Crystal Structures as Geobarometers

Kent Ratajeski

Montana State University

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This page first made public: Aug 7, 2006

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.

Geochemistry, Crystallography | College Lower (13-14), College Upper (15-16)

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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.

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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.

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