Zeb Page

Oberlin College

This exercise is designed for an intermediate-level undergraduate mineralogy course.

Students should have an understanding of crystal chemistry, crystal systems, and solid solution in minerals.

I use this activity late in the course and tie it to a previous optical mineralogy lab, but it could also be done without any prior optics work.

Collect X-ray diffraction data
Use X-ray data to identify minerals and determine unit cell parameters

Use experimental data to identify unknown samples and derive fundamental physical properties.
Relate unit cell parameters with solid solution.
Synthesize results from different methods on the same sample.
Evaluate strengths, weaknesses, and sources of error from different analytical techniques.

Gain experience with an online mineral database.
Learning to deal with ambiguity.

This is a variant on a traditional mineralogy XRD lab with a few twists:

1. Rather than use "black box" search software or IPCC cards, the lab outlines a procedure for students to identify their patterns using the MSA online database with the following benefits:

a. This allows the flexibility to have students do this work alone or in groups, in lab or on their own time. This can be an advantage at institutions where the XRD is not as accessible for individual student use as we might like.

b. Identifying the powder pattern requires trial and error and thinking about the data, rather than simply putting faith in a search/match routine. Students have to exercise judgement as they proceed.

2. I use samples that students have already characterized by other means. Recently I have chosen samples that students have identified in optical labs. In particular we have a Mn-rich tourmaline sample with no pleochroism that students routinely identify as apatite. By revisiting the same sample in the XRD lab students are able to test and revise hypotheses they have made using other means. This also provides them with an important filter in finding matches for their pattern, and make the job more tractable.

3. Samples should be chosen that are of relatively high symmetry to make estimating unit cell dimensions feasible. Students can use reference books to relate unit cell dimensions to solid solutions. The lab could be extended to use a statistical treatment of the unit cell dimensions determined from different peaks.

Students turn in their indexed X-ray patterns, calculations, and a brief report.