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Article

Spectroscopy Characteristics and Color-Influencing Factors of Green Iron-Bearing Elbaite

by
Lianyi Cui
,
Ying Guo
*,
Jun Tang
and
Yushu Yang
School of Gemmology, China University of Geoscience (Beijing), Beijing 100083, China
*
Author to whom correspondence should be addressed.
Crystals 2023, 13(10), 1461; https://doi.org/10.3390/cryst13101461
Submission received: 11 September 2023 / Revised: 29 September 2023 / Accepted: 1 October 2023 / Published: 5 October 2023
(This article belongs to the Section Mineralogical Crystallography and Biomineralization)
Browse Figures
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Abstract

:
The color-influencing factors and spectroscopy of 22 green elbaite samples were investigated using X-Rite SP62 spectrophotometry, ultraviolet–visible (UV–Vis) spectroscopy, laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), and infrared spectroscopy. The chromogenic components iron and manganese were found in the green elbaites; however, the bivariate correlation analysis indicated that the Mn content had no impact on the color, whereas the Fe content significantly affected both the lightness and the hue of green elbaites. The primary factors influencing the color of tourmaline were the absorption band at 720 nm caused by the Fe2+ d-d transitions and the 300 to 400 nm wide absorption edge extending to the visible range due to the O2−-Fe3+ charge transfer. Infrared spectroscopy indicated that the color of tourmalines was also influenced by their structure. As the degree of Y and Z octahedral distortion in the tourmaline lattice increased, the lightness of the tourmaline decreased and its color deepened. The hydroxyl region of the infrared spectra of green elbaite showed three distinct peak positions representing two types of hydroxyl vibrations: O1H at the W position and O3H at the V position. The O1H vibrations are attributed to YLiYAlYAl and XNa or X position vacancy, while the O3H vibrations are assigned to ZAlZAlYAl and ZAlZAlYLi. The presence of three peaks in the hydroxyl vibrational region of the IR spectra indicated that these samples were iron-bearing elbaites.

1. Introduction

The first records of tourmaline were discovered in ancient Ceylon, and it was not until the 1960s in Africa that tourmaline became known as a gemstone material [1]. Because of its rich and unusual color, gem-tourmaline is particularly appealing to a large number of consumers as a mid-range gemstone material. According to the different elements that fill its different lattice positions, tourmaline can be classified into at least 38 mineral species [2,3]. Elbaites make up the majority of gem-quality tourmalines, with only a trace amount of dravites, liddicoatites, and transitional types of the dravite-uvite series.
The color banding of tourmaline in the vertical c-axis direction is indicative of the evolution of the protoliths, and the large number of isotopes contained in tourmaline can be traced to its mineralization and rock-forming environments, so tourmaline has good significance in the exploration of mineral deposits [4,5]. In addition, tourmaline has piezoelectric and thermoelectric properties, which give it a variety of applications in medicine, electronics, chemistry, and environmental protection [6,7,8,9].
Tourmaline is a cyclic silicate mineral, and currently, its internationally recognized chemical formula is XY3Z6(T6O18)(BO3)3V3W. The alkali metal cations Na, K, and Ca mainly occupy the X position with a coordination number of 9. In dravites, the X position is partially filled with Mg2+, but it could also be vacant. The Y sites are Al3+, Fe3+, Cr3+, V3+, Mg2+, Fe2+, Mn 2+, Cu2+, Zn2+, Li+, Ti4+, or vacancy, coordinated to one ion in the V site, one ion in the W site, and four oxygen ions [10]. The Z position is the crystal site, which is almost always filled with Al3+, and some Fe3+, Cr3+, V3+, Mg2+, and Fe2+ ions will substitute for Al3+ in the Z site lattice. The T position is mainly filled with Si4+ with a coordination number of 4. In a few cases, Si4+ at the T position is replaced by B3+ to form boron-rich tourmaline. The B position is B3+ with no obvious substitution, and the coordination number is 3. The W(O1) positions are OH−, F−, and O2−, which have a cubic coordination. The V(O3) sites are OH− and O2− with a coordination number of 3.
The isomorphous substitution of ions forms the different colors of tourmalines [11]. The cations in the structure (such as Fe, Mn, Cr, etc.) exist in a wide range of isomorphous substitutions, which gives tourmaline a very rich color [12,13]. There are no or very few transition metal ions in colorless tourmaline.
The red hue of tourmaline is attributed to the d-d transition of octahedral Mn3+, Mn2+, or Mn2+-Mn3+ intervalence charge transfer (IVCT) in its crystal structure [14,15,16]. The deep blue color is due to Fe2+, Fe3+, and Mn3+ [17]. The “neon” blue color of Paraíba tourmaline is attributed to Cu2+ and Mn3+ [18,19]. The yellow color could be associated with both the Fe2+-Ti4+ and Fe2+-Fe3+ intervalence charge transfers [20]. Regarding the color genesis of green tourmalines, many previous experimental studies have shown that it is due to the isomorphous substitution of transition metal ions, such as Cr, V, and Fe, at the Y or Z position in its structure. It has also been shown that the green color of tourmaline is due to defects in the crystal structure of tourmaline [21,22,23].
The CIE 1976 L*a*b* uniform color space is the most recent phenocopying system for quantitative color characterization, recommended by the International Commission on Illumination [24]. L* represents the lightness. When L* is 0, it represents black, and when L* is 100, it represents white. a* represents red–green hues, where −a* indicates green and +a* indicates red. b* represents the transition from blue to yellow, where −b* represents blue and +b* represents yellow.
With the development of gemstone colorimetry in recent years, researchers can study gemstone color more intuitively and accurately by using colorimetric methods, including quantitative characterization and grading of gemstone color [25], quality evaluation [26], and finding suitable light sources and backgrounds for gemstones [27].
The color of a gemstone is usually influenced by both internal and external factors. The internal factors include transition metal ions, crystal structure, and internal inclusions, and the external factors include the light source and background. Although the effects of the light source and background on green and red tourmalines have been studied by previous researchers using colorimetry methods [28], the relationship between the color and its composition has not been quantitatively discussed in the context of the major chromophores.
In this work, the effects of the transition metal element content and internal structure on the color of green iron-containing elbaites were investigated by LA-ICP-MS, colorimetry methods, and infrared spectroscopy. The color mechanism of green elbaites was analyzed by the UV–visible spectrum.

