Luminescence Characteristics of Green Grossular Garnets

Abstract

Some light green grossular garnets exhibit orange-red luminescence under long-wave and short-wave ultraviolet light. To characterize their luminescence behavior, we studied seven grossular garnets with typical colors ranging from light yellowish-green to intense green by using photoluminescence (PL), ultraviolet–visible (UV–Vis) spectroscopy, electron paramagnetic resonance (EPR), Electron Probe Microanalysis (EPMA) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). In the PL spectra of these grossular garnets samples, a broad band exists at about 589 nm and three sharp peaks appear at 697, 702 and 716 nm. The three-dimensional fluorescence spectra reveal two luminescence emissions. They are (1) a broad band near 600 nm; and (2) a series of sharp peaks centered at 697, 702 and 716 nm. In the UV–Vis spectra, two prominent asymmetrical absorption bands near 430 and 605 nm are related to Cr³⁺ or Cr³⁺/V³⁺, and minor absorption peaks at 408 and 419/420 nm are related to Mn²⁺. EMPA and LA-ICP-MS analysis confirmed the existence of trace elements Ti, V, Cr and Mn. Furthermore, the EPR spectrum excluded the existence of V²⁺ and V⁴⁺ and confirmed the existence of Mn²⁺, Cr³⁺ and Fe³⁺. Regarding V, an interesting phenomenon was reported in which the intensity of luminescence could be suppressed in grossular garnets with higher concentrations of V. These results imply that chromium and manganese are the luminescence activators in grossular garnets, and vanadium is a powerful quencher.

1. Introduction

Grossular, with the chemical formula Ca₃Al₂(SiO₄)₃, is the Ca–Al species of the six frequent members of the garnet group which has more color varieties than any other garnet species. It displays colors ranging from colorless to pink, green, brown, red, yellow and orange. In the global gemstone market, the value of grossular highly depends on its coloration, and the green series are the most popular. Tsavorite, a grossular variety with saturated green, is the most familiar grossular member, which was discovered by Campbell Bridges in 1967 in northeastern Tanzania [1]. Its economic deposits include Tanzania, Kenya, Madagascar and minor occurrences in Pakistan and Antarctica [2]. Except for tsavorite, other varieties in lighter mint-green (“Merelani” garnets), light yellowish (occasionally termed “Kiwi” garnets) and virtually colorless with a faint tinge of green (“Leuco” garnets) garnets are all examples belonging to the green grossular garnet family. All the names mentioned above are commercial, but there still exist some hidden regulations and connections with their color and trace chemical composition. In mineralogy, all these green colorations are due to the substitution of aluminum (Al) by chromium (Cr) and/or some vanadium (V, if any) [3]. Recently, five new chemical types have been defined based on the variation of the V–Mn–Cr contents, which can provide a helpful tool for identifying the geographic origin of tsavorites from East Africa, Madagascar and East Antarctica [4].

Mineral luminescence is a common phenomenon, and transition metal elements (e.g., Cr, Mn, V and Ti) with different valence are important luminescence centers. The luminescence of Cr³⁺ in minerals has been well investigated, where studied minerals include corundum, spinel, topaz, emerald, etc. [5,6,7,8]. Mn²⁺ is also a well-known activator in many minerals. It is responsible for green, yellow and orange-red luminescence [8]. Some Mn-bearing axinite can emit orange luminescence. In addition, Mn²⁺ is an important luminescence activator of garnet.

Grossular garnet is a common luminescent mineral. The red grossular garnets from Mexico showed scarlet red luminescence under shortwave ultraviolet (SWUV) light, and microprobe testing indicated that manganese is the reason for red luminescence [9]. However, the specific luminescence mechanism was not discussed in detail. In 1995, Mazurak et al. found that Cr³⁺ and V³⁺ cause the luminescence of green grossular through absorption and luminescence spectra [10]. Recently, some grossular garnets demonstrated strong pinkish-red luminescence under long-wave ultraviolet (LWUV) light, supposedly induced by Mn²⁺ [11]. In 2013, Gaft et al. studied the luminescence features via the laser-induced time-resolved spectroscopy of grossular from eight origins, which expressed trace elements Mn²⁺, Mn³⁺, Mn⁴⁺, V²⁺, Ni²⁺ and REE³⁺, which could be the luminescence centers of grossular. The above trace elements enter the grossular lattice by substitution, usually at the site of Ca²⁺ (like Mn²⁺) or Al³⁺ (like Cr³⁺ and V³⁺) [12].

