Densities and volumes of hydrous silicate melts: New measurements and predictions

Mohamed Ali M.A. Bouhifd (1) , A.G. Whittington (2) , Pascal Richet (3)

1 LMV - Laboratoire Magmas et Volcans
2 Department of Geological Sciences
3 IPGP - Institut de Physique du Globe de Paris

The equilibrium molar volumes of four series of anhydrous and hydrous aluminosilicate glasses and liquids (0 to 11 mol% H2O) were determined at 1 bar between 300 and 1050 K. The anhydrous compositions range from highly polymerized NaAlSi3O8 to depolymerized synthetic iron-free analogs of tephrite and foidite magma compositions (NBO/T = 0.8 and 1.5, respectively). For each sample the volume was derived from the room-temperature density of the glass and the thermal expansivity of the glass and supercooled liquid from 300 K to a temperature about 50 K higher than the standard glass transition. The partial molar coefficient of thermal expansion of water in hydrous silicate glasses is about (6.2 ± 3.5) × 10− 5 K− 1, and in the melts ranges from 11 × 10− 5 to 36 × 10− 5 K− 1. The present molar volumes of hydrous supercooled liquids are reproduced with the model of Ochs and Lange (1999) to within 1.1%, except for the hydrous foidite series. This agreement confirms that the partial molar volume of water (V H2O) near the glass transition cannot depend strongly on the chemical composition of the silicate end-member, nor on water speciation. In order to reproduce the molar volumes of the foidite series, a combined model (using Lange, 1997 and Courtial and Dingwell, 1999 models and values derived from the new data) is used where an excess volume term between SiO2 and CaO is introduced. Finally, our experimental data are better fit if V H2O = 23.8 ± 0.5 cm3 mol− 1 at 1273 K, and DV H2O/DT = 15.9±1.5 cm^3 mol^-1 K^-1. Contrasting trends are also observed for the configurational contributions to the expansivity with a positive slope of dVi^conf/dT versus water for the most polymerized base compositions (NBO/T ≤ 0.21) and a negative slope for the two most depolymerized base compositions with NBO/T of 0.86 and 1.51.

Mots clés: Hydrous silicate melts; Partial molar volume of water; Vibrational expansivity; Configurational expansivity

Domaines: Sciences de la Terre

Mohamed Ali M.A. Bouhifd, A.G. Whittington, Pascal Richet. Densities and volumes of hydrous silicate melts: New measurements and predictions. Chemical Geology, 2015, 418, pp.40-50. ⟨10.1016/j.chemgeo.2015.01.012⟩. ⟨insu-01443455⟩