2. Materials and Methods

2.1. Materials

All samples were cut parallel to the c-axis and shaped as parallel slabs approximately 3.5 mm thick, and then polished on both facets. The specimens were evenly distributed in color and were internally clean with no obvious inclusions. Some of the tourmaline samples are shown in Figure 1. All specimens underwent color parameter testing, and only 15 specimens with a uniform hue and clean interiors were selected for composition and color testing.
The gemological characteristics of the analyzed tourmalines are given in Table 1. The color of the samples varied from light blue–green to green to yellowish green, and their transparency ranged from transparent to sub-transparent. The RI (refractive index) values were between 1.559 and 1.663, and the DR (double refraction) values were between 0.018 and 0.037. The SG (specific gravity) varied between 2.99 and 3.19 g/cm3. Most of the samples were examined under a gemstone microscope to see “tear-like” gas–liquid inclusions and healing cracks.

2.2. Methods

2.2.1. X-Rite SP62

The color data of the tourmalines were measured using an X-Rite SP62 spectrophotometer from Michigan, USA. The measurements were made against a background of N9 gray Munsell neutral color chips. Using the instrument’s built-in D65 light source, an integrating sphere was used to capture the signal reflected from the sample surface, with the final measurement being the average of three readings. The test is characterized as follows: specular reflections are excluded; the observer’s field of view is 10°; the measurement range is 400~700 nm; the spectral wavelength spacing is 10 nm; the measurement aperture is 4 mm; and the illumination is 6.5 mm.

2.2.2. UV–Vis Spectroscopy

UV–Vis spectral data were collected at the Gem Lab, School of Gemology, China University of Geosciences in Beijing using a PE-Lambda 950 UV–Vis spectrophotometer (PerkinElmer, Waltham, MA, USA). The test parameters are shown below: the test method is transmission; the range of measurement is 300~800 nm; the sampling interval is 1.0 s; the scanning speed is medium; and the scanning mode is single.

2.2.3. LA-ICP-MS

The National Geological Experimental Testing Center of the Chinese Academy of Geological Sciences conducted the in situ LA-ICP-MS experiments. Multi-element quantification of the tourmaline samples was performed using a J-10 femtosecond laser (Applied Spectra Inc., West Sacramento, CA, USA) in conjunction with Thermo X-Series quadrupole mass spectrometry(Thermo Fisher Scientific, Bremen, Germany). The parameters are: laser ablation spot diameter of 50 μm, frequency 8 Hz, and energy 1.08 J/cm2. The analysis time of each sample site is 60 s, the blank acquisition time is 20 s, the continuous collection time of the sample is 40 s, and for each 15 sample points, BCR2G, KL2-G, and NIST610 reference materials are analyzed to correct for quality discrimination and instrument sensitivity drift. All element concentrations are calculated with 29Si as the internal standard. The detection limit of trace elements is (0.05~0.1) × 10−6, and the accuracy is within 10%.

