Working with Electron Microprobe Data from a High Pressure Experiment - Calculating Mineral Formulas, Unit Cell Content, and Geothermometry
Humboldt State University
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In this problem set students use electron microprobe analyses of a peridotite partial melting experiment to determine mineral formulas, calculate unit cell content, plot results on a classification diagram, and use a geothermometer. The exercise is especially suited for a spreadsheet like Excel, but can be done by hand. The bulk of the exercise involves data manipulation and simple calculations, but also includes some open-ended questions. The exercise is appropriate for mineralogy or petrology courses. The problem set helps students gain experience with a spreadsheet, manipulate and use quantitative chemical data, investigate the structure of mineral formulas, and see an application of phase equilibria in petrology.
This exercise is designed for a sophomore to junior level required course in petrology.
Skills and concepts that students must have mastered
Students should know how to use Excel and have a basic understanding of mineralogy (e.g. unit cells) and chemistry.
How the activity is situated in the course
This is a stand-alone exercise.
Content/concepts goals for this activity
The problem set helps students gain experience with a spreadsheet, manipulate and use quantitative chemical data, investigate the structure of mineral formulas, and see an application of phase equilibria in petrology.
Higher order thinking skills goals for this activity
This exercise involves analyzing data.
Other skills goals for this activity
This problem set involves writing and may involve working with Excel.
Description of the activity/assignment
This exercise provides students the opportunity to work with real microprobe data to perform a series of common calculations. It also provides a brief glimpse into a high-pressure experiment. (I hope to expand this opportunity in the future via web activities...stay tuned.) The exercise can be used as a laboratory activity or a problem set. It is ideally suited for the use of a spreadsheet like Excel, but can be completed by hand. This is a great opportunity for students who are unfamiliar with spreadsheets to get their feet wet. For me, trial by fire is the best way to learn a new software program.
The exercise could be used in any undergraduate petrology or mineralogy course and assumes only a general background in mineral chemistry. The goals are for students to: 1) work with real data from an experiment, 2) learn/remind themselves of the relationship between chemistry and
crystal structure as displayed in mineral formula, 3) use a geothermometer to see how phase equilibria can be used to decipher physical properties of rocks.
The exercises include:
- Mineral formula recalculation
- Unit cell content calculation
- Calculating end-member percentage
- Plotting data on a ternary plot
- Geothermometer calculation
The exercise could easily be modified to include other "pet" analyses or questions.
Determining whether students have met the goals
Students have met the goals of this exercise if they answer the problem set completely and accurately.
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