Spectral and Chemical Analysis on Pink-Green Bicolor Tourmaline

Spectral and Chemical Analysis of Pink-Green Bicolor Tourmaline

HUANG Tiantian, GAO Yujie

Abstract: Seventeen pink-green bicolored tourmaline samples were studied by standard gemological testing, Fourier transform infrared spectroscopy, ultraviolet-visible spectroscopy, and chemical analysis by energy dispersive X-ray fluorescence spectrometry. All the samples exhibited both pink and green colors in various ratios. The refractive index of the pink part was 1.615–1.635, while that of the green part was 1.620–1.640. The Fourier transform infrared spectra showed prominent peaks at 4342, 4435, 4537, and 4579 cm⁻¹ along the E∥c direction. The last three peaks become weak in the E⊥c direction. The ultraviolet-visible spectrum of pink part showed a strong Mn-related peak at 511 nm, while the green part showed a Fe-related band at 715 nm. Chemical analysis showed a lower Fe/Mn ratio in the pink part and a higher one in the green part.

Key Words: bicolored tourmaline; fourier transform infrared spectroscopy; ultraviolet-visible spectroscopy; chemical analysis

Tourmaline is a colorful gem species that can produce almost all spectrum colors. Mineralogically, tourmaline represents a series of minerals with a common chemical formula of XY₃Z₆(T₆O₁₈)(BO₃)₃V₃W. Most compositional variability occurs at the X, Y, Z, W, and V sites [1]. The chemical and crystalline complexities give rise to the fancy colors and the varieties of tourmaline, making it a unique gem species in the trade.

Pink to red color tourmaline is appraised as rubellite in the trade owing to their similarity to ruby. A recent study also reports Mn-bearing pink-red tourmaline from Madagascar[2]. Some greenish-blue color tourmalines may exhibit an intense neon visual appearance, which can be attributed to a trace of copper, and they are called Paraiba tourmaline. The neon effect distinguishes Paraiba tourmaline from others, and they demand a high price in the trade. The auction houses keep refreshing the record price of Paraiba in recent years. Previous research has conducted a systematic study decoding the mystery of Paraiba[3-5]. One of the most mesmerizing things about tourmaline is the multiples color tourmaline, containing several kinds of color in one single gemstone. Liddicoatite is a calcium-rich lithium tourmaline. Moreover, it usually exhibited remarkable polychrome and varied geometric patterns, which were described in detail by Dirlam[6].

Since 2010, the color stone market in China has increased tremendously; the end customer starts to purchase tourmaline and tanzanite for jewelry. Large retailers such as Chow Tai Seng, a promoting the color stones through advertising, the trade media, and celebrity endorsement [7]. High-quality red color tourmaline (rubellite) and bi-colored are coveted by gem dealers and collectors. However, But the market’s enthusiasm for tourmaline began to decrease in around 2013-2014, replaced by ruby, sapphire and emerald. Recently, due to various reasons the COVID-19 and unstable sitations in the gem mines, the wave of tourmaline is likely recovering. However, the gemological properties have not well known to the public.

In this study, the authors have conducted a series of the testing method to further our understanding of the color continuation and variation of the pink-green bi-colored tourmaline. We will demonstrate our results and discuss the mechanism with the previous study as well.

1 Samples and methods

In this study, a parcel of seventeen faceted tourmalines was investigated by gemological, spectroscopic, and chemical methods. These samples are all bicolored as a combination of pink and green colors. Each sample was weighted with the total weight being 15.44 carats, ranging from 0.48 to 1.77 ct.

Gemological characteristics were studies by standard instruments. A refractometer with a near-sodium equivalent light source was used to measure refractive indices (R.I.). Fluorescence was observed under 365 nm long-wave and 254 nm short-wave U.V lights in a dark room. Inclusions were studied with an 80× magnification gemological microscope with Leica optics, equipped with different lighting sources. Fourier transform infrared (FTIR) spectroscopy was performed on a TENSOR Ⅱ FTIR spectrometer equipped with a KBr beam splitter and RT-DLaTGS detector. The spectra were recorded in the 400-2,000 cm⁻¹ range with a spectral resolution of 4 cm⁻¹ and 8 scans at 7.5 kHz scanning speed. UV-Vis absorption spectra were recorded in the 200–1000 nm range using a Gem-3000 Ultraviolet-visible spectrophotometer.

