1
Jana Fridrichová#, 1Peter Bačík,
1Petra Rusinová, 2
Marcel Miglierini, 3Valéria Bizovská, 4Peter Antal
1
Comenius University in Bratislava, Faculty of Natural Sciences, Dpt. of Mineralogy and Petrology, Mlynská dolina, 842 15 Bratislava, Slovakia, # [email protected]
2 Slovak University of Technology, Faculty of Electrical Engineering and Information Technology,
Dpt. of Nuclear Physics and Technology, Ilkovičova 3, 812 19 Bratislava, Slovakia
3 Slovak Academy of Sciences, Institute of Inorganic Chemistry, Dúbravská cesta 9, 845 36
Bratislava 45
4 Comenius University in Bratislava, Faculty of Natural Sciences, Dpt. of Inorganic Chemistry,
Mlynská dolina, 842 15 Bratislava, Slovakia
Key words: beryl, heat treatment, Powder X-ray diffraction, Infrared spectroscopy, Mössbauer spectroscopy, Electron microprobe, UV/Vis spectroscopy, DTA
INTRODUCTION
Beryl, ideally Be3Al2Si6O18, space group
P6/mcc, is a cyclosilicate mineral. The crystal
structure was solved by Bragg and West (1926). Structure consists of two tetrahedral sites with Be and Si and one octahedral site with Al. SiO4 tetrahedra form six-membered
rings lying in planes paralel to (0001). The SiO4 rings are linked together with BeO4
tetrahedra and AlO6 octahedra forming a three-
dimensional framework. Structural channels parallel to the Z axis are situated in the centre of the rings where alkali, transition-metal cations, H2O or CO2 can be located. Alkali
cations are also balancing deficiency in positive charge resulting from the replacement of octahedral Al and tetrahedral Be by cations with lower valence such as Fe2+, Mg2+, Li+ (Aurisicchio et al. 1988; Wood and Nassau, 1968). The iron content in natural beryl is about 1 wt. %, but in blue or yellow varieties it can attain 3 wt. %. Ferric ion with ionic radii 0.55 Å prefers octahedral site, whereas ferrous ion with ionic radius 0.78 Å can occupy
tetrahedral, octahedral site and channels. The blue, green and yellow colour of beryl gem stone varieties such as aquamarine and yellow beryl (heliodor) is caused by presence of divalent and trivalent iron and depend on their ratio in the crystal structure (Viana et al. 2002). The aim of this study is to determine optical and crystal-chemical changes in two beryl samples with different colour from Vietnam.
RESULTS
All samples have a gemological quality and were heated at 300, 500, 700, 900, 1100 °C for 12 hours – changes in colour and clarity are demonstrated in Fig.1. Best improvement in colour and clarity applicable for gemological purposes was observed in aquamarine heated at 700 °C. Presuming radiation-induced colourization of yellow beryls (Huong et al. 2012), the heating at 500 °C resulted in resetting of artificial yellow to original very pale blue colour. Basic crystal-chemical properties of studied beryls were determined with EMPA and powder XRD. According to
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c/a ratio (Aurisicchio et al. 1988), all studied
beryls belong to normal beryls (c/a ratio 0.997- 0.998).
Fig. 1. Beryl samples before and after heating.
Both Al octahedral (Al3+ ↔ Me2+, where Me2+ = Mg2+, Fe2+) and Be tetrahedral substitution Be2+ ↔ Li+) substitutions have a common effect on the beryl composition. Aquamarines are generally enriched in Fe (< 0.25 apfu) and alkali (< 0.08 apfu) than yellow beryls (< 0.07
apfu Fe, < 0.04 apfu alkali). Both beryl
samples displayed only weak zoning in BSE images, but no changes in chemical composition were observed in heated samples. However, samples heated at 900 and 1100 °C show system of cracks which are very likely crystallographically oriented. The results from XRD and EMPA suggest that the heating had no effect on the structure of samples, since no significant changes of lattice parameters were observed during the heating.
Mössbauer spectroscopy could have been applied only on aquamarine samples due to insufficient Fe content in yellow beryl. In aquamarine, reduction of Fe was observed in samples heated at 300 to 700 °C (from 68:32 to 78:22 Fe2+:Fe3+ ratio) and subsequent oxidation from 900 to 1100 °C (to 8:92) which could induce changes in their colour and clarity. The results of Mössbauer spectroscopy were confirmed by data from Vis/NIR spectroscopy. In both samples, an increase in absorption of broad band at 810 nm occurring between 500 and 700 °C which can be attributed to Fe2+
in octahedral (Wood & Nassau 1968). After the heating at 900 and 1100 °C, its absorption decreased with significant increase of total
absorption as result of decrease in transparency. Moreover, small bands at 375 and 425 nm attributed to octahedral Fe3+ (Wood & Nassau 1968) were pronounced.
The behaviour of water was studied with IR spectroscopy and DTA/TG. Two types of structural arrangement of water (H2O I – 3694
cm-1, H2O II - 3655 and 3592 cm -1
) were detected by IR spectroscopy. The absorption of 3655 cm-1 band distinctly decreases between 300 and 500 °C. It can be described as release of one of double coordinating H2O II and this
event is confirmed also by decrease in TG curve. After the heating at 900 °C, the decrease in absorption displays at both 3694 and 3592 cm-1 bands. Moreover, the 3655 cm-1 band disappears at all. Finally all water from channels is released which is documented on samples heated at 1100 °C with no distinct absorption bands present. The release of water from channels in large crystals might be a relatively violent process which may be the reason of cracks development in samples heated at temperatures higher than 900 °C.
REFERENCES
AURISICCHIO, C.,FIORAVANTI, G.,GRUBESSI, O., ZANAZZI, P. F. (1988): Reappraisal of the crystal chemistry of beryl. Am. Mineral. 73,
826-837.
BRAGG W.L. AND WEST J.(1926):The crystal structure of beryl. Proceedings of the Royal Society London,3A,691-714.
HUONG L. T.-T., HÄGER T.,HOFMEISTER W., HAUZENBERGER CH.,SCHWARZ D., LONG V. P.,WEHMEISTER U.,KHOI N.N.,
NHUNG N. T. (2012): Gemstones from Vietnam: An Update. Gems & Gemology, 48,
158-176.
VIANA R.R., DA COSTA G.M.,DE GRAVE E., JORDT-EVANGELISTA H.,STERN W.B.(2002): Characterization of beryl (aquamarine variety) by Mössbauer spectroscopy. Phys. Chem. Min.
29, 78-86.
WOOD D. L. AND NASSAU K. (1968): The characterization of beryl and emerald by visible and infrared absorption spectroscopy.
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