Tanzania, one of the high-value economic deposits in the Neoproterozoic Mozambique Belt, has an extensive geologic history and hosts a variety of gem and mineral deposits [13,14,15]. Some green grossular garnets from Tanzania exhibit orangey-red luminescence under LWUV light and yellowish-orange luminescence under SWUV light. Therefore, this work aims to explore the cause of luminescence in green grossular garnets from Tanzania and to compare these properties with different hues and saturations of luminescence.

2. Materials and Methods

Seven faceted grossular samples (BH1-7) with colors ranging from light yellowish-green to intense green (Figure 1) are listed in

Table 1. Gemological properties of green grossular garnets.

Sample No.	Weight (ct)	Size (mm)	Specific Gravity	Refractive Index	Color
BH1	2.00	8.19 × 6.55 × 4.76	3.60	1.738	light yellowish-green
BH2	1.27	7.05 × 4.96 × 3.79	3.61	1.739	light yellowish-green
BH3	1.62	7.59 × 5.96 × 3.97	3.64	1.737	light yellowish-green
BH4	1.12	5.63 × 5.56 × 3.92	3.61	1.738	light mint-green
BH5	0.86	5.54 × 5.33 × 3.34	3.64	1.739	light mint-green
BH6	0.73	6.79 × 4.99 × 2.80	3.65	1.739	green
BH7	0.84	5.87 × 4.09 × 3.64	3.65	1.738	green

. These natural grossular samples were collected from a gem dealer who claimed they were mined from Tanzania. Some samples show different intensities of orange-red luminescence under LWUV light and yellowish-orange luminescence under SWUV light (Figure 1).

Sample photos and luminescence images were captured by a Nikon D810 camera in Madelight DIB-1612N LED light box (approximately 5500 K color temperature) in the Gemmological Institute of China University of Geosciences (Wuhan, China). The external UV light source is a traditional gemological mercury lamp.

Following the luminescence images, the three-dimensional (3D) fluorescence spectra of samples BH1 and BH5 were obtained using a JASCO FP-8500 fluorescence spectrometer (JASCO, Ishikawamachi Hachioji-shi Tokyo, Japan) in the Gemmological Institute of China University of Geosciences (Wuhan, China). The following parameters were used: excitation (Ex) range from 350 to 700 nm with 5 nm bandwidth; emission (Em) range from 350 to 700 nm with 2.5 nm bandwidth; photomultiplier (PMT) voltage was set to 700 V; and the resolution was controlled within 2.5 nm.

To obtain deeper insight into the distribution of luminescence for its trace elements in the next step, a mapping of luminescence on sample BH3 was accessed by JASCO NRS7500 Raman spectrometer with a 532 nm laser in the Gemmological Institute of China University of Geosciences (Wuhan, China).

Mineral chemical analyses were performed using a JEOL JXA-8230 electron probe microanalyzer (EPMA) with four wavelength-dispersive spectrometers (WDS) at the Center for Global Tectonics, School of Earth Sciences, China University of Geosciences (Wuhan). The following parameters were used: 15 kV accelerating voltage; 20 nA probe current; and a 10 μm beam diameter. Dwell times were 10 s on element peaks and half that on background locations adjacent to peaks. Raw X-ray intensities were corrected using a ZAF (atomic number, absorption, fluorescence) correction procedure. Trace elements analysis was performed using a GeoLasPro Agilent 7900a LA-ICP-MS at Wuhan Sample Solution Analytical Technology Co., Ltd., Wuhan, China. The laser beam spot diameter was 44 μm with a frequency of 5 Hz and an intensity of 5.5 J/cm². NIST 610, BHVO-2G, BCR-2G and BIR-1G were used as calibration reference materials. The raw data were processed by ICPMS Data Cal software, and the multi-external standards combined internal standard calibration strategy for oxide was used for quantitative calculations.

