An Examination of the Phase Transformation in Clinopyroxene

Robert Downs, University of Arizona

Introduction

There are several phases of pyroxene that display different symmetries. The most important include orthopyroxene, and clinopyroxene. In this exercise, we will examine the structures of 3 different clinopyroxene crystals as a function of cell volume in order to understand the mechanism behind their phase changes. Arlt and Armbruster (1997) and Arlt et al, (1998) reported the structure of kanoite, ideally MnMgSi2O6, as a function of pressure and temperature. We will make an animation that follows the volume changes of this pyroxene.

Information for instructors

Please see the Teaching Notes page.

Directions

  • Go to the American Mineralogist Crystal Structure Database, search for kanoite in the mineral field and Arlt in the author field. You should retrieve 4 datasets. Chose the one at high pressure, ambient conditions and high temperature. You should notice that the structures at P and T display C2/c symmetry, while the one at room conditions displays P21/c.
  • Download the files into XtalDraw. And hit alt-g to display polygons.
  • We are aiming for a certain view, shown below

  • This is obtained by expanding the structure by hitting alt-1, choosing lattice, enter a value of 1 to expand the view to +/- 1 unit cell in all direction, then hit OK.

  • Hit alt-f and alt-a to set a view down the facepole a*, and then hit alt-up arrow to rotate the image by 90°.

  • Now we will select a portion of the view by holding the right-button of the mouse down and dragging a box and clicking on crop. This feature deletes all atoms outside of the box, and keeps the ones that are inside the box. Note that the cut selection deletes the contents of the box.

  • The results of cropping this selection should look like the following,

  • Hit alt-down arrow to rotate 90° down and crop another selection



    to get the following section of the structure.
  • The M2 site is the red octahedron in the middle, right click on it and turn it into a sphere, I used a radius of 0.6

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