Dehydration and thermal stability of elpidite: An in-situ single crystal X-ray diffraction study

• We tracked the dehydration behavior of elpidite between 25 and 350 °C.
• Hydrogen positions of elpidite at RT were determined.
• Rehydration of the elpidite was found to be extremely slow.
• Structure of partially hydrated elpidite was solved.
• We determined the driving force of the Pbcm-Cmce transition.

Elpidite Na2ZrSi6O15·3H2O is a natural zirconosilicate with space group Pbcm. A sample from Mt. Saint Hilaire (Canada) was investigated between 25 and 350 °C under different humidity conditions (RH = 0% and at RH = 100%). The largest structural channels are eight- and ten-rings delimited by corner-sharing SiO4 tetrahedra and ZrO6 octahedra.Under dry conditions, the structure started liberating water at 75 °C. At 100 °C, when it lost one H2O molecule, elpidite underwent a structural change from space group Pbcm to Cmce.The structure continuously released water up to 225 °C until it became anhydrous. From 225 °C to 350 °C no structural changes were observed. Associated with the transition from Pbcm to Cmce symmetry, the unit-cell volume reduced by 2.7%. An additional decrease is observed up to 225 °C when the contraction reached 3.4% of the starting volume. After release of all H2O molecules, increase of temperature led to gradual expansion of the unit cell from 2940.79 (7) Å3 at 225 °C to 2947.4 (5) Å3 at 350 °C.Supplementary ex-situ heating experiments showed that the structure was retained up to 950 °C but after heating at 1200 °C the sample melted.Under humid conditions a dehydration-trend similar to that found under a dry N2-atmosphere was observed but the structural changes occurred at slightly higher temperatures.Immersion of a dehydrated sample in water for 30 days led to only marginal rehydration and the Cmce symmetry was preserved.

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