STRUCTURE OF HAPLOBASALTIC LIQUIDS AT MAGMATIC TEMPERATURES - IN-SITU, HIGH-TEMPERATURE STUDY OF MELTS ON THE JOIN NA2SI2O5-NA2(NAAL)2O5

In situ, high-temperature Raman spectroscopy has been used to determin e the effect of temperature (from 25 to 1319-degrees-C) and bulk Al/(A l + 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 p olymerization (NBO/T, NBO = nonbridging oxygen, T = Al + Si) of the me lts resembles that of molten basalt, and the Al/(Al + Si) range covers that of most naturally occurring magmatic liquids. In Al-free Na2Si2O 5 melt, only three structural units coexist and expression 1, T2O5(2Q3 ) double arrow line pointing left and right TO3(Q2) + TO2(Q4), can be used to describe the equilibrium. When Al3+ is substituted for Si4+ in the charge-balanced form (NaAl)4+ double arrow line pointing left and right Si4+, structural units less polymerized than TO3 are observed i n the glasses and melts. An additional equilibrium (2), 3TO3(3Q2) doub le arrow line pointing left and right T2O7(2Q1) + TO2(Q4), is needed t o describe the structural relationships in those melts. Aluminum appea rs to substitute principally for Si4+ in the fully polymerized structu ral units. No temperature effect on this preference can be discerned w ithin the sensitivity of the structure probe. Above the glass transiti on temperature (430-540-degrees-C for these compositions), the mol fra ctions of the structural units, X(i), can be described as simple linea r functions of temperature with partial-derivativeX(i)/partial-derivat iveT between 6 and 12.10(-5) degrees-C-1 for i = T2O7(Q1) and TO2(Q4), and partial-derivativeX(i)/partial-derivativeT in the range -13 to -1 5.10(-5) degrees-C-1 for i = T2O5(Q3). Whereas (partial-derivativeX(i) /partial-derivativeT) for i = TO3(Q2) is 9.10(-5) degrees-C-1 for sodi um disilicate melt, this slope is negative (partial-derivativeX(i)/par tial-derivativeT = -3.0 to -18.10(-5) degrees-C-1) for Al-bearing melt s in the Al/(Al + Si) = 0.05-0.3 range. The partial-derivativeX(i)/par tial-derivativeT generally increases slightly the more aluminous the m elt. Above the glass transition temperature, equilibrium 1 shifts to t he right with temperature in Al-free compositions with an enthalpy of reaction (assuming ideal mixing) among the coexisting structural units , DELTAH(x)1 = 16.7 +/- 0.7 kJ/mol. Substitution of Al3+ for Si4+ resu lts in DELTAH(x)1 in the range -11.5 +/- 0.6 to -14.8 +/- 0.5 kJ/mol. This change results from formation of T2O7(Q1) units together with TO2 (Q4) at the expense of TO3(Q2) (reaction 2). The DELTAH(x)2 is, theref ore, positive, with DELTAH(x)2 = 23.2 +/- 2.8-37.7 +/- 2.9 kJ/mol with a general positive correlation between DELTAH(x)2-value and Al/(Al Si)].