Europhys. Lett.
Volume 67, Number 2, July 2004
Page(s) 212 - 218
Section Interdisciplinary physics and related areas of science and technology
Published online 01 July 2004
Europhys. Lett., 67 (2) , pp. 212-218 (2004)
DOI: 10.1209/epl/i2003-10286-8

Coordination changes in magnesium silicate glasses

M. C. Wilding1, C. J. Benmore2, J. A. Tangeman3 and S. Sampath2, 4

1  University of California at Davis, Department of Geology Davis, CA 95616, USA
2  Argonne National Laboratory, Intense Pulsed Neutron Source Argonne, IL 60439, USA
3  Containerless Research Inc. - 906 University Place, Evanston, IL 60201, USA
4  University of Wyoming, Department of Chemistry - Laramie, WY 82071, USA

(Received 12 November 2003; accepted in final form 12 May 2004)

Glasses made from the magnesium silicate minerals enstatite ( $\chem{MgSiO_3}$) and forsterite ( $\chem{Mg_2SiO_4}$) and three intermediate compositions can be considered as analogues of quenched melts from the Earth and Lunar mantle. Combined neutron and X-ray diffraction data show an abrupt change in glass structure in the narrow compositional range 38% $\chem{SiO_2}$ to 33% $\chem{SiO_2}$ ( $\chem{Mg_2SiO_4}$). These structural changes reflect a change from a glass characterized by corner-shared $\chem{SiO_4}$ tetrahedra and an approximately equal mixture of $\chem{MgO_4}$ and $\chem{MgO_5}$ polyhedra, to one in which the average coordination of magnesium by oxygen is increased from $4.5
\pm 0.1$ to $5.0\pm 0.1$. Both these local environments are very different from that of their crystalline counterparts. The change in structure is associated with a discontinuous change in the rheological properties of these glass-forming liquids close to the forsterite composition.

61.10.Nz - X-ray diffraction.
61.12.Ld - Neutron diffraction.
61.43.Fs - Glasses.

© EDP Sciences 2004