Heat capacity, entropy, configurational entropy, and viscosity of magnesium silicate glasses and liquids

In this work, fve Mg-silicate glasses with compositions between MgSiO3 and Mg2SiO4 were synthesized by aerodynamic levitation combined with laser melting. Low-temperature heat capacity (Cp) was measured (by relaxation calorimetry in the range 2–310 K) for all of them, with the resulting vibrational entropies at T = 298.15 K: sample MG50 with composition Mg0.996SiO2.996 and entropy 72.88 J mol−1 K−1 ; MG54 Mg1.174SiO3.174 78.54; MG58 Mg1.364SiO3.364 85.05; MG62 Mg1.611 SiO3.611 91.40; and MG67 Mg1.907SiO3.907 102.75. Heat capacity of the glasses is higher than that of the corresponding crystal mixtures below 200 K but plunges below the Cp,crystal at higher temperatures. High-temperature Cp was measured (by differential scanning calorimetry in the range 300–970 K) for MG50 and MG67 up to ≈ 1000 K. Using our Cp data, selected data for entropies of fusion, Cp of crystals, and fctive temperatures, the confgurational entropy (Sconf) at glass transition temperature (Tg) were calculated. For the near-forsterite glass MG67, the Sconf is 1.9 J mol−1 K−1 at Tg = 1040 K. As this small value is a diference of several large numbers, its uncertainty is relatively high; we consider a conservative estimate of 15 J mol−1 K−1 . Using the expression log 휂 = A + B∕[TSconf(T)], the available experimental viscosities (휂) and the temperaturedependent confgurational entropy from our work, we refned the parameters A = −2.34 and B = 76, 500 for this equation, with Sconf(T) = 1.90 + (83.7 ln(T∕1040)). The confgurational entropy for the enstatitic MG50 glass is 16.8 J mol−1 K−1 at Tg = 1063 K. The presented data can be combined with enthalpies of formation and thermophysical properties of Mg-silicate glasses for models that could elucidate geological and geophysical observations in the crust and mantle of the Earth.