Structure of haplobasaltic liquids at magmatic temperatures: In situ, high-temperature study of melts on the join Na2Si2O 5-Na 2(NaAl) 2O 5
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 Na2Si2O5- Na2( NaAl) 2O5. The degree of polymerization ( NBO/ T, NBO = nonbridgingoxygen, 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 Na2Si2 O5 melt, only three structural units coexist and expression 1, T2O5(2 Q3) ⇔ TO3( Q2) + TO2( Q4), 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 TO3(3 Q2) ⇔ T2O7(2 Q1) + TO2( Q4), 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, Xi, can be described as simple linear functions of temperature with ∂Xi/ ∂T between 6 and 12.10 -5 °C -1 for i = T2O7( Q1) andTO2( Q4), and ∂X i/∂T in the range -13 to -15 · 10 -5 °C -1 for i = T2O5( Q3). Whereas ( ∂X i/∂T) for i = TO3( Q2) 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 A1/(A1 + 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, ∆Hx1 = 16.7 ± 0.7 kJ/ mol. Substitution of Al 3+ for Si 4+ results in ∆Hx1 in the range -11.5 ± 0.6 to -14.8 ± 0.5 kJ/mol. This change results from formation of T 2O 7(Q 1) units together with TO 2(Q 4) at the expense of TO 3(Q 2) (reaction 2). The ∆x2 is, therefore, positive, with ∆Hx2 = 23.2 ± 2.8-37.7 ± 2.9 kJ/ mol with a general positive correlation between ∆Hx2-value and A1/(A1 + Si)].