The chemical composition analyses were performed on a Spectro Midex type energy dispersive X-ray fluorescence (EDXRF) equipped with calculating software X-LabPro 5, using a Ta target with the spot size of 2 mm. The machine was calibrated by MCA(Multi-Channel Analysis) with a reference material supplied by the equipment supplier. The Modified RoHS+Bigspot method was used at an acceleration voltage of 19 KV and a beam current of 0.30 mA for Al, Si, K, Ca; while 48 kV, 0.60 mA for elements Ti, V, Cr, and Fe. Each sample was tested 2-3 points for accuracy.

2 Results and discussion

2.1 Gemological Properties

All the samples show distinct color sections of pink and green, usually with light to medium saturation and medium to high tone. Dichroism was observed in both parts, with the pink exhibiting purplish pink and light pink and the green part showing yellowish-green to bluish-green. All the samples were inert under long-wave fluorescence (365nm). The gemological properties are summarized in Table 1. Microscopic observation reveals contrasting scenarios in the pink and green parts. Distinct long parallel tubes containing fluids were only found in the green part. In contrast, the pink part usually contains many colorless mineral crystals, which show interference color under cross-polarized lightening conditions, as shown in Figure 4 and Figure 5.

Table 1 General Gemological Properties of Bicolor Tourmaline in this study

2.2 FTIR Spectrum

Both reflectance and transmission FTIR spectrum of all samples were collected. The reflectance spectrums were practiced on the pink and green parts separately. The results showed that these two parts generally show similar if not identical patterns, with prominent peaks at 504,715,790,992, 1114 cm⁻¹, accompanied by several small peaks at 597, 635, 1282 1360 cm⁻¹. A gentle shoulder was detected at 1039 cm-1, as shown in figure 6. No distinct differences in FTIR fingerprint were observed.

As mentioned above, these samples all contain long parallel tubes. Since tourmaline is a trigonal three-fold ring silicate mineral, the tube can serve well as good indicators of the optical axis of the tourmaline host. To better understand the structural O.H. and other possible inclusions, we have recorded the transmission spectrum from two directions: parallel to the c- axis (E∥c) and perpendicular to the c-axis (E ⊥ c).

Generally, A series of small peaks between was detected in the 2400-3300 cm⁻¹ perpendicular to c-axis, while they almost disappeared and leaving only several, with an increasing peak at 3010 cm⁻¹ and two shoulders ate 3154 and 3258 cm⁻¹ in the E∥c direction. A noisy broad band centered at 3550 cm⁻¹ was tested in both directions, without a distinct difference. A similar situation happened to several tiny peaks at 4878, 5140, and 5197 cm⁻¹. In the parallel to c-axis direction, four prominent peaks were detected at 4342, 4435, 4537, and 4579 cm⁻¹. The last three become weak when the testing direction changes to perpendicular to the c-axis. A prominent peak at 6998 cm⁻¹ was tested in the E∥c direction, accompanied by two small peaks at 6747 and 7140 cm⁻¹; when the direction convert into E ⊥ c, the 6998 cm⁻¹ peaks decreased a little, and the other two increased accordingly.

Table 2 Summary of the transmission FTIR spectrum features tested in two directions

2.3 UV-Vis Spectrum

UV-Vis spectroscope is a valuable tool to study the colors and their relevance with trace elements. We have collected two spectrums from each sample for all samples in this research, one from the pink part and another from the green part. Two representative spectrums were illustrated in figure 8, with the red line representing the pink color part and the green line for the green part accordingly. The prominent band centered at 511 nm in the pink part, a tiny peak accompanied nm at 450 nm, both originating from Mn³⁺[8]. The weak peak due to Cr³⁺ was positioned at 615nm, consistent with chrome tourmaline, and the 690nm peak due to Fe²⁺. Mn-related absorption takes away most of the blue and green part of the visible light, while the Fe-related peaks have little influence on the orange and red color part, thus, resulting in pink color. In the green part, by comparison, the distinct broadband centered at 715nm was caused by the Fe²⁺ absorbing most of the red color, with only a fragile band at 560 nm attributed to Mn²⁺[9]. Once again, these results prove UV-Vis as powerful to unveil the mysterious origin of color in a gemstone.

