Thermal stability and dehydration of armstrongite, a microporous zirconium silicate

The dehydration of armstrongite from Khan Bogdo (Mongolia) was investigated by combining thermal analysis, in situ HT X-ray powder diffraction (XRPD) and Fourier Transform InfraRed (FTIR) spectroscopy. The process starts at ∼380 °C and is completed within few tenths of degrees. It involves a mass loss of 6.1 wt% and a cell volume decrease of 7%. Armstrongite at RT has C2/m symmetry with (in Å) a = 14.010 (2), b = 14.115 (1), c = 7.838 (1), β = 109.387 (3)° V = 1462.2 (2) Å3. XRPD data in the T-range 370–400 °C show a significant contraction of the cell volume without any symmetry change. At 400 °C the dehydrated phase has cell dimensions (in Å): a = 13.425 (2), b = 13.752 (1), c = 7.818 (1), β = 110.246 (3)° V = 1354.2 (2) Å3. The patterns collected in the T-range from 800 to 30 °C show that armstrongite rehydrates quickly at T ∼320 °C; unit cell parameters and volume refined at the end of the heating/cooling cycle point to a complete reversibility of the dehydration process. Fast rehydration upon cooling is also evident in the FTIR spectra; a complete recovery of the OH-stretching and bending signals is observed at T ∼280–300 °C. Notably, this process can be monitored on single-crystals, while powders embedded in KBr pellets do not recover the structural water content. The thermal expansion of armstrongite is more pronounced along the b axis, with αa: αb: αc= 1.09 × 10−6: 1.69 × 10−5: 7.61 × 10−7at 90 °C and 7.73 × 10−6: 8.94 × 10−6: 5.85 × 10−6at 800 °C.