EL MARCO AUTO-REFERENCIAL

1_{Jana Fridrichová}#

, 1Peter Bačík,

2_{Ján Štubňa,} 3

Peter Antal

1 _{Comenius University in Bratislava, Faculty of Natural Sciences, Dpt. of Mineralogy and Petrology,}

Mlynská dolina, 842 15 Bratislava, Slovakia, # [email protected]

2 _{Institute of Gemmology, of the Faculty of Natural Sciences of the Constantine the Philosopher}

University in Nitra, Nábrežie mládeže 91, 949 74 Nitra, Slovakia

3 _{Comenius University in Bratislava, Faculty of Natural Sciences, Dpt. of Inorganic Chemistry,}

Mlynská dolina, 842 15 Bratislava, Slovakia

Key words: tourmaline, heat treatment, Vis/NIR spectroscopy, Raman spectroscopy

INTRODUCTION

Heat treatment is routinely performed on gemstones to enhance optical properties, mainly colour and clarity, to make them more valuable and marketable (Castan᷉eda et al. 2006; Bačík et al. 2011). The aim of this study is to use a nondestructive and noninvasive methods and devices to identify and describe faceted gemstones.

RESULTS

All 8 samples except T4 are faceted gemstones and were described and analyzed before and after heating at 700 °C for 24 hours. Vis/NIR and Raman spectroscopy were performed at Dpt. of Inorganic Chemistry, Comenius University. Sample T1 is a rectangular cut, greyish green colour, its clarity is VVS, T2 is a cabochon of pale green colour with clarity grade SI (significant fissure viewed from the side). T3 is a brilliant of dark green colour, it is the darkest specimen from all. T4 is an angular shape tourmaline with VVS clarity grade, T5 is a dark green brilliant with VS clarity, T6 is a pear cut of pale green colour and VS clarity, T7 is

yellowish green oval with VVS clarity, T8 is a pink pear with VVS clarity grade. Clarity was graded under 10x magnification. After heat treatment most of the samples did not change in colour and clarity, except T7 and T8. Both samples faded in saturation of colour, while T7 faded only in hue and T8 lost its colour almost completely. Tourmalines were identified and process of partial but not full heating-induced dehydration was confirmed with Raman analysis. However, for more detailed analysis is necessary to use other nondestructive techniques e.g. XRF for determination of chemical composition.In Vis/NIR spectra T1 sample shows broad absorption band at 730 nm. It corresponds to Fe2+ (Laurs et al. 2008; Merkel & Breeding 2009). Significant absorption band is also visible at 970 nm and continues to higher wavenumbers. It can be an overtone of OH vibrations (Laurs et al. 2007). T1 specimen is likely elbaite since the green colour in elbaite is often caused by Fe2+. Presence of Cu2+ can be excluded, however Cu2+ exhibits two significant peaks at 700 and 900 nm (Merkel & Breeding 2009). Spectrum of T2 and T3 have a similar pattern to T1, T2

37 displays significant peak around 710 nm a T3 shows two peaks at 680 and 750 nm. All of these peaks can be assigned to Fe2+. There are no significant changes in spectrum before and after treatment in all three samples, it suggests that no oxidation process did not occur. Sample T4 is different to all previous, it exhibits significant peak at 730 nm, attributable to Fe2+. In addition this sample displays two peaks at 440 and 600 nm which can be assigned to V3+ (Merkel & Breeding 2009). T4 is probaly uvite or dravite, because colour in uvite and green dravite is caused by V3+ and Cr3+ (Schmetzer & Bank 1979) and both of them never occur in elbaite (Rossman et al. 1991). Spectra of T5–T8 samples shows no significant bands and can not be relevantly interpreted. These samples are faceted which produce reflections inside the specimens influencing absorption. Moreover, direct crystallographic orientation is almost impossible. It is recommended to measure in directions parallel or perpendicular to the Z axis (Laurs et al. 2008; Merkel & Breeding 2009). This directions correspond to minimal (parallel to Z axis) a maximal (perpendicular to Z axis) absorption. Tourmaline has significant pleochroism (darkest colour is visible in the direction of ordinary ray – perpendicular to Z axis, or paralel – but only in the direction of the ordinary ray). The darkest colour is the result of the highest absorption. This can be explained by geometrical arrangement of chromophore-bearing octahedral sites. In the view in the direction of Z axis, octahedral positions occupy larger area than in the view perpendicular to the Z. Thus the probability of light absorption in the direction parallel to Z on chromophores located in octahedral positions is higher than in the perpendicular direction, which explains the differences in absorption in different directions. This are reasons why spectroscopic methods may have limits in their use on faceted gemstones.Although it was not possible to determine the cause of

decolourization in originally pink T8 spectroscopically, it can be attributed to change in Mn valence (Abduriyim et al. 2006).

REFERENCES

ABDURIYIM, A., KITAWAKI, H., FURUYA, M., SCHWARZ, D. (2006): Paraíba-type copper-bearing tourmaline from Brazil, Nigeria, and Mozambique: Chemical fingerprinting by LA-ICP-MS. Gems &

Gemology, 42, 1, 4-21.

BAČÍK, P., OZDÍN, D., MIGLIERINI, M., KARDOŠOVÁ,P.,PENTRÁK,M.&HALODA, J.(2011):Crystallochemical effects of heat treatment on Fe-dominant tourmalines from Dolní Bory (Czech Republic) and Vlachovo (Slovakia).Phys. Chem. Min.38,

599-611.

CASTAN᷉ EDA, C., EECKHOUT, S.C., DA COSTA, G.M., BOTELHO, N.F. & DE GRAVE,E.(2006): Effect of heat treatment on tourmaline from Brazil. Phys. Chem.

Min. 33, 207-216.

LAURS, B.M., SIMMONS, W.B., ROSSMAN, G.R.,FRITZ,E.A.,KOIVULA,J.I.,ANCKAR, B. & FALSTER, A.U. (2007): Yellow Mn- rich tourmaline from the Canary mining area, Zambia. Gems & Gemology, 43, 4, 314-331.

LAURS, B.M., ZWAAN, J.C., BREEDING, C.M., SIMMONS, W.B., BEATON, D., RIJSDIJK, K.F., BEFI, R. & FALSTER, A.U. (2008): Copper-Bearing (Paraíba-Type) Tourmaline From Mozambique. Gems &

Gemology, 44, 1, 4-30.

MERKEL P.B., BREEDING C.M., 2009: Spectral differentiation between copper and iron colorants in gem tourmalines.

Gems & Gemology, 45, 2, 112-119.

ROSSMAN, G.R., FRITSCH, E. & SHIGLEY, J.E. (1991): Origin of color in cuprian elbaite from São José da Batalha, Paraíba, Brazil.Am. Mineral.76,1479-1484.

SCHMETZER, K & BANK, H. (1979): East African tourmalines and their nomenclature. Journal of Gemmology, 16, 5, 310-311

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