Physics
Scientific paper
Jan 1980
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1980gecoa..44..109t&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 44, Issue 1, pp.109-117
Physics
9
Scientific paper
The distribution of interatomic distances in amorphous NaAlSi 3 O 8 has been determined at 805°C by X-ray radial distribution analysis to investigate structural differences between the glass (T < 763°C) and the supercooled liquid (763°C < T < 1118°C). Except for slight differences attributable to thermal expansion, no significant changes were observed. The sample crystallized during the course of the experiment, but at least one crystal-free data set was obtained. The transition from the inferred six-membered ring structure of the supercooled liquid to the four-membered ring structure of the crystal was clearly visible in radial distribution function (RDF's) determined before and after crystallization. RDF's were also determined at 25°C for two NaAlSi 3 O 8 glasses with different histories. The first was derived from a melt that had been cooled slowly from 1600 to 32°C above the melting point ( T f = 1118° C ) to detect possible repolymerization to a more `crystal-like' structure as the melt approached T f . The second glass was prepared by holding a single crystal of Amelia albite at 50°C above T f to see if the crystalline four-membered ring structure was preserved in melts at temperatures just above the liquidus. No significant differences were observed between these two RDF's and one obtained from a glass quenched from 1800°C. These results suggest that in addition to the destruction of formation of a periodic structure, melting and crystallization in NaAlSi 3 O 8 also involves a repolymerization of tetrahedra. This would explain the observed kinetic barrier to melting and crystallization in the anhydrous system and the catalytic effect of small amounts of water or alkali oxide.
Brown Gordon E. Jr.
Fenn Philip M.
Taylor Mark
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