Structural and vibrational properties of agrellite

Abstract

Abstract Agrellite, NaCa 2 Si 4 O 10 F, is a tubular silicate mineral which crystal structure is characterized by extended [Si 8 O 20 ] 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 H 2 O or OH − into the channel system. We construct structural models of water-containing NaCa 2 Si 4 O 10 F and identified H 2 O positions. The derivation of H 2 O 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 (H 2 O) in minerals.