Since the beginning of the 19th century the polarized light microscope (PLM) was the instrument of choice for mineral identification. Generations of geologists have used the polarized light microscope in the study of rocks in thin sections and of sediments in grain mounts. In the last decades the use of the polarized light microscope (PLM) has declined sharply with the rise of other analytical techniques (XRD and SEM-EDS). The declining popularity of the PLM has led to a limited offer of courses in optical mineralogy and nowadays only a few professionals remain with a profound knowledge of the instrument.
Despite the declining popularity of the polarized light microscope, it still offers advantages over modern analytical techniques. The main advantages are outlined below:
A polarized light microscope is affordable compared to other analytical instruments like XRD and SEM-EDS. The high purchase price of XRD and SEM-EDS instruments causes the unit price per analysis to go up. Of course this can be overcome by a higher productivity, but in the end most of the time is spend on sample preparation instead of the measurement itself. Moreover the results of the measurements of the XRD and SEM-EDS instruments still need to be interpreted. Interpretation of XRD and SEM-EDS results is not always unambiguous and requires time as well. So when the quantity of samples to be analyzed is not too large, the polarized light microscope could provide a reliable and cheap alternative.
Detection limit of XRD-technique
X-ray powder diffraction (XRD) is a rapid analytical technique for the study of crystal structures. A beam of X-rays is directed towards a crystalline sample and the angles and intensities of diffracted beams are recorded. Based on the produced diffraction pattern the mineral phase can be determined.
The detection limit of the XRD-technique is generally regarded as 1%. Because heavy minerals typically represent only 1% of a bulk sample of SAND, they are hardly detected during a regular XRD-analysis. At the other hand a XRD-analysis on a heavy mineral concentrate might cause so many diffraction peaks to overlap that distinction between different mineral phases becomes a very challenging task. And even in heavy mineral concentrates occur certain trace minerals in low quantities that they won’t be detected because of the detection limit of the XRD-technique.
Minerals of similar chemical composition in SEM EDS-technique
SEM-EDS is an analytical technique where a scanning electron microscope (SEM) is equipped with an energy dispersive spectrometer (EDS). The energy dispersive spectrometer (EDS) detects x-rays emitted from the sample during bombardment by an electron beam. Because each element got its unique electromagnetic emission spectrum, the presence of different elements can be detected. Based on the relative abundance of elements in the emission spectrum, the mineral phases can be determined.
The major drawback of the SEM-EDS technique is the existence of minerals of similar chemical compositions. For instance the common aluminosilicate minerals Kyanite, Sillimanite and Andalusite have a similar chemical composition of Al2SiO5. Furthermore mineral phases of the garnet group, epidote group, pyroxene group and amphibole group have overlapping chemical compositions and are difficult to distinguish too.
Combination of techniques
The best solution is the combination of techniques. For example by polarized light microscopy you can determine easily whether you deal with a amphibole or pyroxene. By using additional information from an energy dispersive spectrometer (EDS), it is possible the determine the mineral phase based on the relative abundance of certain elements. Nevertheless the polarizing light microscope still offers a reliable and cheap solution for a regular heavy mineral analysis.