Photoluminescence (PL) spectra of samples were obtained using HORIBA LabRAM HR Elolution spectrometer in the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences (Wuhan, China). The testing method was used with the following parameters: 325 nm; 600 grating; and 325–1000 nm range.

Electron paramagnetic resonance spectra have been recorded on an electron paramagnetic resonance (EPR) spectrometer (A300, Bruker) at 77 K in Detection of Technical Sousepad (Wuhan, China). The magnetic field was scanned from 500 to 5000 G.

Ultraviolet–visible (UV–Vis) absorption spectra were obtained by using a Lambda 650S spectrometer in the Gemmological Institute of China University of Geosciences (Wuhan, China). The wavelength covers the ultraviolet-to-near-infrared band, i.e., from 350 to 800 nm.

3. Results and Discussion

3.1. Photoluminescence Features

Figure 1 shows that the luminescence excited from LWUV light is orange-red, while that from SWUV light is yellowish-orange (samples BH1–5). The luminescence intensity under LWUV and SWUV decreased as the green hue strengthened. This rough regulation implies that the green chromophores are negatively correlated with luminescence. A typical instance is sample BH3, which contains distinguished green color zones where we can hardly recognize luminescence under SWUV.

To investigate the luminescence features, PL spectra of green grossular samples were collected. Overall, the PL spectra of samples with intense luminescence (sample BH1–5) are stronger than those of samples which are almost without luminescence (sample BH6 and 7). The spectra show a broad peak at ~589 nm and a series of sharp peaks around 700 nm (Figure 2). There are two sharp, independent peaks, centered at 697 and 702 nm, and an asymmetric band at 716 nm. In addition, two shoulder peaks are located at approximately 721 nm and 746 nm. Combining the photoluminescence spectra with previous literature data, the broad band, ~589 nm, turns out to be related to Mn²⁺, sharp peaks at 697 and 702 nm are associated with Cr³⁺ and the asymmetric band at 716 nm may be attributed to V²⁺ [12].

The 3D fluorescence spectra (i.e., EEM, excitation–emission matrix, or excitation–emission mapping) of sample BH1 (light yellowish green one) and sample BH5 (light mint-green one) were collected at room temperature, as shown in Figure 3.

The 3D fluorescence spectra show that the two samples have the same luminescence features (numbers and positions) but with different intensities (Figure 3). The intensity of sample BH5 is significantly lower than that of sample BH1, just as we expected according to the luminescence phenomena in Figure 1. Under the 434/437 nm excitation, sample BH1 and BH5’s emission spectra have two luminescence emissions. One emission in the orange-red region consists of a series of shape peaks centered at 697, 702 and 716 nm. Another emission is a broad band near 600 nm with lower intensity in the yellow-orange region; this broad band is more evident under the 409 nm excitation (the orange curves in the emission spectra).

As mentioned above, the grossular may have different luminescence features, which can be manifested through the 3D fluorescence spectrum. The emission spectra of the grossular in this work have a weak band at near 600 nm and a series of peaks at around 700 nm, which imply manganese-, chromium- and vanadium-induced luminescence. Pink grossular from Mogok, Myanmar, can present strong red luminescence under LWUV light; its emission spectrum under 325 nm excitation displayed a strong narrow band at 604 nm and some weaker bands at 583, 595, 617, 631, 645 and 660 nm [16]. Some natural grossular garnets from California, USA, also have a similar luminescence spectrum, i.e., with a narrow vibrational structure and a short decay time, and their emission is supposedly related to Mn²⁺ accompanied by F-centers [11].

A green zone crossing sample, BH3, provides a good example of the spatial relationship between luminescence and coloration. The following mapping of 532 nm laser PL from sample BH3 (Figure 4) shows a strong positive correlation between the sharp peak series (697, 702 and 716 nm), indicating that these three peaks are related to the same origin (Figure 4b–d). Conversely, the 589 nm luminescence is concentrated outside the green zone (Figure 4a).

