Optical Mineralogy in a Modern Earth Sciences Curriculum

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ABSTRACT

Optical mineralogy is a subject firmly integrated into geoscience programs that offer mineralogy and petrology modules. Polarized-light microscopy remains a powerful and cost-effective analytical method, both at the educational and the professional level. It is the ideal analytical tool for the teaching laboratory. Virtually any petrographic work that does not specifically require electron-microscope-scale analysis involves an optical microscope, whether in conjunction with other analytical equipment, or not. However, changes in the perspectives of geoscience education and the necessity to accommodate students with interdisciplinary interests alongside those who opt for a classic geology degree create a need for an optical mineralogy course that is concise, but still meets the demands of subsequent course modules that build on it. There is a range of resources that we can make use of to maintain reasonably high levels of theoretical and practical skills in polarized-light microscopy, such as application-focused lab materials and practice-oriented teaching with a strong interactive component, as well as computer-based teaching aids.

INTRODUCTION

Polarized-light microscopy is of potential interest to any science that is concerned with crystalline materials (geology, mineralogy, materials science, biology, forensic science, to name the most obvious ones). It is traditionally taught as a mineralogy module, even though optical crystallography makes no distinction between natural minerals and synthetic crystalline materials. This article emphasizes geoscience aspects, simply because that is still the main field of application of polarized-light microscopy. Bloss (1999) aptly outlines the significance of mineral optics for geoscientists with his statement "The polarizing light microscope remains the premier tool for rapidly identifying the minerals and mineral reactions that occur in petrographic thin sections of rocks". However, it must be kept in mind that the use of this analytical tool is by no means restricted to petrography, or even geoscience.

What organisms are for biology, what chemical elements and their compounds are for the chemist, earth materials are for geoscientists (if we, for the purpose of clarity, restrict the term "geoscience" or "earth science" to subjects concerned with the solid earth, including unconsolidated sediments). There is a clear and indisputable demand on geoscientists to have a fundamental understanding of earth materials, irrespective of one's preference for basic research or for applied aspects of geoscience. The fact that earth materials, with few exceptions (such as melts, fluids, glasses, and organic substance), are composed of minerals underlines the significance of mineral science education for any aspiring geoscientist. The characterization of rocks and minerals remains a basic objective of geoscience education.

We have means to identify minerals on the basis of chemical composition (e.g., electron microprobe), or structure (e.g., X-ray diffraction), or both. Optical mineralogy employs specific physical properties that reflect both composition and structure. These are optical properties in the strict sense (refractive indices, color, birefringence, optic class, optic sign, optic axial angle), but also morphological-structural characteristics (form, habit, cleavage, twinning) and the relation between the two (sign of elongation, extinction behavior). I will restrict myself here to the discussion of transmitted-light microscopy, even though much of what is stated would apply to reflected-light microscopy as well. However, reflected-light microscopy is a more specialized subject commonly taught in conjunction with ore deposits, and is not necessarily part of a standard geoscience education program.

OPTICAL MINERALOGY: STAPLE DIET OR LUXURY SIDE DISH?

For many decades, optical mineralogy has been a core subject in most earth sciences departments. …