State of water molecules and silanol groups in Opal minerals: A near infrared spectroscopic study of opals from Slovakia Miroslav Bobon 1, Alfred A. Christy 2, Daniel Kluvanec 1 and Ludmila Illasova 3 1 Department of Physics, Faculty of Natural Sciences, Constantine the Philosopher University in Nitra, Slovakia 2 Department of Science, Faculty of Engineering and Science, University of Agder, Serviceboks 422, 4604 Kristiansand, Norway 3 Gemological Institute, Faculty of Natural Sciences, Constantine The Philosopher University in Nitra, Slovakia

Points to discuss 1. Opal 2. Chemical structure of Opals 3. Classification of Opals 4. Silica gel and Near Infrared spectroscopy of silica gel 5. Chemistry of silica gel and its relationship to opal minerals 6. Experimental 7. Results and discussion 8. Conclusion

Chemical structure of Opals Chemically, opal is an amorphous hydrated silicon dioxide with a chemical formula SiO 2.nH 2 O. The number of water molecules vary. The structure of opal is similar to silica gel and any analytical techniques that were used in analysing silica gel could be used in analysing opal minerals too. The chemical composition of opal minerals is similar to silica gel. However, they have smaller surface areas.

Woods opals

Common opals

Classification of Opals Opals are classified as A, CT or C according to the nomenclature used by Jones and Segnit (1971) based on the X- ray diffraction of the opal minerals. (A table with the samples used in this work)

Silica gel and Near Infrared spectroscopy of silica gel

Near Infrared spectrum of silica gel

Near infrared Spectroscopy of silica gel The near infrared spectrum of a silica gel sample give rise to a peak composed of three bands having maximum at 5314, 5270 and 5119 cm -1. These bands arise because of the OH stretchings of water molecules adsorbed to surface silanol groups. For the first time Christy was able to prove that the band at 5314 cm -1 was due to the OH groups of water molecules hydrogen bonded to free silanol groups and the band at 5270 cm -1 was due to the OH groups of water molecules hydrogen bonded to vicinal silanol groups on the surface. Equal proportions of free and hydrogen bonded silanol groups on the surface provides optimal adsorption. This finding opened the possibilities for the investigation of silica gel samples and its adsorption properties using Near Infrared Spectroscopy.

Model for water adsorption on Silica gel

Hydrothermal treatment of silica gel The same silica gel sample hydrothermally treated at 160 ºC with water. The sample was dried under vacuum at 200 ºC to remove adsorbed water. The near infrared spectra of the hydrothermally treated silica gel sample measured during the adsorption of water molecules results in the peaks at 5314 and 5270 cm -1 with completely different proportionalities than the peaks of the original sample. Water adsorption effectivity also decreased. This agrees with the adsorption model of Christy.

Mechanism

Aim of the investigation The chemical composition of opal minerals is similar to silica gel. The surface and bulk functional groups of opal have similar interactions with water molecules. A similar approach used by Christy (2010) would reveal the nature of surface and bulk hydroxyl groups and water molecules in opal samples. The aim of the present investigation is to get insights into the silanol groups and water molecules on the surface and inside the bulk of the opal minerals. It is also the intention of the investigation to check whether near infrared spectroscopy could give clues regarding the types of opal minerals.

Experimental Wood and common opal samples from the Slovak Republic. All the samples used in this work were classified as A, CT or C according to the nomenclature used by Jones and Segnit. The opal samples were powderised in a mortar with pestle. The near infrared spectra of the samples were measured on the original samples first. Then each of the samples was dried under vacuum at 200 ºC and its infrared spectrum was recorded. Samples were also heated to 750 ºC for detailed study of the behaviour of the functional groups on the opal mineral surface and in the bulk.

Results and discussion The near infrared spectra of two opal samples randomly selected from the list before drying (opal 12 and 15).

Results and discussion The features of the near infrared spectra changes and the absorption due to the silanol groups are visible in the spectra of samples dried at 200 ºC. The spectra clearly show the presence of free silanol groups in the dried opal samples. The peak is almost invisible in the absorption spectra of the samples. Furthermore, water molecules are in the bulk (micropores).

Results and discussion The near infrared spectra and the second derivative spectral profiles of the samples heated at 750 ºC. The broad absorption near 7316 cm -1 has disappeared in opal 4. The opal sample 5 has a broad peak that overlaps with the free silanol absorptions. The broad absorption may arise from the water molecules in free and hydrogen bonded state in the closed pores and cavities of the opal sample.

Results and discussion The bands arising from the OH combination vibration of water molecules attached to vicinal Si-OH groups in the Opal Type A samples are centered around 5270 cm -1. There is a peak appearing around the same frequency with a slight red shift in frequency for the CT- type opals. This is true with all the CT samples analysed in this work. The OH combination frequency suggests that the water molecules form stronger hydrogen bonding with a particular type of silanol groups other than vicinal silanol groups. The only possible explanation for this behaviour is that the water molecules are in hydrogen bonding with geminal OH groups in the CT type opal samples.

Results and discussion-model for water molecules in opal samples

Conclusion New insights into the state of water in opal minerals have been revealed by the analysis of the samples by near infrared spectroscopy. The absorption peaks arising around 5200 cm -1 give indication of the type of water present in the opal samples. There is distinction between the type hydrogen bonding water molecules are engaged in A types of opal samples and CT type opal samples. The evidence clearly indicate the silanol groups in the CT samples engaged in hydrogen bonding with water molecules are of geminal types and in the A type of samples are of vicinal types.

Conclusion The surface and the bulk contain free and hydrogen bonded silanol groups. The concentrations of these two different types of OH groups vary. However, the concentration of free silanol groups appears to be higher in the CT type samples than the A type samples. The experimental set up used in this paper makes it possible to identify the status of water on the surface and in the cavities of the opal samples. There is clear evidence from this spectroscopic study that some of the water molecules are trapped in closed cavities and are hard to eliminate by using vacuum conditions.

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