Thermal equation of state of lawsonite up to 10 GPa and 973 K

Abstract

Thermal equation of state (EoS) of synthetic lawsonite [CaAl₂Si₂O₇(OH)₂·H₂O] has been established using in–situ X–ray diffraction methods under high pressure and high temperature. Sodium chloride NaCl was used as the pressure standard in the experiments. The unit–cell volumes were measured up to 10 GPa and 973 K after the deviatric stress was released at high temperature. The P–V–T dataset was analyzed using a Birch–Murnaghan equation of state, yielding the room pressure volume V₀ = 674.2(2) ų and the isothermal bulk modulus at room temperature K₀ = 129(2) GPa (K′ set to 4). These values are comparable with the previous studies. When fitting the high temperature data, a second order temperature derivative of the bulk modulus was considered. Unlike Daneil et al. (1999) who reported a minimum value of bulk modulus at ~500 K, the bulk modulus decreases with increasing temperature at least up to 973 K. The dataset yields: V₀ = 674.3(4) ų, K₀ = 128.7(15) GPa, (∂K_T/∂T)_P = −0.047(8) GPa K⁻¹, (∂²K_T/∂T²)_P = 0.028(6) × 10⁻³ GPa K⁻¹, α = 3.13(25) × 10⁻⁵ K⁻¹, assuming K′ = 4. These data can be used to calculate the density and the stability of lawsonite under high pressure and high temperature conditions.