SYNTHESIZING DIOPSIDE-CATS CLINOPYROXENES IN INTRODUCTORY MINERALOGY LAB

We incorporate experimental investigations in our undergraduate mineralogy class. Recently, we have had students synthesize diopside-CaTs clinopyroxenes. The most important part of these investigations is the process, not the knowledge content. Still, this project greatly helps students understand the nature of solid solutions, the relationship between moles and grams, the effect of atom size on crystal structures, the basics of X-ray diffraction, and the nature of experimental science.

We break the project into small parts, carried out over a 6-8 week period. Initially students fire pure reagents at high temperature, compare X-ray patterns before and after, and determine weight loss. They compare predictions with actual experimental results. Each student is then assigned a clinopyroxene composition and must mix the appropriate amount of reagents so that after firing the mix will have the correct stoichiometry. They weigh and X-ray the mix before and after firing, and again compare predictions with results. They then make a pellet and let it react at high temperature for a week. If X-raying reveals they made a clinopyroxene, diffraction angles are measured and used to determine unit cell parameters.

Student works on their own compositions, so there are no “free rides.” Yet, they can apply logic to compare results. For example, weight loss on firing varies with composition. So, students plot all their weight loss figures together and easily see if someone is way off track. Similarly, once done with their experiments they compare unit cell parameters for the various compositions.

These experiments are not trivial. Often, students have to start over to get reasonable results. Some never get the targeted results. The underlying cause of most “failures” is sloppy lab technique, but simple math errors, contamination, mixing up samples, and many other problems come up. Still, even when experiments fail, they are a success because students learn a great deal.

Many students have trouble because the project is not “cook book.” They must make many decisions and may become uncomfortable when they don’t get expected results. Their abilities to be innovative and confront problems are not well developed. Surprisingly, despite the difficulties, after a decade of doing these experiments, we have yet to have a student go to the library to find “the correct result” or to see how others “succeeded.”