2.2.4. Infrared Spectroscopy

A Bruker tensor 27 Fourier transform infrared spectrometer was used to collect the sample’s infrared spectrum (IR). Here are the details of the experimental conditions: the test method is the reflection method; the wavelength range is 400–4000 cm−1; the scanning is repeated 32 times; and the resolution is 4 cm−1.

3. Results

3.1. Color Analysis

The color (E//c) of 22 tourmaline samples was measured by applying a CIE D65 standard light source and an N9 Munsell neutral background as the test condition. Their lightness L* (32.11 to 73.45), colorimetric coordinates a* (−4.02 to −17.32) and b* (2.11 to 20. 24), chroma C* (7.93 to 21.67), and hue angle h° (112 to 170.9) were measured. The results indicate that the color data are consistent with the appearance of green tourmalines.
In order to facilitate the observations of the overall color distribution of the samples, the three-dimensional color data of the 22 samples were plotted as a three-dimensional scatter diagram, as shown in Figure 2, in which the X, Y, and Z axes represent a*, b*, and L*, respectively. According to Figure 2, the samples in this work had a medium to high lightness and hue ranging from blue–green to green to yellow–green.
The relationship between chroma, hue angle, and color coordinates in 22 samples was analyzed by bivariate correlation. To better quantify the degree of association between two random variables, the Pearson correlation coefficient r was introduced. The range of values for r is (−1, 1), with positive and negative values corresponding to the positive and negative correlations of two variables. The higher the absolute value of r, the stronger the correlation [29].
The chromaticity coordinate b* was substantially positively related to its chroma C* through an analysis of the color parameters of 22 tourmalines, with a Pearson correlation coefficient r of 0.850. By curve fitting the relationship between the chroma coordinate b* and its chroma C*, the goodness of fit R2 was obtained as 0.899, as shown in Figure 3a. The R2 goodness of fit measures how well the regression line fits the observations, with a maximum value of 1. A value of R2 closer to 1 indicates a better fit between the regression line and the observations. The +b* axis represents yellow, and the chroma C* increases almost linearly with an increase of color coordinate b*, which indicates the yellow hue primarily influences the chroma of the tourmalines.
In addition, according to Figure 3b, the hue angle h° decreases with an increase of b*, where r is −0.824 and R2 is 0.909. In contrast to b*, a* has a much weaker correlation with the hue angle h° and the chroma C*. Therefore, both the hue angle and the chroma of green tourmalines are primarily determined by the color coordinate b*, which is the yellow hue.

