THE THERMODYNAMIC PROPERTIES OF SR3MGSI2O8(S) FROM T = (0 TO 1500)K

The low-temperature adiabatic heat capacity of Sr3MgSi2O8(s) has been measured from T = (4 to 420) K, yielding for C-p degrees and S degrees at T = 298.15 K the values (257.35 +/- 0.60)J.K-1.mol(-1) and (280.15 +/- 0.60)J.K-1.mol(-1), respectively. The enthalpy increment function for temperatures above the ambient one has been obtained using high-t emperature drop calorimetry measurements from T = (525 to 910) K. In a ddition, the standard molar enthalpy of formation has been derived fro m the standard molar enthalpies of solution in (3.1 mol.dm(-3) HCl + 3 .2 mol.dm(-3) HF)(aq) using an isoperibol solution calorimeter. From t hese measurements and the auxiliary data, the following value has been obtained: Delta(f)H(m) degrees(298.15 K){Sr3MgSi2O8(s)} = -(4575.3 +/ - 5.1) kJ.mol(-1). A table containing the thermodynamic properties at selected temperatures from (298.15 to 1500) K is provided. (C) 1998 Ac ademic Press Limited.