Abstract
In this work, five Mg-silicate glasses with compositions between MgSiO_3 and Mg_2SiO_4 were synthesized by aerodynamic levitation combined with laser melting. Low-temperature heat capacity (C_p) 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 Mg_{0.996}SiO_{2.996} and entropy 72.88 J mol^{-1} K^{-1}; MG54 Mg_{1.174}SiO_{3.174} 78.54; MG58 Mg_{1.364}SiO_{3.364} 85.05; MG62 Mg_{1.611}SiO_{3.611} 91.40; and MG67 Mg_{1.907}SiO_{3.907} 102.75. Heat capacity of the glasses is higher than that of the corresponding crystal mixtures below 200 K but plunges below the C_{p,mathrm{crystal}} at higher temperatures. High-temperature C_p was measured (by differential scanning calorimetry in the range 300–970 K) for MG50 and MG67 up to approx 1000 K. Using our C_p data, selected data for entropies of fusion, C_p of crystals, and fictive temperatures, the configurational entropy (S_mathrm{conf}) at glass transition temperature (T_mathrm{g}) were calculated. For the near-forsterite glass MG67, the S_mathrm{conf} is 1.9 J mol^{-1} K^{-1} at T_mathrm{g} = 1040 K. As this small value is a difference 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 eta = A + B/[T S_mathrm{conf}(T)], the available experimental viscosities (eta ) and the temperature-dependent configurational entropy from our work, we refined the parameters A = -2.34 and B = 76,500 for this equation, with S_mathrm{conf} (T) = 1.90+(83.7 ln (T/1040)). The configurational entropy for the enstatitic MG50 glass is 16.8 J mol^{-1} K^{-1} at T_mathrm{g} = 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.
Highlights
Silicate glasses are important materials in geology, mineralogy, geochemistry, and geophysics (Mysen and Richet 2005)
Physics and Chemistry of Minerals (2021) 48:28 glasses were studied by nuclear magnetic resonance (NMR) spectroscopy (e.g., Kroeker and Stebbins 2000), Raman spectroscopy (e.g., Kalampounias et al 2009), X-ray and neutron scattering (e.g., Wilding et al 2008), and Brillouin spectroscopy (Speziale et al 2009)
The magnesium silicate glasses were synthesized by a technique using aerodynamic levitation combined with laser melting, described previously by Tangeman et al (2001)
Summary
Silicate glasses are important materials in geology, mineralogy, geochemistry, and geophysics (Mysen and Richet 2005). Physics and Chemistry of Minerals (2021) 48:28 glasses were studied by nuclear magnetic resonance (NMR) spectroscopy (e.g., Kroeker and Stebbins 2000), Raman spectroscopy (e.g., Kalampounias et al 2009), X-ray and neutron scattering (e.g., Wilding et al 2008), and Brillouin spectroscopy (Speziale et al 2009). Vibrational properties of these glasses were studied by molecular dynamics (Belonoshko and Dubrovinsky 1996) and ab initio simulations (Spiekermann et al 2013). These data were used, in conjunction with previously published Cp for crystalline substances and estimates of Cp for supercooled liquids, to calculate configurational entropies of these compositions
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