3.2. Chemical Composition

The structure of tourmaline can be described by the coordination of ions to the cation as the coordination center, which has been divided into [XO9], [YO4VW], [ZO5V], [Si6O18], and [BO3] as the five main components [30]. Assuming that the five significant lattice positions where the cations of the coordination centers are located are considered a “structural unit,” then the crystal of tourmaline is actually the result of the continuous stacking of such structural units in three-dimensional space, as shown in Figure 4.
Figure 5 shows the lattice positions of the basic structural units of tourmaline, with the silica–oxygen tetrahedron SiO4 forming the Si6O18 hexagonal ring, while the Y-site ions form a layered magnesium hydroxide-type configuration with O2− and OH−. The [Y-O4(OH)2] coordinated octahedra are connected to the Si6O18 double tripartite ring by O2− at the vertices of the SiO4 tetrahedra. The three Y-octahedra are co-prismatic and intersect to form an octahedral layer, with the location of the intersections just below the center of the Si6O18 double tripartite ring, which is the W site in the lattice. Between the Si6O18 hexagonal ring and the Y-octahedral layer, there are three planar [BO3] triangular sheets composed of B atoms with surrounding oxygen atoms, and the [BO3] triangular sheets are connected to the Y-octahedron by O2−. Z ions connect these complex anions. [Z-O5(OH)] octahedrons share a prism with [Y-O4(OH)2] octahedra in the octahedral layer. The X ion with coordination number 9 is in the upper vacancy in the center of the Si6O18 double tripartite ring.
Laser ablation inductively coupled plasma mass spectrometer (LA-ICP-MS) analysis is becoming a popular method for measuring the chemical composition of gemstones [31], as it offers a cheap, clean, fast, and mostly non-destructive analysis, and it also allows the detection of some elements with a light atomic weight, such as Li.
This work performed laser ablation inductively coupled plasma mass spectrometer tests on 15 specimens, and the results are shown in Table 2. Although certain elements (e.g., H, F, and B) and their contents that may be present in some tourmaline crystals could not be measured due to the limitations of the instrumental testing range, they do not affect this research or the discussion of the greening genesis of tourmaline.
The transition elements Fe, Mn, Cr, V, and Cu are known as important chromogenic and chromophores in tourmaline. The results showed that the tourmaline samples contained V (0 to 16.9 ppm), Cr (1.4 to 9.0 ppm), Mn (3063 to 18,160 ppm), Fe (1996 to 32,085 ppm), and Cu (0 to 9.5 ppm), and the content of iron and manganese was the total content detected, regardless of the valence state. The detected contents of Cr, V, and Cu were very small, and their contribution to the tourmaline color was very weak; therefore, this work mainly analyzed the effects of the transition metal elements Fe and Mn on the tourmaline color.
The atoms per formula unit were calculated for each sample using the method proposed by Ziyin [31] et al. The results are shown in Table 2. This method is based on the calculations of Henry et al. with modifications for the additional limitations of LA-ICP-MS. LA-ICP-MS uses 40% SiO2 as the internal standard for testing, so the atoms per formula unit of Si is 6. The fluor or oxy species could not be determined due to the inability of LA-ICP-MS to measure the F content and the valence states of Fe and Mn. Therefore, fluor-elbaite is also classified as elbaite in this work.
The following assumptions are necessary for the classification of tourmalines based on raw LA-ICP-MS data: first, the W and V sites are completely occupied by hydroxyl groups; second, there is no tetrahedral boron in the lattice, and the atoms per formula unit of B is 3; and third, manganese and iron are both divalent. The assumption that iron is divalent makes it impossible to determine the iron-dominant species, such as povondraite and bosiite. However, these tourmaline subspecies are generally not gem-quality.
The species of the tourmaline samples in this work were determined according to the simplified classification process for tourmaline species proposed by Ziyun et al. Figure 6 shows a flowchart of the tourmaline species classification. The Ca-Na (+K)-vacancy ternary plot establishes the dominance of the X-site in tourmaline (Figure 6a), which is the first step in species classification. All of the tourmaline samples belonged to the alkali-primary group, according to Figure 6a. The ternary plot of the Al-V-Cr subsystem determines the Z-site dominant element (Figure 6b), which further separates the tourmaline species. Al3+ dominated the Z-site in all samples. The dravite-schorl-elbaite ternary subsystem determines the Y-site dominant element (Figure 6c). Li+ was the dominant monovalent cation at the Y-site of the tourmaline samples. Consequently, all of the samples of tourmaline were classified as elbaite.

