observed emission in an attempt to relax the increased energy imparted to the specimen. This emission is commonly observed in the light optical system of the electron microprobe during the chemical analysis of minerals but rarely ever seen in the SEM due to the lack of suitable optics. Although not always seen or noticed due to low emission intensity, the CL signal can be put to important uses in the chemical characterization of minerals. The example below suggests that CL is an indispensable tool for the understanding of the subtle compositional variations that cannot quickly and easily be imaged in any other way in an electron microprobe or SEM.

The scanning electron microscope is an ideal tool in which to demonstrate this effect on small areas of samples. SEM-CL images have been acquired at length scales of less than 0.1 µm to several millimeters in many minerals. Although many non-metallic minerals luminesce when the electron beam strikes them, the intensity and wavelength variation of the emission can be large. It is well known that many mineralogical materials emit at very low intensity levels which cannot be observed with any other technique. However, recent advances in the SEM-CL technique allow very low intensity emission to be observed and recorded. Therefore, low electron beam excitation conditions can be used resulting in an increased spatial resolution.

Variation in the CL emission wavelength from mineral samples can be due to many factors however the emission from a single mineral grain is more constrained.

Figure 1 shows a color CL image of a sample of sandstone in which quartz grains emit in hues of blue, purple and red. Such CL images are now easily and relatively quick to acquire using the ChromaCL - the live, true-color CL imaging system from Gatan. These colors are believed to be generally related to the provenance and thermal history of the various grains as well as factors that affect individual crystals. These intra-crystal factors include variations in properties such as trace element type and level, crystal defects and internal strain.

USE OF CATHODOLUMINESCENCE IN MINERAL COMPOSITION STUDIES

Few studies have been carried out to correlate a quantitative compositional fingerprint of a single mineral grain with the corresponding CL image. One such study was reported by Student et al. (2006) who studied a single quartz phenocryst grain from a volcanic rock in Michigan. Figure 2 shows the highly chemically zoned quartz grain that was analyzed for this work. This gray level image is a display of only the blue CL signal generated by the quartz grain and is called a monochromatic image. An optical light microscope image of this grain would show only a uniform quartz crystal and a backscattered electron image would show a uniform grey quartz grain since the minor element content occurs at such low concentrations. We show the grey level image here so that the line of dots can be more easily distinguished.

The study by Student et al. highlighted the role of the titanium (Ti) trace element content in the color and intensity of the emitted CL signal (see figure 3). The blue color and varying shades of blue in this image are typical of quartz crystals of volcanic origin. Blue fine-grained matrix quartz can also be seen surrounding the phenocryst.

In the CL images there is a line of black dots (seen better in the grey monochromatic image) starting at the upper right hand edge of the grain and extending into the core of the crystal. These dots indicate locations of the microanalytical characterization by electron microprobe analyzer (EMPA).

The table below gives the results of EMPA analysis for Ti dissolved in the quartz structure. As can be seen by the data, there is a direct correspondence of the intensity of the blue CL emission color and the titanium content in this crystal. Clearly the deep blue color of the core of this phenocryst indicates that it is significantly enriched in the titanium cation.

Although it may not always be possible to see such a striking correlation of trace or minor element content in mineral grains, the CL emission wavelength or color gives a very clear and visual map of compositional variations and gradients at the ppm level that cannot be visualized in any other way.

Student, JJ., Wark, D.A., Mutchler, S.R., and Bodnar, R.J., (2006) Pristine rhyolite glass melt inclusions in quartz phenocrysts from the 1.1 Ga Midcontinent Rift System, Keweenaw peninsula, Michigan,
Eos Trans. AGU, 87(52), Fall Meet. Suppl., Abstract V23C-0619.

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