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Evidence for an I2/a to Imab phase transition in the silica polymorph moganite at ~570 K

¹ Department of Geosciences, 309 Deike, Penn State University, University Park, Pennsylania 16802, U.S.A.
² Department of Mineral Sciences, National Museum of Natural History, Smithsonian Institution, Washington, D.C. 20560-0119, U.S.A.

Correspondence: ^* E-mail: heaney{at}geosc.psu.edu

Rietveld analysis of synchrotron X-ray powder diffraction data for the silica mineral moganite from 100 K to 1354 K has revealed a reversible phase transition from space group I2/a to Imab at approximately 570 K. The thermal expansion behavior of the lattice parameters alters sharply at the transition point, and the monoclinic ß angle decreases to 90°. The displacive transition from - to ß-moganite is effected by the rotation of apparently rigid tetrahedra about the [010] axis, and the linear temperature dependence of the volume strain and of the non-symmetry-breaking e₁₁ and e₂₂ strains indicates that the character of the transition is second-order. The continuous increase in the b axis over the entire temperature range reveals a concomitant rotation of tetrahedra about [100] that does not affect the overall symmetry. In addition, we present a refinement without structural constraints for -moganite at room temperature using time-of-flight neutron diffraction data.

This article has been cited by other articles:

				P. J. Heaney, D. A. McKeown, and J. E. Post Anomalous behavior at the I2/a to Imab phase transition in SiO2-moganite: An analysis using hard-mode Raman spectroscopy American Mineralogist, April 1, 2007; 92(4): 631 - 639. [Abstract] [Full Text] [PDF]

				T. MOXON and S. RIOS Moganite and water content as a function of age in agate: an XRD and thermogravimetric study European Journal of Mineralogy, April 1, 2004; 16(2): 269 - 278. [Abstract] [Full Text] [PDF]