The Polarized Light Microscope: Should We Teach The Use Of A 19Th Century Instrument In The 21St Century?

The Polarized Light Microscope: Should We Teach the use of a 19th Century Instrument in the 21st Century?
Mickey E. Gunter ABSTRACT
The polarizing light microscope (PLM) has no doubt contributed more to our knowledge of minerals and rocks than any other single instrument. Then why is the use of the PLM, and the teaching of optical mineralogy in general, decreasing? Probably one of the mainreasons is educators seek to present the newest, most technologically advanced techniques and methods to their students. Also, the geoscience curriculum has changed to include environmental geology, essentially hydrogeology. However, in presenting these newer materials, or new courses, we must exclude something, and it appears that one of the things excluded is instruction in use of the PLM. Anotherpossibility is the professors teaching these courses do not have an adequate understanding of optical mineralogy. Excluding the subject of optical microscopy will be the biggest mistake we ever make in the geosciences curriculum. This statement is justified because of the fundamentally important concepts presented in optical mineralogy: 1) three-dimensional visualization, 2) inquiry-basedlearning, and 3) hands-on use of an analytical instrument. No other single course in our curriculum provides so many of these fundamental skills to our students. An example of inquiry-based learning is determining the best technique to identify a mineral. For example, powder X-ray diffraction provides a diffraction pattern searchable in a database, but the result might not be correct. In the past 20-30years, the spindle stage has allowed for more detailed single crystal studies than ever before. Optical techniques are also used to study such diverse mineralogical problems as cation-diffusion in zeolites and asbestos identification, or they may be incorporated into other areas of research, such as synchrotron experiments on oriented single crystals. Also, any serious petrologic study mustbegin with a thorough examination of the samples by PLM, before other types of characterization can proceed. A diversity of employment opportunities exist for students who are trained in use of the PLM, such as in the fields of forensics, material science, manufacturing, the food industry, medical technology, and the emerging field of environmental mineralogy. Department of Geological Sciences,University of Idaho, Moscow, Idaho 83844-3022, mgunter@uidaho.edu computer-based instruments (e.g., X-ray diffractometers and electron microscopes). The overarching theme is that students, and researchers, must understand the limitations of the various instruments and be able to select the most appropriate one for the problem at hand. Also, equally important, we should teach how to integrate severaldifferent analytical methods to answer the question at hand. The PLM is one of the few instruments students will ever get to use independently. This itself warrants its teaching as we strive for more interactive methods of instruction. No other single instrument provides chemical, structural, and morphological information on a single sample and with so little sample preparation, albeit thechemical and structural data are found indirectly. It is this “indirect method” of inferring chemical and structural data that is the double-edged sword for the use of the PLM. Microscopists must infer this information based on their experiences. When data are misinterpreted, it is often (incorrectly) the microscopic techniques and not the microscopist who is blamed. Clearly, the PLM cannot directlydetermine a material’s crystal structure or chemical composition; diffraction methods are commonly used for the former and spectroscopic methods for the latter (e.g., energy dispersive spectroscopy in an electron beam instrument). However, once the structure and composition are known, generally speaking, they can be related to some feature that is observable or measurable with a PLM. For instance,...