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

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.