Crystal Structures as Geobarometers

Kent Ratajeski
Montana State University
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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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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


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.

More information about assessment tools and techniques.

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.