3.3. UV–Vis Spectral

To further investigate how transition metal elements affect the color of tourmalines, the samples were tested by UV–visible spectroscopy. According to Figure 7, the UV–visible spectrum of the tourmalines largely agrees with that of the green elbaite studied by Agostinho et al. [32]. Minor differences in the UV–visible spectral characteristics of the tourmalines numbered Tg-1 to 5 (Figure 7a) and Tg-7 to 14 (Figure 7b) can be attributed to the higher transparency and lower iron content of the tourmalines numbered Tg-1 to 5.
The UV–vis spectra of the tourmaline samples mainly show two strong absorbing broad bands (300~400 and 720 nm), one shoulder peak (670 nm), and four weak absorbing bands (415 nm, 470 nm, 497 nm, and 583 nm). The peaks at 300 to 400 nm and 400 to 500 nm primarily absorb blue–violet light; the peaks at 583 and 720 nm mainly absorb orange–red light. The transmission window in the green region in the 510 to 550 nm range contributes to the greenish hue of tourmaline.
The strong absorption edge extending into the visible region around 300–400 nm is consistent with the optical absorption band observed in green tourmalines by Faye et al. They suggest that the presence of this band is indicative of the presence of O2−-Fe3+ or O2−-Fe2+ charge transfer transitions in tourmaline. However, Faye found that in the majority of Fe-bearing minerals, the charge transfer between O2− and Fe3+ had a stronger effect on the color. Agostinho et al. attribute the weak absorption band at 413 nm to the Fe2+-Ti4+ IVCT interaction of the green tourmaline [32]. In green elbaites, this band is weaker. Weak absorption bands at 470, 497, and 583 nm are, respectively, due to Fe3+ d-d electron transition, crystal field-allowed transition 6A1 → 4E1 + 4A1 (4G), and Fe2+-Fe3+ IVCT interactions.
The absorption of Fe2+ in the octahedral lattice of tourmaline has two manifestations in the UV–visible spectrum: the absorption peak at 670 nm is indicative of Fe2+ in the Z-site, and the absorption band at 760 nm is due to Fe2+ in the Y-site [33]. Therefore, the absorption at 670 nm in this paper is indicative of a spin-forbidden transition of Fe2+ occupying the Z site. The broad absorption band near 720 nm, located between 670 and 760 nm, indicates that Fe2+ is present in both Y and Z octahedrons. The crystal field transition 5T2g → 5Eg of a single Fe2+ ion is responsible for this absorption peak [34].
In summary, due to the weak absorptions at 400~500 nm and its proximity to the invisible region, as well as the weak intensity of the 583 nm absorption bands and the 670 nm shoulder peaks, the contribution of all of these absorption bands to the color of tourmaline is very weak. Consequently, in green elbaites, the absorption edge at 300~400 nm extending into the visible region due to O2−-Fe3+ charge transfer and the absorption broadband near 720 nm caused by Fe2+ d-d transitions are the primary contributors to the green coloration.

3.4. Infrared Spectral

Ten tourmaline samples were examined by infrared spectroscopy along the vertical c-axis direction to further investigate the group vibrational properties of the tourmalines. The infrared spectra of the ten green tourmaline samples have approximately the same main peak position in the fingerprint region as that of ordinary green tourmaline (Figure 8), and their molecular skeletons are consistent with tourmaline-like minerals, whose absorptions in the fingerprint region of the samples are mainly as shown in Figure 7 (400 to 1500 cm−1). The vibrational bands of metal cations in the infrared spectra are generally lower than 400 cm−1 or close to 400 cm−1, and the vibrational frequency of each coordination polyhedron is [MgO6]—470 cm−1, [FeO6]—400 cm−1, [CaO8]—380 cm−1, and [NaO8]—270 cm−1 [35]. All samples have weak absorptions at 428 and 449 cm−1, which may be associated with [FeO6] and [MgO6], respectively. The bending vibration of the [BO3] group is responsible for the absorption at 503 cm−1 [36]. The absorption peak at 500~600 cm−1 is influenced by Si-O stretching vibrations [37]. The absorption at 600~800 cm−1 is related to the νs Si-O-Si bond produced by its bond distortion. The absorptions at 988 cm−1 and 1031 cm−1 are attributed to the νs and νas of O-Si-O, respectively. The νas of Si-O-Si are responsible for the absorptions around 1056 cm−1 and 1112 cm−1. The [BO3] stretching vibrations are responsible for the two absorption peaks around 1300 cm−1.
There are three peaks in the region of 3000 to 3750 cm−1 for structural water and adsorbed water, and the intensity of the higher wavenumber absorption band is extremely low and difficult to observe, which is attributed to the interference of adsorbed water, according to Yunhui Tang. The IR spectra show three peaks in the hydroxyl vibrational region, indicating the samples are iron-containing elbaites.
In Figure 7, the ordinate scale in the 3400 to 3800 cm−1 range is reduced to facilitate observation. There are two different OH− ions in the structure of the tourmaline, occupying either the W or V site. The O1H stretching vibrations at the W position are responsible for the vibrational bands with wavenumbers above 3600 cm−1, while the O3H stretching vibrations at the V position are responsible for the vibrational bands with wavenumbers below 3600 cm−1 [38]. Thus, the absorption around 3636 cm−1 of the tourmaline samples is associated with the O1H stretching vibration band at the W position in the tourmaline lattice, and the absorptions around 3562 and 3449 cm−1 are due to the O3H stretching vibration at the V position.
O1H, which occupies the W site, is the intersection of three Y-octahedra and is located directly below the X site cation. On the other hand, O3H occupying the V site is coordinated with one Y site and two Z sites, and its vibrational frequency is also influenced by the cations at the Y and Z sites. In elbaite, the Li+ and Al3+ are in the Y-site, while Al3+ occupies the Z-site. Therefore, due to the influence of both the Y position and the overlying X position cations, the 3636 cm−1 OH vibration generated at the O1 site is assigned to YLiYAlYAl and XNa or X site vacancy. The OH vibrational spectra of 3449 and 3562 cm−1 produced by the green tourmaline samples at the O3 point position can be attributed to ZAlZAlYAl and ZAlZAlYLi.

