Haploandesitic melts at magmatic temperatures: In situ, high-temperature structure and properties of melts along the join K2Si4O9-K-2(KAl)(4)O-9 to 1236 degrees C at atmospheric pressure

The structure of haploandesitic magma was modeled via in situ, high-temperature Raman spectroscopy along the join K2Si4O9-K-2(KAl)(4)O-9 from 25 to 1236 degrees C. These glasses and melts consist of Q(4), Q(3), and Q(2) structural units. Aluminum substitutes for Si in all units. The disproportion reaction, 2Q(3) double left right arrow Q(4) + Q(2), shifts to the right with increasing temperature (above the glass transition temperature) and with increasing Al/(Al + Si). Stepwise linear regression yields the temperature- and Al/(Al + Si)-dependence for the equilibrium constant for the disproportionation reaction, ln K = -2.7 +/- 0.1 - 1172 +/- 153/T(K) + 12.6 +/- 0.9 . Al/Al + Si + 14.3 +/- 3 .(Al/Al + Si)(2). The enthalpy of the reaction decrease with increasing Al/(Al + Si) from similar to 11 kJ/mol at Al//(Al + Si) = 0 to about 3 kJ/mol at Al/(Al + Si) similar to 0.1 before increasing with further Al3+ double left right arrow Si4+ substitution.