Armstrongite at non ambient conditions: An in-situ high temperature single crystal X-ray diffraction study

The dehydration process of armstrongite, CaZr[Si6O15]·2H2O, from Khan Bogdo deposit (Gobi, Mongolia) was studied by in-situ High Temperature Single Crystal X-ray Diffraction (HT SCXRD) in air from 25 to 500 °C and in N2atmosphere from 25 to 375 °C. An abrupt discontinuity in the trend of the a and b parameters and of the unit-cell volume was observed at T = 275 °C in dry conditions and at T = 450 °C in air. This discontinuity is associated to dehydration and to a first-order transition. When compared to RT armstrongite, the dehydrated phase structure (obtained at 275 °C only under dry conditions) is characterized by the same space group (C2/m), cell volume decrease of ∼7.5%, compatible with the loss of the two water molecules, positional disorder of Ca over three sites, splitting of some of the heteropolyhedral framework oxygen atoms, tilting of Zr octahedra and Si tetrahedra and, distortion of four-, six- and eight-membered channels. Differently from other Zr-silicate structures, the channels dimension of the dehydrated structure (defined as the ratio between the longest and shortest diagonals of the channels) allowed the armstrongite structure to completely recover the structural water after 21 days exposure to humid conditions.