Crystal Structures as Geobarometers

Kent Ratajeski, kratajeski@montana.edu
Department of Earth Sciences, Montana State University, Bozeman, MT
Published Jan. 4, 2005

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Wouldn't it be nice to know the depth of crystallization of this clinopyroxene? Image courtesy of Dr. Ruth Siddall, University College of London. Sample shown is a leucitophyre from Mt. Vesuvius, Italy. Details

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.

In this exercise, 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. The students are instructed on how to examine the structures to determine pressure-sensitive crystallographic parameters and are asked a series of questions related to what they learn.

References

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.

This exercise is one of several examples featured in the Teaching Mineralogy with Crystal Structure Databases and Visualization Software module at SERC. Each example is designed to give instructors and students direct practice for using crystal structure databases and visualization programs to teach crystallography and mineralogy in the earth sciences. Students should begin with the Directions page.