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The results of a combined electron probe microanalysis, single-crystal X‑ray diffraction, and Fourier transform infrared study of a crystal of armstrongite from Khan Bogdo deposit (Gobi, Mongolia) are reported. Major element analysis provided (wt%): CaO 9.2(1), ZrO2 20.9(2), and SiO2 62.5(2). Significant concentrations of REE (0.45 wt%) were also detected. From single-crystal structural refinement, armstrongite resulted monoclinic [space group C2/m, a = 14.0178(7), b = 14.1289(6), c = 7.8366(3) Å, b = 109.436(3)°, V = 1463.6(1) Å3, Z = 4] and twinned with two individuals rotated around a twin twofold axis parallel to [100]. The analyzed crystal was refined up to R = 3.3% (Rw = 2.9%). The structural refinement showed that the investigated armstrongite has only two water groups per formula unit consistent with the infrared analysis. Indeed, the occurrence in the infrared spectrum of the armstrongite (here reported for the first time) of two bending vibration bands at about 1640 and 1610 cm–1 testifies to the presence of two water groups environments. The results of this integrated approach converged to the following empirical formula (based on Si = 6 atoms per formula unit): (Ca0.96Ce0.01Yb0.01)Zr0.99Si6O14.97·2.02H2O. Finally, the studied mineral shows a framework density (FD = 21.86) lying in the range of zeolites and microporous heterosilicates with tetrahedral-octahedral frameworks. The determined crystal chemical features are relevant for the possible employment of this mineral or of its synthetic analogs for technological applications

The crystal structures of tobelite and NH4+-rich muscovite from the sedimentary rocks of the Armorican sandstones (Brittany, France) have been solved for the first time by single crystal X-ray diffraction. The structural study was integrated by electron probe microanalyses, X-ray photoelectron and micro-Fourier transform infrared spectroscopy. The crystals belong to the 2M2 polytype with the following unit-cell parameters: a = 9.024(1), b = 5.2055(6), c = 20.825(3) Å and β = 99.995(8) for tobelite and a = 9.027(1), b = 5.1999(5), c = 20.616(3) Å and β = 100.113(8)° for NH4+-rich muscovite. Structure refinements in the space group C2/c converged at R1 = 8.01%, wR2 = 8.84% and R1 = 5.59%, wR2 = 5.63% for tobelite and NH4+-rich muscovite, respectively. \ud X-ray photoelectron spectroscopy revealed nitrogen environments associated either to inorganic (B.E. 401.31 eV) and to organic (B.E. 398.67 eV) compounds. Infrared spectra showed, in the OH- stretching region (3700-3575 cm-1), two prominent bands, centered at ~ 3629 and ~ 3646 cm-1, and two shoulders at ~ 3664 and ~ 3615 cm-1 which were assigned to Al3+Al3+-OH- arrangements having OH- groups affected by different local configurations. In addition, a series of overlapping bands from about 3500 to 2700 cm-1 characteristic of the NH4+-stretching vibrations, a main band at ~ 1430 and a shoulder at ~ 1460 cm-1 which were associated to the NH4+ bending vibration (ν4) were also present.\ud The ammonium concentration was semi-quantitatively estimated in both crystals from the absorbance of the OH--stretching and NH4+-bending vibrations in the infrared spectra. Additional estimate was obtained for the NH4+-rich muscovite by considering the normalized peak area between K2p3/2 and N1s in the X-ray photoelectron spectrum. The obtained values were also in agreement with those derived from the interlayer spacing in the simulated X-ray powder diffraction spectra. \ud The results of this integrated approach converged to (K0.18Na0.01NH4+0.62)Σ=0.81\ud (Al1.98Fe2+0.02)Σ= 2.00(Si3.19Al0.81)Σ= 4.00O10.00OH2.00 for tobelite and to (K0.46Na0.03Ba0.01NH4+0.36)Σ=0.86\ud (Al1.98Mg0.01Fe2+0.01V3+0.01)Σ=2.01(Si3.13Al0.87)Σ=4.00O10.00F0.08OH1.92 for NH4+-rich muscovite

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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 P 1: 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 2Ca2þ (M1þM2) +(F, OH)(O20) ↔ Ti4þ (M1) +Naþ(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

Three samples of (Tl,Sb,As)-rich pyrite from the ore deposits of southern Apuan Alps (Tuscany, Italy) 15were studied through a multi-technique approach in order to constrain the speciation and incorporation 16 mechanism of thallium in pyrite. High concentrations of Tl, Sb, and As were detected in all the studied 17 samples through laser ablation inductively-coupled plasma mass spectrometry (LA-ICP-MS). Average 18 Tl contents were 1,299 ppm, 1,967 ppm, and 2,623 ppm in samples from Sennari, Canale della Radice, 19 and Fornovolasco, respectively. The LA-ICP-MS time-resolved down-hole ablation profiles were 20 smooth indicating that Tl, Sb, and As are dissolved in the pyrite matrix, or occur in homogeneously 21 distributed nanoparticles (NPs). X-ray absorption spectroscopy (XAS) data revealed that Tl, Sb, and As 22 occurs as Tl + , Sb 3+ , As 3+ , and As 1-. In all the studied samples, bond distances and coordination numbers 23 for Sb 3+ and As 3+ are constant, whereas Tl displays a range of coordination numbers (~3 to ~6),