3.2. Chemical Analysis and Type Identification

The chemical analyses of major and trace elements of these samples were performed by EPMA and LA-ICP-MS. Each sample was analyzed at five and two points, respectively, to obtain an average.

The EPMA results (

Table 2. Chemical composition of green grossular garnets.

Composition		BH1	BH2	BH3	BH4	BH5	BH6	BH7
Oxide (wt.%)
CaO	Range	36.62–36.96	36.85–37.04	36.12–36.59	36.86–36.99	36.36–36.97	36.35–36.70	36.55–37.13
	Average	36.77	36.95	36.27	36.93	36.69	36.52	36.83
SiO2	Range	39.52–41.41	39.73–40.08	39.50–39.74	39.18–39.45	39.69–40.26	39.03–39.13	37.84–38.28
	Average	40.10	39.96	39.60	39.29	39.94	39.07	38.05
Al2O3	Range	21.19–21.75	22.26–22.48	22.41–22.72	22.04–22.36	22.07–22.37	22.20–22.52	21.79–22.23
	Average	21.45	22.37	22.60	22.20	22.22	22.38	22.01
MgO	Range	0.13–0.53	0.56–0.60	0.47–0.51	0.53–0.56	0.60–0.67	0.41–0.48	0.49–0.54
	Average	0.29	0.58	0.50	0.54	0.62	0.44	0.53
FeO	Range	0.02–0.09	0.03–0.09	0.02–0.11	0.04–0.10	0.03–0.10	0.06–0.11	0.01–0.10
	Average	0.05	0.06	0.06	0.06	0.07	0.08	0.04
MnO	Range	0.20–0.30	0.34–0.44	0.24–0.32	0.38–0.44	0.28–0.34	0.61–0.70	0.54–0.65
	Average	0.26	0.38	0.27	0.41	0.30	0.66	0.58
K2O	Range	0.00–0.02	0.00–0.02	0.01–0.02	0.01–0.02	0.02–0.02	0.01–0.02	0.00–0.02
	Average	0.01	0.01	0.01	0.01	0.01	0.01	0.01
TiO2	Range	0.26–0.49	0.34–0.37	0.28–0.35	0.24–0.27	0.45–0.56	0.34–0.38	0.46–0.52
	Average	0.38	0.35	0.31	0.26	0.51	0.37	0.49
Cr2O3	Range	0.00–0.04	0.00–0.05	0.02–0.04	0.06–0.11	0.01–0.09	0.02–0.10	0.05–0.09
	Average	0.01	0.01	0.03	0.08	0.03	0.06	0.07
Na2O	Range	0.00–0.02	0.01–0.02	0.03–0.04	0.01–0.04	0.01–0.03	0.01–0.04	0.00–0.02
	Average	0.01	0.01	0.02	0.02	0.02	0.02	0.01
NiO	Range	0.02–0.08	0.02–0.06	0.04–0.06	0.01–0.02	0.02–0.08	0.01–0.05	0.01–0.05
	Average	0.04	0.02	0.02	0.01	0.03	0.02	0.02
V2O3	Range	0.01–0.05	0.01–0.05	0.03–0.07	0.03–0.13	0.11–0.32	0.43–0.65	0.61–0.70
	Average	0.01	0.02	0.04	0.10	0.23	0.51	0.67
Total	Range	98.18–101.40	100.46–100.89	99.36–100.25	99.87–100.04	100.33–101.23	100.03–100.31	99.02–99.94
	Average	99.39	100.72	99.71	99.91	100.68	100.13	99.31

) show that the main components are CaO (36.12–37.13 wt.%), SiO₂ (37.84–41.41 wt.%) and Al₂O₃ (21.19–22.72 wt.%), with low concentrations of impurity elements (Mg, Mn, Ti and V). The chromophores in green grossular garnets are Cr and V. As the vanadium oxide content gradually increases to 0.67 wt.%, the intensity of the green color gradually increased to deep green. The calculated chemical formulae of the studied garnets are listed in

Table 3. List of chemical formulae of all the studied green grossular garnets.

