[S-MP47_1PO1] Physics and Chemistry of Minerals

[SMP47-P11] Structural study on the phase transformation of natural scolecite with increasing temperature

*Takahiro UCHIDA¹, Takahiro KURIBAYASHI¹, Toshiro NAGASE² (1.Department of Earth Science, graduate school of science, Tohoku University, 2.The Tohoku University Museum, Tohoku University)

Keywords:scolecite, dehydration, phase transformation, single crystal X-ray diffraction, twin, high temperature

Scolecite, CaAl₂Si₃O₁₀●3H₂O is classified to fibrous zeolite group. The sequence of general phase transformation with increasing temperature has been reported for natural scolecite: scolecite➞ meta-scolecite ➞ amorphous phase and decomposes to An + Qtz (Rykl et al. 1986; Gottardi and Galli 1985). In this study, the high-T evolution of the structure of natural scolecite from Poona, India were studied up to 573 K to reevaluate the dehydration process of scolecite using TG-DTA and in situ single crystal X-ray diffraction experiments. As the results from structual refinement at room temperature, the lattice constants of the sample are determined as follows : a= 18.504(3)A, b= 18.971(2) A, c= 6.5262(9) A and β = 90.558(5)⁰. The crystal structure of scolecite, CaAl₂Si₃O₁₀●3H₂O, was refined with the space group F1d1 from 3567 reflections with Io > 2σ(I), yielding R= 4.62%, wR= 11.41%. At 〜423 K, the space group was changed to Fdll from F1d1, and scolecite underwent a phase transformation to meta-scolecite phase.As the results from structual refinement at 523 K , the lattice constants of the sample are determined as follows : a= 18.122(3)A, b = 18.847(3) A, c = 6.5408(11) A and α = 88.948(7)⁰. The crystal structure of scolecite, CaAl₂Si₃O₁₀●2H₂O, was refined with the space group Fd11 from 2782 reflections with Io > 2σ(I), yielding R= 10.72%, wR= 28.85%. When phase transformation occurs, OW2 in scolecite is expelled and then the half of Ca ions move by 〜1/2c. At 573 K, the number of ovserved reflections was decreased dramatically.Under high-T experiments from 423 to 523 K, two reciprocal lattices were observed, each lattice is corresponding to twin component with the [00-1] twin law. The [00-1] twinning could be associated with the dehydration mechanism. The X-ray diffraction data suggest the possibility of exist of a new H₂O site in meta-scolecite phase. This may be a key to solve the dehydration process of scolecite.