A transmission electron microscopy (TEM) investigation was performed on Ca-rich feldspars along the join CaAl2Si2O8-SrAl2Si2O8 (An-SrF). Two series of samples were synthesized by dry (T = 1450 °C, t = 96 h) and hydrothermal (PH2O = 2 kbar, T = 600 °C, t = 300 h) annealing with compositions between An100 and An50SrF50. TEM investigation at room temperature showed the presence of sharp c-type reflections only in hydrothermal An100 and An80SrF20 feldspars, whereas diffuse and elongated c reflections were found in all other feldspars in the range An80SrF20-An50SrF50. The intensity of c reflections decreases with increasing Sr content, and fully disappears only at An50SrF50.
In-situ, high-temperature TEM investigation (HT TEM) on hydrothermally treated An100 and An80SrF20 feldspars, showed a significant decrease in the temperature (Tc) of the P1̅-I1̅ phase transition with increasing Sr content. In An80SrF20 feldspar, a precise critical temperature cannot be defined, and the transition is smeared over a T range between 100 and 150 °C.
A P1̅-I1̅ 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.