HR: 1340h
AN: MR13B-1261    [Abstracts]
TI: Glass or Solution Structures can not be Validated by Radial Distribution Analysis
AU: Boonman, K N
AF: Utrecht University, Budapestlaan 4, Utrecht, 3584CD, Netherlands
AU: * de Jong, B H
EM: bernard@geo.uu.nl
AF: Utrecht University, Budapestlaan 4, Utrecht, 3584CD, Netherlands
AB: The manufacture of crystalline phases from first principle calculations has been hampered by uncertainties in describing the nucleation and growth process. For aluminosilicates the principal working hypothesis has been that differences or similarities in molecular moieties between glass or melt and crystalline precipitate as inferred from radial distribution analysis, originally designated the Prins function (Zernike & Prins, 1927, Bernal, 1964) affect the activation energy for nucleation. Thus anorthite composition glass, a fragile system, crystallizes easily presumably due to the presence of 4-membered rings as inferred from radial distribution analysis, whereas albite composition glass, a robust system, not at all due to the presence of 6-membered rings. Unease exists about the precise information content of such analysis as for instance manifested by the long standing discord about the presence of boroxyl rings vis a vis linear boron oxide chains, both of which being compatible with the radial distribution of B2O3 glass. To settle the issue concerning the information content of a radial distribution for amorphous solids or liquids once and for all, we have calculated this function for all known 14 crystalline single and mixed alkali disilicates (R2O-2SiO2). These silicates contain the, by stoichiometry preordained, local environment Q3, i.e. tetrahedra with 3 bridging and one non-bridging oxygen, as is characteristic for sheet silicates. Some of these compounds form sheet silicates with 6-membered rings, others contain 4, 4-8, 4-6-8; 4-8-12, or 14-membered rings occurring either as sheets, tubes, or 3-D networks. We calculated total and partial pair distributions eliminating the alkalis in the latter to enhance details of the silica moieties. Our results show that no ring system assignment could be made to any of the calculated PDF's, nor that the large variety of ring systems in the different phases could be distinguished from one another. Validating Molecular Dynamic calculations using as criterion the quality of fit with observed radial distributions for glasses or aqueous solutions, is therefore incompatible with the information content available in such analysis. Thus structural assessments of extended moieties in melts and aqueous solutions remain elusive.
DE: 3954 X-ray, neutron, and electron spectroscopy and diffraction
SC: Mineral and Rock Physics [MR]
MN: 2007 Fall Meeting