Computer Science
Scientific paper
Apr 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994gecoa..58.1711m&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 58, Issue 7, pp.1711-1733
Computer Science
2
Scientific paper
In situ, high-temperature Raman spectroscopy has been used to determine the effect of temperature (from 25 to 1319°C) and bulk Al /( Al + Si ) (from 0.0 to 0.3) on the equilibria that describe the anionic structure of melts on the join Na 2 Si 2 O 5 - Na 2 ( NaAl ) 2 O 5 . The degree of polymerization ( NBO / T , NBO = nonbridging oxygen , T = Al + Si ) of the melts resembles that of molten basalt, and the Al /( Al + Si ) range covers that of most naturally occurring magmatic liquids. In Al-free Na 2 Si 2 O 5 melt, only three structural units coexist and expression 1, T 2 O 5 (2 Q 3 ) TO 3 ( Q 2 ) + TO 2 ( Q 4 ), can be used to describe the equilibrium. When Al 3+ is substituted for Si 4+ in the charge-balanced form ( NaAl ) 4+ / Si 4+ , structural units less polymerized than TO 3 are observed in the glasses and melts. An additional equilibrium (2), 3 TO 3 (3 Q 2 ) T 2 O 7 (2 Q 1 ) + TO 2 ( Q 4 ), is needed to describe the structural relationships in those melts. Aluminum appears to substitute principally for Si 4+ in the fully polymerized structural units. No temperature effect on this preference can be discerned within the sensitivity of the structure probe. Above the glass transition temperature (430-540°C for these compositions), the mol fractions of the structural units, X i , can be described as simple linear functions of temperature with X i / T between 6 and 12.10 -5 °C -1 for i = T 2 O 7 ( Q 1 ) and TO 2 ( Q 4 ), and X i / T in the range -13 to -15 · 10 -5 °C -1 for i = T 2 O 5 ( Q 3 ). Whereas ( X i / T ) for i = TO 3 ( Q 2 ) is 9· 10 -5 °C -1 for sodium disilicate melt, this slope is negative ( X i / T = -3.0 to -18· 10 -5 ° C -1 ) for Al-bearing melts in the A 1/( A 1 + Si ) = 0.05-0.3 range . The X i / T generally increases slightly the more aluminous the melt. Above the glass transition temperature, equilibrium 1 shifts to the right with temperature in Al-free compositions with an enthalpy of reaction (assuming ideal mixing) among the coexisting structural units, H x 1 = 16.7 ± 0.7 kJ / mol . Substitution of Al 3+ for Si 4+ results in H x 1 in the range -11.5 ± 0.6 to -14.8 ± 0.5 kJ/mol. This change results from formation of T 2 O 7 (Q 1 ) units together with TO 2 (Q 4 ) at the expense of TO 3 (Q 2 ) (reaction 2). The x 2 is, therefore, positive, with H x 2 = 23.2 ± 2.8-37.7 ± 2.9 kJ / mol with a general positive correlation between H x 2 -value and A 1/( A 1 + Si )].
Frantz John D.
Mysen Bjørn O.
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