Olivine Crystal Structure

Kent Ratajeski

Montana State University

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Initial Publication Date: August 7, 2006 | Reviewed: December 14, 2014 (see revision history: 3 events)

Summary

In this exercise, students use the American Mineralogist Crystal Structure Database and a crystallographic visualization program such as Xtaldraw or CrystalMaker to study the temperature-dependent ordering of Mg and Fe in the M1 and M2 sites of olivine.

Crystallography, Crystal Chemistry, Island Silicates | College Lower (13-14), College Upper (15-16)

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Context

Audience

mid-level mineralogy course

Skills and concepts that students must have mastered

students should be familiar with the olivine crystal structure
students should be familiar with the concept of order/disorder within crystals
students should know how to use Excel to make x-y graphs

How the activity is situated in the course

this activity could supplement class lectures on order/disorder
this activity could supplement class lectures on solid solutions
this activity could supplement class lectures on solid solutions nesosilicate mineral structures

Goals

Content/concepts goals for this activity

students should be able to use the American Mineralogist Crystal Structure Database to obtain crystal structures of synthetic olivines
students should be able to use a crystallographic visualization program (such as CrystalMaker or Xtaldraw) to identify which elements are housed in which crystallographic sites in the olivine structure
students should be able to use downloaded olivine compositions and Excel to determine the ordering of M1 and M2 cations within olivine

Higher order thinking skills goals for this activity

application of the abstract concept of crystallographic order to a dataset in which the amount of order within two cation sites is temperature-dependent
recognition of the relationship between crystallographic order and entropy

Other skills goals for this activity

using crystallographic software to solve geologic problems
using an Excel program to plot data
writing answers to open-ended questions

Description of the activity/assignment

This exercise is based on recent crystallographic research on the olivine crystal structure published by Redfern et al. (2000). The authors of this study synthesized Fa50 olivine olivine (MgFeSiO4) in an experimental apparatus at temperatures ranging from 100 to 1250 °C, quenched the experiments, and used in situ neutron powder diffraction techniques to investigate changes in the synthesized olivines as a function of temperature.
While this study reports cutting-edge materials research carried out with the latest crystallographic techniques, the results are educationally instructive and illustrate important concepts normally covered in an undergraduate mineralogy course.

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 manipulate the structures and are asked to plot some of the crystallographic data from this study on graphs using a spreadsheet program such as Excel.

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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