ContextAudience: mid-level mineralogy course.
Skills and concepts that students must have mastered: This activity assumes students are familiar with the concepts of coordination number, chemical bonding, and basic symmetry. It may be necessary for you to provide a demonstration to the class, including a brief tutorial for using the software available to the students. We recommend XtalDraw or CrystalMaker if you have PC's, or CrystalMaker if you have Macs. You may also want to point out one or more of the following relationships that can be seen in a given model:
- show how to rotate the crystal in different orientations
- identify the location of close packed layers of oxgyen
- point out similarities among crystallographically-equivalent sites
- note the presence of voids in the structure
- demonstrate and explain the benefits and limitations for employing the various rendering options provided by the visualization software (i.e., polygonal, ball-and-stick, and space-filling).
How the activity is situated in the course: This activity could supplement or take the place of class lectures on Pauling's Rules. This exercise could be run as a lab activity during a single lab session, or split up as smaller modules administered throughout the course to accompany lectures introducing the various mineral groups. Alternatively, this exercise could be assigned as homework for the students to complete at their own pace.
GoalsContent/concepts goals for this activity: Students who complete this exercise should be able to use a crystallographic visualization program such as CrystalMaker or XtalDraw to determine properties of mineral structures, including:
- coordination polyhedra
- element occupancies in various crystallographic sites
- symmetry operations within crystal structures.
Higher order thinking skills goals for this activity: This exercise requires students to apply the abstract concepts of symmetry and coordination to actual crystal structures. Spatial thinking skills are also used to visualize the three-dimensionality of the crystal models with the computer (3-D glasses can be used with CrystalMaker to facilitate spatial learning). At the conclusion of the exercise, the students are required to synthesize their specific observations into a set of general principles governing crystal structures (i.e., Pauling's Rules).
EvaluationThe exercise is formatted as a self-paced exercise where students can check their own answers by clicking on "Show answer" tabs. The exercise could be reformatted as a normal homework assignment without the answers given. If used as an evaluative instrument, outcomes could be any of the following:
- written short answers to the questions
- longer integrative writing assignments
- verbal articulation of answers to peers or instructors
The questions that accompany this exercise could easily be amended to meet your own instructional needs. You may wish to use these questions as models for developing similar exercises for other crystal structures. Longer-term uses of this exercise might be achieved if you refer to this exercise throughout the course in order to make connections between different content areas, for example, when you...
- talk about the determinative properties of minerals
- introduce new mineral groups in hand sample
- introduce the techniques of x-ray diffraction
- explain why different minerals give different powder (or precession) patterns