Crystal Structures as Geobarometers
Summary
In this exercise, designed for undergraduate igneous petrology or advanced mineralogy courses, students download a series of crystal structures from the American Mineralogist Crystal Structure Database to explore the crystal-chemical basis that allows the clinopyroxene crystal structure to be used as a geobarometer.
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Context
Audience
mid to upper level undergraduate petrology course
Skills and concepts that students must have mastered
- students have already had a course in mineralogy
- students are familiar with the crystal structure of clinopyroxene
- students should also know how to use Excel
How the activity is situated in the course
- this activity is a stand-alone exercise
- this activity could supplement class lectures on crystallization of magma, basaltic magmas, alkalic magmatism of continental interiors, or igneous geothermobarometry
Goals
Content/concepts goals for this activity
- use the American Mineralogist Crystal Structure Database to obtain crystal structures of natural clinopyroxenes from a suite of alkalic mafic rocks in Victoria, Australia
- use a crystallographic visualization program (such as CrystalMaker or Xtaldraw) to identify which crystallographic sites contain various elements within clinopyroxene
- use downloaded clinopyroxene compositions and the Excel program CpxBar to determine depths of clinopyroxene crystallization within the samples
Higher order thinking skills goals for this activity
- This exercise requires students to decide which calibration of the clinopyroxene geobarometer is the most appropriate for a suite of samples, based on the whole-rock compositions of the samples.
- Students are also asked to think about the effects of intensive variables (such as T and XH2O) on pressure estimates from the clinopyroxene geobarometer.
Other skills goals for this activity
- use crystallographic and petrology software to solve geologic problems
- use an Excel program to generate data
- write answers to open-ended questions
Description of the activity/assignment
This exercise is based on the recent formulation of a geobarometer based on the crystal structure of clinopyroxene (Nimis, 1995; 1998; 1999). This method relates structural parameters (e.g., the volumes of the unit cell and the M1 polyhedron) to the pressure at which the mineral crystallizes within basic and ultrabasic magmas.
- students are guided into the American Mineralogist Crystal Structure Database to retrieve and download published crystal structure data for viewing in either the CrystalMaker or Xtaldraw visualization software packages.
- students are instructed on how to examine the structures to determine pressure-sensitive crystallographic parameters
- students are then asked a series of questions related to what they learn.
Determining whether students have met the goals
- This activity 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.
Teaching materials and tips
Other Materials
Supporting references/URLs
The problem set can be found at Crystal Structures as Geobarometers. This site provides a tutorial with step by step instructions on how to find, download and render the data to emulate findings reported by Nimis (1995, 1998, and 1999).
- Nimis, P. (1995) A clinopyroxene geobarometer for basaltic systems based on crystal-structure modeling. Contributions to Mineralogy and Petrology, 121, 115-125.
- Nimis, P., and Ulmer, P. (1998) Clinopyroxene geobarometry of magmatic rocks. Part 1. An expanded structural geobarometer for anhydrous and hydrous basic and ultrabasic systems. Contributions to Mineralogy and Petrology, 133, 122-135.
- Nimis, P. (1999) Clinopyroxene geobarometry of magmatic rocks. Part 2. Structural geobarometers for basic to acid, tholeiitic and mildly alkaline magmatic systems. Contributions to Mineralogy and Petrology, 135, 62-74.