4. Discussion

4.1. Influence of the Fe and Mn Content on Color

The relationship between the Fe and Mn content and color parameters was analyzed using bivariate correlation analysis in the green tourmaline samples, and their Pearson’s r correlations are shown in Table 3. The analysis shows that the Fe content significantly affects the L* and h° of green elbaites, while the Mn content has a weak correlation with its color parameters. It is probable that Mn has no influence on the color of tourmalines. The above speculation is further supported by the absence of Mn-caused absorption observed in the UV–visible spectra of green elbaites. Analysis of the tourmaline crystal structure shows that Mn does not occupy the Y site, but rather undergoes an isomorphous substitution with Al3+ on the Z site, and thus does not contribute to the tourmaline color.
The lightness showed a significant negative correlation with the Fe content, with a Pearson’s r of −0.882. The hue angle also showed a moderately significant negative correlation with the Fe content, with a Pearson’s r of −0.639. With an increase of Fe content, the lightness L* of the tourmaline samples decreased almost linearly (Figure 9a), and the color deepened. At the same time, the hue angle h° also showed a decreasing trend (Figure 9b), and the color changed from light blue–green to yellow–green. Mn does not contribute to the color of green elbaites, while iron is the major color-causing element.
To further investigate how the iron content affects the color of tourmalines, the relationship between the intensity of the 720 nm absorption band of the UV–visible spectrum and the iron content was analyzed, as shown in Figure 9c. The intensity of the 720 nm absorption band increases as the iron content increases. The more energy that the tourmaline absorbs in the visible region, the less energy it transmits, which leads to a reduction in the lightness of the tourmaline. The absorption band at 720 nm is caused by the crystal field transition of Fe2+. This indicates that the more iron ions enter the tourmaline lattice, the more energy is absorbed by the electron transition, and its lightness decreases. This is consistent with the finding that the iron content and lightness L* of a tourmaline are strongly negatively correlated.

4.2. Influence of Crystal Structure on Tourmaline Color

The large numbers and values of the O3H hydroxyl absorption bands indicate a more pronounced structural distortion of the ligand associated with O3. O3 has one Y ion and two Z ions as ligands. Thus, the high number of spectral bands and the large values of the O3H vibrational bands indicate the presence of large structural distortions in both the Y- and Z-octahedra in green tourmalines. The magnitude of the wavenumber of the O3H vibrational bands provides a measure of the degree of Y and Z octahedral distortion in the lattice. According to Figure 10a, the color of a tourmaline deepens as the number of O3H vibrational bands increases, and it gradually shifts toward the long wave direction.
The wavenumbers of the O3H vibrational bands show a significant negative correlation with their lightness L*, as shown in Figure 10b. In other words, structural aberrations can significantly influence the color of a tourmaline. As the degree of Y and Z octahedral distortion in the tourmaline lattice increases, the lightness of the tourmaline decreases, and the color deepens. The structural distortion of the Y-site octahedron leads to a larger M-O distance and a consequent decrease in the octahedral field splitting parameter value [39]. As the octahedral field splitting coefficient value decreases, the wavenumber of light absorbed by electron leaps grows, and the color of the tourmaline becomes darker. The structural distortion of the Z-octahedron affects the distortion of the Y-octahedron co-ribbed with it through O3, which indirectly affects the color of the tourmaline.

5. Conclusions

In the lattice of the tourmaline samples, the elements that predominantly occupied the X position were Na+ and K+, initially classifying the samples into alkali groups. The ion that dominated the Z-site was Al3+, further determining the species of the samples. Li+ dominated at the Y-site, ultimately classifying the tourmaline samples as elbaite. Mn, as a transition metal element, did not contribute to the color of the green elbaite, while iron was the dominant color-causing ion. The lightness L* of the tourmaline samples decreased almost linearly with increasing iron content, and the color deepened, while the hue angle h° also tended to decrease in general, with the color shifting from light blue–green to yellow–green. In green elbaites, the absorption edge at 300~400 nm extended into the visible region due to O2−-Fe3+ charge transfer and the absorption broadband near 720 nm caused by Fe2+ d-d transitions were the primary contributors to the green coloration. Structural aberrations can significantly influence the color of a tourmaline. The wavenumbers of the O3H vibrational bands showed a significant negative correlation with their lightness L*. As the degree of Y and Z octahedral distortion in the tourmaline lattice increased, the lightness of the tourmaline decreased and the color deepened.

