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Abstract The dehydration process of armstrongite, CaZr[Si 6 O 15 ]·2H 2 O, 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 N 2 atmosphere 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 (C 2/ 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. Graphical abstract Image 1 Highlights • The dehydration of armstrongite, a zeolite-like mineral, was studied. • The mineral dehydrates at 450 °C in air and at 275° in N 2 atmosphere. • The process involves a phase transition without symmetry change. • The dehydrated structure is disordered and distorted with respect to the hydrated one. • The mineral, differently from similar compounds, truly behaves as a zeolitic material. [ABSTRACT FROM AUTHOR]

The structures of tokkoite, K2Ca4[Si7O18OH](OH,F) and tinaksite, K2Ca2NaTi[Si7O18OH]O from the Murun massif (Russia) were refined from single-crystal X-ray diffraction data in the triclinic space group P1̄: Average crystallographic data are a ≈ 10.423, b ≈ 12.477, c ≈ 7.112 Å, α ≈ 89.92°, β ≈ 99.68°, γ ≈ 92.97°, V ≈ 910.5 Å3 for tokkoite; a ≈ 10.373, b ≈ 12.176, c ≈ 7.057 Å, α ≈ 90.82°, β ≈ 99.22°, γ ≈ 92.80°, V ≈ 878.5 Å3 for tinaksite. The substantial similarities between the geometrical parameters of the tokkoite and tinaksite structures led us to conclude that the two minerals are isostructural. However, major differences of tokkoite with respect to tinaksite are larger lattice constants, especially concerning the b parameter, longer distances, especially ; larger values of the M1-M3 and O20-O2 bond lengths, and a stronger distortion of the M1 polyhedron. Mössbauer analysis showed that significant trivalent iron is present, VIFe3+ 40.0(7)% in tokkoite and 12.8(3)% in tinaksite. It is confirmed that 2Ca(M1+M2)2+ + (F, OH)-(O20) ↔ Ti4+(M1) +Na+(M2) +O-(O20) is the exchange reaction that describes the relation between tokkoite and tinaksite. In addition, this exchange reaction causes local stress involving mainly the M1 site and its interaction with the M2 and M3 sites. [ABSTRACT FROM AUTHOR]

A crystal chemical study of narsarsukite from the Murun alkaline massif, Russia has been carried out combining single-crystal X-ray diffraction, electron microprobe analyses, micro-Fourier transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS). The narsarsukite single crystals are tetragonal (space group I4/m) with unit-cell parameters: 10.7140(1) ≤ a ≤ 10.7183(2) Å and 7.9478(1) ≤ c ≤ 7.9511(1) Å. The XPS analysis showed that Fe occurs in the mineral as Fe3+, whereas the FTIR spectrum showed that the sample studied is anhydrous. The average crystal chemical formula of the Murun narsarsukite is: Na2.04K0.01(V5+0.01Ti0.74Zr0.01Al0.01Fe3+0.22Mg0.01)1.00Si4.00(O10.74F0.23OH0.03)11.00. Structural disorder at octahedral and interstitial sites was modelled and also discussed in consideration of the main substitutional mechanism Ti4+ + O2− ↔ Fe3+ + (F−, OH−) active in the structure of the mineral. [ABSTRACT FROM AUTHOR]

A transmission electron microscopy (TEM) investigation was undertaken in order to elucidate the nature of the structural disorder in a tobelite specimen from the sedimentary rocks of the Armorican sandstones (western France). This structural disorder may be the origin of residual electron-density maxima in difference-Fourier maps reported previously in single-crystal XRD studies of tobelite. The TEM investigation of tobelite confirmed that it is the 2M2 polytype in subfamily-B, but the ordered sequence is interrupted by numerous stacking faults parallel to (001), for which the stacking vectors belong to both subfamilies-A and -B of mica polytypes, with a prevalence of the latter. Chemical heterogeneity depending on the Si/Al ratio and Na and Mg concentration was observed at the nanoscale among different mica lamellae in a single crystal. The observed variations are consistent with a change in mica chemistry leading to interlayer vacancies which may cause shortening of the interlayer separation, as revealed by the singlecrystal structure refinements. [ABSTRACT FROM AUTHOR]