Using High-Precision Specific Gravity Measurements to Study Minerals in Undergraduate Geoscience Courses

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ABSTRACT

This article describes ways to incorporate high-precision measurements of the specific gravities of minerals into undergraduate courses in mineralogy and physical geology. Most traditional undergraduate laboratory methods of measuring specific gravity are suitable only for unusually large samples, which severely limits their usefulness for student projects involving minerals in ordinary rocks of the sort usually encountered by working geologists. To overcome this limitation, a custom-built apparatus is described that, when combined with a precision analytical balance of the type commonly present in academic research laboratories, can be used to determine the specific gravities of samples as small as several milligrams. For a balance with precision to 0.01 mg, G can typically be measured with an accuracy of ±0.01 or better for specimens weighing several tens of milligrams and ±0.03 or better for specimens as small as 5-10 milligrams. The apparatus is easy to make and easy to use. It provides students with a simple and effective way to use quantitative methods to characterize and identify minerals in hand specimen, including small single crystals separated from common medium-grained rocks.

INTRODUCTION

Mineral characterization and identification in hand specimen have long been important parts of laboratory exercises in mineralogy and physical geology. Hands-on activities involving the careful examination and identification of minerals can help students develop observational, analytical, and critical skills as they familiarize themselves with common rock-forming and ore minerals (e.g. Moecher, 2004; Hollocher, 2008). Learning and experimenting with the relevant analytical methods simultaneously helps students hone their practical skills and critical thinking abilities (Wulff, 2004). Examples of specific classroom and laboratory exercises for undergraduates, culled from an NSF-sponsored workshop on "Teaching Mineralogy", are provided by Brady et al. (1997) and have been compiled on a website (Science Education Resource Center: Teaching Mineralogy, 2009).

In mineralogy and physical geology classes, hand specimen mineralogy typically comes very early in the term and helps to establish the tone of the course (e.g. Dyar et al., 2004; Swope and Gieré, 2004; Wirth, 2007). Standard techniques of hand specimen characterization and identification are detailed in practically all mineralogy textbooks and are also described briefly in most introductory physical geology texts (e.g. Marshak, 2008; Tarbuck and Lutgens, 2008). Most of the techniques are essentially qualitative, dealing with properties such as form habit, color, streak, luster, cleavage, fracture, and hardness. This early emphasis on qualitative methods is entirely reasonable but may contribute, unfortunately, to the widespread perception of geoscience as being "remedial science" rather than the highly quantitative field in which modern geoscientists actually work (Manduca et al, 2008). In this context, measurement of specific gravities of minerals provides a special opportunity to emphasize quantitative approaches early in the curriculum. Specific gravity (G) is a quantifiable but intuitively simple property that can be used to characterize and identify minerals in introductory geology courses for non-science students as well as in mineralogy courses for geoscience majors. If sufficiently accurate and precise, specific gravity measurements can be used to estimate the chemical compositions of simple binary solid solution minerals such as olivines, orthopyroxenes, and plagioclase feldspars. This compositional information provides an opportunity to create a bridge between mineralogy and the fields of petrology and geochemistry. Such an emphasis on quantitative skills, and on the resulting links that can be established across the geosciences curriculum (e.g. Nelson and Corbett, 2000), contributes to the general goal of preparing students to deal thoughtfully with quantitative issues and problems in all fields of academics as well as in the world outside the classroom (Science Education Resource Center: Teaching Quantitative Skills in the Geosciences, 2008). …