Author Contributions

Conceptualization, L.C.; methodology, L.C., Y.G., J.T. and Y.Y.; validation, Y.G., J.T. and Y.Y.; formal analysis, L.C.; investigation, L.C.; resources, L.C.; data curation, L.C.; writing—original draft preparation, L.C.; supervision, Y.G., J.T. and Y.Y. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

Not applicable.

Acknowledgments

The rest of the experiments in this research were conducted in the laboratories of the Gemological Institute, China University of Geosciences, Beijing.

Conflicts of Interest

The authors declare no conflict of interest.

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Crystals 13 01461 g001
Figure 1. Photograph under standard D65 light of some samples used in the present study.
Figure 1. Photograph under standard D65 light of some samples used in the present study.
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Figure 2. The tourmaline samples in a CIE 1976 L*a*b* uniform color space project plot.
Figure 2. The tourmaline samples in a CIE 1976 L*a*b* uniform color space project plot.
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Crystals 13 01461 g003
Figure 3. Color analysis of the samples. (a) The color coordinate b* has a strong positive correlation with its chroma C*; (b) The color coordinate b* is highly negatively correlated with its hue angle h°.
Figure 3. Color analysis of the samples. (a) The color coordinate b* has a strong positive correlation with its chroma C*; (b) The color coordinate b* is highly negatively correlated with its hue angle h°.
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Crystals 13 01461 g004
Figure 4. Crystal structure of tourmaline in the vertical c-axis direction (2 × 2 × 2 cell).
Figure 4. Crystal structure of tourmaline in the vertical c-axis direction (2 × 2 × 2 cell).
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Figure 5. Lattice positions of the basic structural units of tourmaline. (a) Individual lattice positions along the vertical c-axis direction; (b) Lattice positions in the parallel c-axis direction.
Figure 5. Lattice positions of the basic structural units of tourmaline. (a) Individual lattice positions along the vertical c-axis direction; (b) Lattice positions in the parallel c-axis direction.
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Figure 6. The flowchart of the tourmaline species classification. (a) Ternary system for the primary tourmaline groups based on the dominant occupancy of the X site; (b) Ternary diagram for the Al-V-Cr subsystem; (c) Ternary dravite-schorl-elbaite subsystem.
Figure 6. The flowchart of the tourmaline species classification. (a) Ternary system for the primary tourmaline groups based on the dominant occupancy of the X site; (b) Ternary diagram for the Al-V-Cr subsystem; (c) Ternary dravite-schorl-elbaite subsystem.
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Crystals 13 01461 g007
Figure 7. The UV–Vis spectrum of the samples. (a) The UV–Vis spectra of samples Tg-1 to 5. (b) The UV–visible spectra of Tg-7 to 14.
Figure 7. The UV–Vis spectrum of the samples. (a) The UV–Vis spectra of samples Tg-1 to 5. (b) The UV–visible spectra of Tg-7 to 14.
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Figure 8. The infrared spectra of green elbaites (ν: stretching vibrations; νs: symmetric stretching vibrations; νas: asymmetric stretching vibrations; δ: deformation vibration).
Figure 8. The infrared spectra of green elbaites (ν: stretching vibrations; νs: symmetric stretching vibrations; νas: asymmetric stretching vibrations; δ: deformation vibration).
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Figure 9. Relationship between the iron content and the color. (a) The iron content is strongly negatively correlated with L*; (b) The iron content is negatively correlated with h°; (c) The absorption intensity at 720 nm is positively correlated with the iron content.
Figure 9. Relationship between the iron content and the color. (a) The iron content is strongly negatively correlated with L*; (b) The iron content is negatively correlated with h°; (c) The absorption intensity at 720 nm is positively correlated with the iron content.
