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TEM observations on the P-I phase transition in feldspars along the join CaAl₂Si₂O₈-SrAl₂Si₂O₈

¹ Dipartimento di Scienze Mineralogiche e Petrologiche, Via Valperga Caluso 35, I-10125 Torino, Italy
² Centro di Studi sulla Geodinamica delle Catene Collisionali, Via Accademia delle Scienze 5, I-10123 Torino, Italy

Correspondence: ^* E-mail: triba{at}dsmp.unito.it

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 (P_H2O = 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 (T_c) of the P-I 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 P-I 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.

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