Armstrongite from Khan Bogdo (Mongolia): Crystal structure determination and implications for zeolite-like cation exchange properties

Abstract

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

Structure refinement and crystal chemistry of tokkoite and tinaksite from the Murun massif (Russia)

“…The size of the eight-membered rings is 7.116(4) × 4.979(5) Å 2 for tok_1 and 7.057(2) × 5.035(2) Å 2 for tin_1, while the size of the four-membered rings is 3.735(5) × 3.732(5) Å 2 for tok_1 and 3.603(2) × 3.835(2) Å 2 for tin_1. These values are similar to those found in other silicates with heterogeneous frameworks (Mesto et al, 2014). The eight-membered Si-O rings host K2 atoms whereas K1 atoms are shifted along the [001] direction (Fig.…”

Crystal chemistry of fluorcarletonite, a new mineral from the Murun alkaline complex (Russia)

“…The size of the eight-membered rings is 7.178(1) × 4.664(1) Å, while that of four-membered rings is 4.027(1) × 3.203(1) Å. These values are quite close to those found in other silicates from the Murun charoitites (see, for example, Lacalamita et al, 2017) and silicates with heterogeneous frameworks (see, for example, Mesto et al, 2014). There is a further eight-membered ring with a size of 4.466(1) × 4.466(1) Å in the straighter windows and 6.144(1)×6.144(1) Å in the wider ones configured with the equivalent eight-membered ring of the adjacent SiO 4 layer.…”

Single-crystal X-ray diffraction, EMPA, FTIR and X-ray photoelectron spectroscopy study of narsarsukite from Murun Massif, Russia

“…Similar values have been also found for other microporous heterosilicates with mixed octahedral-tetrahedral framework, e.g. vlasovite (Sokolova et al, 2006), elpidite (Grigor'eva et al, 2011), armstrongite (Mesto et al, 2014), tinaksite and tokkoite (Lacalamita et al, 2017). Here we refer to the most recent structure refinement of these minerals.…”