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Figure 10. The figure shows the shift of the infrared spectrum in the high-frequency range. (a) The tourmaline color deepens as the O3H vibrational band is shifted toward the long wave direction. (b) The O3H vibrational band wavenumber is negatively correlated with its lightness L*.
Figure 10. The figure shows the shift of the infrared spectrum in the high-frequency range. (a) The tourmaline color deepens as the O3H vibrational band is shifted toward the long wave direction. (b) The O3H vibrational band wavenumber is negatively correlated with its lightness L*.
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Table 1. Gemological properties of the tourmaline samples.
Table 1. Gemological properties of the tourmaline samples.
PropertiesResults
ColorLight bluish green-green-yellowish green
Weight0.799~6.828 ct
Specific gravity2.99~3.19 g/cm3
Refractive index1.618~1.663
Birefringence0.018~0.037
TransparencySub-transparent to transparent
InclusionsTear-shaped gas-liquid inclusions
Table 2. Chemical composition of the tourmaline samples by LA-ICP-MS.
Table 2. Chemical composition of the tourmaline samples by LA-ICP-MS.
Tg-1Tg-2Tg-3Tg-4Tg-5Tg-6Tg-7Tg-8Tg-9Tg-10Tg-12Tg-14Tg-18
Obtained and normalized from LA-ICP-MS * (ppm)
Li10,70510,52511,14010,81510,720884910,30911,35411,07210,23411,59798239685
Na17,89518,79520,46917,596179,4417,04318,24519,50320,24921,79621,15821,77819,539
Mg0.614.633.60.018.71.191.5220121497277511991399
Al228,663230,511228,571238,938215,007234,878215,074228,060219,771232,427233,992224,552215,408
K157164201136134110127130173181139177164
Ca381128926280388461299745961279912,1053326806930259064
Ti6.259.757.05.626.013.71313288134891318521912
V0.20.10.30.20.20.20.32.75.44.516.95.05.0
Cr9.05.06.55.53.72.83.13.46.93.54.53.81.9
Mn10,485987711,689965814,064429018,160363989563063860930858826
Fe350410,15916,048199676023494802412,85823,75730,12720,89932,08525,801
Cu0.34.01.51.20.14.90.49.56.64.42.53.32.7
Atoms per formula unit
Na0.6920.7260.7910.6800.6930.6590.7050.7540.7830.8420.8180.8420.755
K0.0040.0040.0050.0030.0030.0030.0030.0030.0040.0040.0030.0040.004
Ca0.0850.0640.1400.0860.1360.0220.1320.0620.2690.0740.1790.0670.201
Li1.3591.3361.4151.3731.3611.1241.3091.4421.4061.3001.4731.2471.230
Mg0.0000.0000.0010.0000.0010.0000.0030.0080.0450.0360.0290.0440.052
Ti0.0000.0010.0010.0000.0000.0000.0020.0060.0150.0090.0240.0100.017
Mn0.1690.1600.1890.1560.2270.0690.2930.0590.1450.0500.1390.0500.143
Fe0.0560.1610.2550.0320.1210.0550.1270.2040.3770.4780.3320.5090.410
Cu0.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.000
Al7.5287.5897.5257.8667.0787.7337.0817.5087.2357.6527.7037.3937.092
V0.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.000
Cr0.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.000
* If normalized data are greater than or equal to 100 ppm, they are rounded to zero decimal places. If normalized data are less than 100 ppm, they are rounded to one decimal place.
Table 3. Correlation analysis of the tourmaline color parameters with w(Fe) and w(Mn). ** Significant at the 0.01 level (two-tailed).
Table 3. Correlation analysis of the tourmaline color parameters with w(Fe) and w(Mn). ** Significant at the 0.01 level (two-tailed).
Transitional Elements L*C*h°
FePearson’s r−0.882 **0.229−0.639 **
Sig.0.0000.5250.010
MnPearson’s r0.2040.3120.126
Sig.0.4650.6580.655
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Cui, L.; Guo, Y.; Tang, J.; Yang, Y. Spectroscopy Characteristics and Color-Influencing Factors of Green Iron-Bearing Elbaite. Crystals 2023, 13, 1461. https://doi.org/10.3390/cryst13101461

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Cui L, Guo Y, Tang J, Yang Y. Spectroscopy Characteristics and Color-Influencing Factors of Green Iron-Bearing Elbaite. Crystals. 2023; 13(10):1461. https://doi.org/10.3390/cryst13101461

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Cui, Lianyi, Ying Guo, Jun Tang, and Yushu Yang. 2023. "Spectroscopy Characteristics and Color-Influencing Factors of Green Iron-Bearing Elbaite" Crystals 13, no. 10: 1461. https://doi.org/10.3390/cryst13101461

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