Structural and vibrational properties of agrellite

Ekaterina Kaneva,Roman Shendrik,Alexandr Bogdanov

doi:10.1038/s41598-020-72631-1

Ekaterina Kaneva, Roman Shendrik + Show 1 more

Open Access

https://doi.org/10.1038/s41598-020-72631-1

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Abstract

Agrellite, NaCa2Si4O10F, is a tubular silicate mineral which crystal structure is characterized by extended [Si8O20]8– tubes and has a two-dimensional channel system. The mineral is a representative of a complex silicate family which contains some structural voids but cannot be considered as microporous because of small channel widths. However, the channel system of such minerals is able to host single guest atoms, molecules or radicals which can affect their physical properties. Presently, the exact mechanism of such hosting is undetermined. However, such information could be quite useful for materials’ application as zeolites as well as for a better understanding of their formation mechanisms. In this work we couple X-ray diffraction, infrared (IR) spectroscopy and ab initio calculations to identify structural features in agrellite from Malyy Murun massif (Russia) caused by incorporation of either H2O or OH− into the channel system. We construct structural models of water-containing NaCa2Si4O10F and identified H2O positions. The derivation of H2O sites is based on simulation of IR-spectra. Infrared spectroscopy in combination with the ab initio calculation has proven to be an effective tool for the identification of the structural positions of hydroxyl anions (OH−) and neutral water groups (H2O) in minerals.

Highlights

Introduction of OH group substitutingF atom produces a peak at about 3611 cm−1 associated with OH− stretching vibrations and has only a slight effect on the other part of the IR spectrum.The changes caused by adding water molecules affect two parts of spectra. H2O bending vibrations appear in the 1500–1700 cm−1 spectral region
The symmetric and asymmetric stretching vibrations of H 2O appear in the region of 800–2200 cm−1 depending on the water position in the crystal structure
The present study was concerned with the crystal structure and water contamination features in agrellite, complex silicate mineral from Murun massif, Russia

Summary

Introduction

Introduction of OH group substitutingF atom produces a peak at about 3611 cm−1 associated with OH− stretching vibrations and has only a slight effect on the other part of the IR spectrum.The changes caused by adding water molecules affect two parts of spectra. H2O bending vibrations appear in the 1500–1700 cm−1 spectral region. F atom produces a peak at about 3611 cm−1 associated with OH− stretching vibrations and has only a slight effect on the other part of the IR spectrum. H2O bending vibrations appear in the 1500–1700 cm−1 spectral region. The symmetric and asymmetric stretching vibrations of H 2O appear in the region of 800–2200 cm−1 depending on the water position in the crystal structure. Asymmetric vibrations have lower frequencies than symmetric ones; this is not a rule in agrellite structure. The calculated frequencies of H2O in agrellite are collected in Table 4 with vibration types assigned. The simulated crystal structures need to be examined in the space group P1 since the entry of water molecules does not have a symmetrical principle and affects the structural units by different distortion mechanisms. The number of atoms in the asymmetric unit of the structure when described in sp. Atomic coordinates of simulated models C1, C2, C3 and C4 are reported in Supplementary Tables S9–S12, respectively

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