Armstrongite, CaZr[Si6O15]·2H2O, is a natural “zeolite-like” Zr-silicate with a heteropolyhedral framework consisting of SiO4 tetrahedra and ZrO6 octahedra that form cavities occupied by Ca-exchangeable cations (Mesto et al., 2014). The behavior at non ambient conditions of armstrongite from Khan Bogdo deposit (Gobi, Mongolia) was studied by in-situ High Temperature Single Crystal X-ray Diffraction (HT SCXRD), both in air and under dry conditions up to 500°C and 375 °C respectively, and by in-situ High Pressure Single Crystal X-ray Diffraction (HP SCXRD) using synchrotron X-ray diffraction data (collected up to 8.01 GPa), a diamond anvil cell and the mix methanol:ethanol:water as hydrostatic pressure-transmitting fluid. On heating an abrupt discontinuity in the trend of the cell parameters and unit-cell volume occurs at T = 275°C in dry condition and at T = 450°C in air. The cell volume decreased by ~7.5%, compared to that measured at RT, and is compatible with the loss of the two water molecules. The dehydrated phase (solved and refined at 275°C only under dry conditions) exhibits the same space group (C2/m) as RT armstrongite, significantly shortened a and b cell dimensions, increased β angle, and smaller unit-cell volume (a = 13.406(3), b = 13.752(3), c = 7.811(2) Å, β = 110.22(3)°, V = 1351.3(5) Å3) with respect to the hydrated phase (a = 14.0135(7), b = 14.1234(6), c = 7.8388(4) Å, β = 109.401(4)°, V = 1463.4(1) Å3) at RT. The process is also accompanied by the distortion of the cavities as a consequence of Ca splitting and positional disorder of tetrahedral framework oxygens. The dehydration/rehydration process of armstrongite is completely reversible as also found from previous HT XRPD investigation (Schingaro et al., 2018). HP SCXRD data show a first-order phase transition between 4.01(5) and 5.07(5) GPa. In the high-pressure polymorph, the unit-cell volume triplicates. The bulk compression of armstrongite is mainly accommodated through the tilting of both SiO4 tetrahedra and ZrO6 octahedra around the shared oxygen hinges. The high-P polymorph of armstrongite is found to be stiffer (KV0 increase of ~ 66%), and a remarkable change of the elastic anisotropic scheme occurs. No evidence of crystal-fluid interaction, with a selective sorption of molecules of the pressure-transmitting fluid through the cavities, was observed. Mesto, E., Kaneva, E., Schingaro, E., Vladykin, N., Lacalamita, M. & Scordari, F. (2014): Armstrongite from Khan Bogdo (Mongolia): crystal structure determination and implications for zeolite-like cation exchange properties. Am. Mineral., 99, 2424-2432. Schingaro, E., Lacalamita, M., Mesto, E. & Della Ventura, G. (2018): Thermal stability and dehydration of armstrongite, a microporous zirconium silicate. Micropor. Mesopor. Mat., in press.
|Titolo:||An in situ HT-HP single crystal X-ray diffraction study of armstrongite, a microporous zirconium silicate|
|Data di pubblicazione:||set-2018|
|Parole Chiave:||armstrongite; dehydration; compressibility; in situ HT-SCXRD.|
|Settore Scientifico Disciplinare:||Settore GEO/09 - Georisorse Miner.Appl.Mineral.-Petrogr.per l'amb.e i Beni Cul|
|Enti collegati al convegno:||Società Geologica Italiana - Società Italiana di Mineralogia e Petrologia|
|Citazione:||An in situ HT-HP single crystal X-ray diffraction study of armstrongite, a microporous zirconium silicate / E. Mest, G. Cametti, M. Lacalamita, D. Comboni, G.D. Gatta, P. Lotti, M. Merlini, M. Hanfland, E. Schingaro.. ((Intervento presentato al convegno SGI-SIMP tenutosi a Catania nel 2018.|
|Appare nelle tipologie:||14 - Intervento a convegno non pubblicato|