Coordination changes in magnesium silicate glasses

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: 12 May 2004

Abstract

Glasses made from the magnesium silicate minerals enstatite () and forsterite () 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% to 33% (). These structural changes reflect a change from a glass characterized by corner-shared tetrahedra and an approximately equal mixture of and polyhedra, to one in which the average coordination of magnesium by oxygen is increased from to . 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.