Teaching Mineralogy with Crystal Structure Databases and Visualization Software
Integrating Research and Education > Crystallography > Clinopyroxene Phase Transition

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

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

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