State of water molecules and silanol groups in opal minerals: a near infrared spectroscopic study of opals from Slovakia |
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Authors: | Miroslav Boboň Alfred A Christy Daniel Kluvanec L’udmila Illášová |
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Institution: | (1) Department of Physics, Faculty of Natural Sciences, Constantine the Philosopher University in Nitra, Nitra, Slovakia;(2) Department of Science, Faculty of Engineering and Science, University of Agder, Serviceboks 422, 4604 Kristiansand, Norway;(3) Gemmological Institute, Faculty of Natural Sciences, Constantine the Philosopher University in Nitra, Nitra, Slovakia; |
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Abstract: | Recently, near infrared spectroscopy in combination with double derivative technique has been effectively used by Christy
(Vib Spectrosc 54:42–49, 2010) to study and differentiate between free and hydrogen bonded silanol groups on silica gel surface. The method has given some
insight into the type of functionalities, their location in silica gel samples, and the way the water molecules bind onto
the silanol groups. The important information in this respect comes from the overtones of the OH groups of water molecules
hydrogen-bonded to free silanol groups, and hydrogen-bonded silanol groups absorbing in the region 5,500–5,100 cm−1. Chemically, opal minerals are hydrated silica and the same approach was adapted to study the state of water molecules, silanol
functionalities, and their locations in opal samples from Slovakia. Twenty opal samples classified into CT and A classes and
one quartz sample were used in this work. The samples were crushed using a hydraulic press and powderized. Each sample was
then subjected to evacuation process to remove surface-adsorbed water at 200°C, and the near infrared spectrum of each sample
was measured using a Perkin Elmer NTS FT-NIR spectrometer equipped with a transflectance accessory and a DTGS detector. The
samples were also heated to 750°C to remove the hydrogen-bonded silanol groups on the surface to reveal their locality. Second
derivative profiles of the near infrared reflectance spectra were obtained using the instrument’s software and used in the
detailed analysis of the samples. The analysis of the near infrared spectra and their second derivative profiles had the aim
in finding relationships between the surface chemical structure and the classification of opal samples. The dry opal samples
were also tested for their surface adsorption effectivity toward water molecules. The results indicate that the opal samples
contain (1) surface-adsorbed water, (2) free and hydrogen-bonded silanol groups on the surface, (3) trapped water molecules
in the bulk, and (4) free and hydrogen-bonded silanol groups in the cavity surfaces in the bulk. A part of the water molecules
in the bulk of opal minerals are found as free molecules and the rest are found in hydrogen-bonded state to either free or
vicinal or geminal silanol groups. |
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