TEM observations on theP1̅-I1̅ phase transition in feldspars along the join CaAl2Si2O8-SrAl2Si2O8

Abstract

A transmission electron microscopy (TEM) investigation was performed on Ca-rich feldspars along the join CaAl 2 Si 2 O 8 -SrAl 2 Si 2 O 8 (An-SrF). Two series of samples were synthesized by dry ( T = 1450 °C, t = 96 h) and hydrothermal ( P H 2 O = 2 kbar, T = 600 °C, t = 300 h) annealing with compositions between An 100 and An 50 SrF 50 . TEM investigation at room temperature showed the presence of sharp c -type reflections only in hydrothermal An 100 and An 80 SrF 20 feldspars, whereas diffuse and elongated c reflections were found in all other feldspars in the range An 80 SrF 20 -An 50 SrF 50 . The intensity of c reflections decreases with increasing Sr content, and fully disappears only at An 50 SrF 50 . In-situ, high-temperature TEM investigation (HT TEM) on hydrothermally treated An 100 and An 80 SrF 20 feldspars, showed a significant decrease in the temperature ( T c ) of the P 1- I 1 phase transition with increasing Sr content. In An 80 SrF 20 feldspar, a precise critical temperature cannot be defined, and the transition is smeared over a T range between 100 and 150 °C. A P 1- I 1 phase transition is therefore observed both as a function of composition and temperature. The transition dependence on composition is induced by the increase in the non-tetrahedral cation radius that causes a static disorder and affects the intensity of c reflections. Dynamical disorder adds to the static disorder induced by Sr across the transition at HT.