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 Å and a bonding radius of 2.7 Å. Change the center of the image by right-clicking on the M2 sphere, and then hitting center, to place M2 at the middle of the window.
• Choose a screen size, say 15 Å, by hitting ctrl-S and entering 15 in the edit box.
• We are now ready for our first frame in the animation. Hit the bitmap menu, then movie, then make movie. Save the movie file in a suitable directory. We use a unique directory for each animation. Hit the frames button and append the current image.
• Hit file, then next, to get the next dataset, the P21/c dataset. Follow a similar sequence of steps, noting that it has a different origin.
  
  
  
• Click on the movie dialog box and append this frame. You can hit OK and play to see what you have done so far.
• Click on the XtalDraw window to put it back into focus, and get to the next dataset (ctrl-N). Repeat the steps to make a 3-frame movie that you play in ping-pong mode.
  
  

You should observe that the M2 atom changes its coordination by breaking bonds with O3 and forming new ones. When the M2 is 6 coordinated then a two-fold axis of symmetry passes through the M2 atom and the structure displays C2/c symmetry. When the structure expands or shrinks, the tetrahedra in the chain rotate and the bonding changes around M2 to 5 coordinated. The 2-fold rotation axis disappears and the symmetry changes. It is easy to imagine that a change in the chemistry of the M2 site that involves a change in atomic size can also lead to this symmetry change. This was the focus of the Arlt et al (1998) study. Downs (2003) laid out the systematics of the different symmetries of pyroxene and related them to the bonding around M2.

Optional Assignment

Investigate the phase transition between orthopyroxenes (enstatite or ferrosilite) and the clinopyroxene phases using the same procedure outlined above for the 3 clinopyroxene phases. You can find the appropriate data at the American Mineralogist Crystal Structure Database and construct some images and animated gif's and come up with a model. Incidently, this phase transtion occurs at about 225 km depth in the Earth's mantle, and appears to be responsible for one of the minor discontinuities in seismic wave velocity. Good luck!

References

Arlt T, Angel R J, Miletich R, Armbruster T, Peters T (1998) High-pressure P21/c-C2/c phase transitions in clinopyroxenes: Influence of cation size and electronic structure. American Mineralogist 83, 1176-1181

Arlt T and Armbruster T (1997) The temperature-dependent P2_1/c - C2/c phase transition in the clinopyroxene kanoite MnMg[Si₂O₆]: a single-crystal X-ray and optical study. European Journal of Mineralogy 9, 953-964

Downs, RT (2003) Topology of the pyroxenes as a function of temperature, pressure, and composition as determined from the procrystal electron density. American Mineralogist 88, 556-566