2.4 EDXRF Chemical Analysis

Tourmaline is a very complex mineral, and abundant kind of elements are evolving in the crystalline structure. We have carefully selected four elements to discuss, of which Mn and Fe are most important. The results are present separately based on the pink and green regions in Table 3. No considerable copper was detected in these samples. Cr was generally at a deficient level, insufficient to give rise to green color. Although not directly responsible for coloration, Zn is generally higher in the green part than in the pink region, with an average Zngreen/Znpink=6.74. The interesting pattern among Mn and Fe were discovered, as shown in figure 9. The Fe/Mn content ratio in the pink part is designated as PinkFe/Mn, while green will be designated as GreenFe/Mn. Generally, all the samples show PinkFe/Mn much lower than GreenFe/Mn, except for No.2 and No.3 samples. These abnormal values found in No.2 and No.3 can be attributed to the relatively small area of pink and the large size of the testing beam of the EDXRF machine.

Table 3 Chemical Compositions of Bicolor Tourmaline by EDXRF

3 Conclusions

This article investigated the samples by a series of gemological, spectral, and chemical testing methods. The pink and green showed quite different R.I. values and very different inclusion features. The Raman spectrum will be applied in future research to identify and analyze this crystal inclusion and the fluids within the long tube. The spectrum and chemical analysis clearly showed that Mn and Fe are the main coloration agents in the bi-colored pink and green tourmaline. However, the mechanism of color is still not clear. We will carry more experiments to further study the gradient of traces element and their relation with the color.

Acknowledgment

The authors are grateful to Ms. Ma Rui for kindly provide somes smaples. Huixin Zhao is thanked for photographing. We also thank Han Qi for their assitance during the spectral and chemical testing.

References

[1] Henry D J, Novak M, Hawthorne F C, et al. Nomenclature of the tourmaline-supergroup minerals[J]. American Mineralogist, 2011, 96(5):895-913.

[2] Bosi F, Celata B, Skogby H, et al. Mn-bearing purplish-red tourmaline from the Anjanabonoina pegmatite, Madagascar[J]. Mineralogical Magazine, 2021, 85(2):242–253.

[3] Abduriyim A, Kitawaki H, Furuya M, et al. “Paraíba” -Type Copper-Bearing Tourmaline from Brazil, Nigeria, and Mozambique: Chemical Fingerprinting by LA-ICP-MS[J]. Gems & Gemology, 2006.

[4] Laurs, Brendan M, Zwaan, et al. Copper-bearing (paraíba-type) tourmaline from mozambique[J]. Gems & Gemology, 2008.

[5] Katsurada Y, Sun Z, Breeding C M, et al. Geographic Origin Determination of Paraiba Tourmaline[J]. Gems and Gemology, 2019, 55(4).

[6] Dirlam D.M., Laurs B.M., Pezzotta F. and Simmons W.B. Liddicoatite tourmaline from Anjanabonoina, Madagascar[J]. Gems & Gemology, 2002.

[7] Qingyuan, Yu, Zhili, et al. Exploring the Chinese Gem and Jewelry Industry[J]. Gems and Gemology, 2014, 50(1):2-29.

[8] Fan J, Fe Ng X, Guo S, et al. Optical absorption spectra of tourmaline crystals[J]. Journal of the Chinese Ceramic Society, 2009, 37(4):523-530.

[9] Castaneda, C., Eeckhout S G, GMD Costa, et al. Effect of heat treatment on tourmaline from Brazil[J]. Physics and Chemistry of Minerals, 2006, 33(3):207-216.