Sample No.	Chemical Formula
BH1	(Ca2.98Mg0.03Mn0.02)3.03(Al1.91Ti0.02)1.93Si3.03O12
BH2	(Ca2.95Mg0.06Mn0.02)3.03(Al1.96Ti0.02)1.98Si2.97O12
BH3	(Ca2.92Mg0.06Mn0.02)3.00(Al2.00Ti0.02)2.02Si2.98O12
BH4	(Ca2.97Mg0.06Mn0.02)3.05(Al1.96Ti0.01)1.97Si2.95O12
BH5	(Ca2.93Mg0.07Mn0.02)3.02(Al1.95Ti0.03V0.01)1.99Si2.98O12
BH6	(Ca2.94Mg0.05Mn0.04)3.03(Al1.98V0.03Ti0.02)2.03Si2.93O12
BH7	(Ca2.99Mg0.06Mn0.04)3.09(Al1.96V0.04Ti0.03)2.03Si2.88O12

, and they are classified as grossular garnets [17].

Although the main reason for the green color of tsavorite is the presence of Cr and/or V, other elements such as Mn exist in significant quantities, sometimes higher than those of V and Cr [4]. In 2014, Feneyrol et al. proposed five grossular chemical types according to the variation of the V–Mn–Cr contents, including (1) vanadium grossular with V > Cr > Mn; (2) vanadium grossular with V > Mn > Cr; (3) Mn-bearing vanadium grossular with Mn > V > Cr; (4) Mn-bearing chromium grossular with Mn > Cr > V; and (5) Cr- and Mn-bearing grossular with Cr > Mn > V [4]. LA-ICP-MS results (

Table 4. Trace elements of green grossular garnets obtained by LA-ICP-MS (in ppmw).

	Mg	P	Ti	V	Cr	Mn	Fe
BH1	3531	157	2893	121	128	2232	613
BH2	3463	136	2127	257	102	3266	597
BH3	3534	71.6	1850	149	189	2287	620
BH4	3259	54.0	1733	281	133	2328	579
BH5	3653	197	2926	2470	318	2641	591
BH6	3156	87.6	2281	3582	310	4947	710
BH7	3400	137	3068	4721	425	4750	612

) indicate that the samples BH2 and BH4-7 are type 3 (Mn > V > Cr); they should be renamed as Mn-bearing vanadium grossular. In addition, the ratio of V/Cr indicates that these tsavorites might have originated from Tanzania. The V/Cr ratios in this work ranged from 0.8 to 11.6, which are near consistent with the V/Cr ratios between 0.7 and 10.1 of Tanzanian tsavorites in a previous study [4]. Our results correspond to tsavorites from Komolo, Namalulu and Kimwengan in Tanzania [4]. For the samples BH1 and BH3, the contents of Cr and V have no noticeable difference and are hard to classify.

3.3. Assignment of Luminescence and Coloration

By using EPMA and LA-ICP-MS, it was confirmed that trace elements related to luminescence, i.e., Cr, V and Mn, exist in the grossular garnets. However, their role in luminescence and coloration needs further definition. The absorption spectra of representative green grossular garnets are shown in Figure 5. Two asymmetrical absorption bands have centers at approximately 430 and 605 nm. Previous studies have confirmed that the bands at 430 and 605 nm are related to Cr³⁺ or Cr³⁺/V³⁺ [18]. Combined with Figure 1, samples BH6–7, with the intense green hue, have stronger absorption peaks at nearly 430 and 605 nm, which also showed the correlation between the trace elements (Cr and V) and the green color.

Two shoulders or minor absorption peaks at 408 and 419/420 nm were detected, which are prominent in the sample with stronger luminescence. Previous studies assigned these bands to the absorption of Mn²⁺ [4,19]. A relatively weak band at 514 nm was also observed in the UV–Vis spectra. This band was reported once in “raspberry” grossular from Mexico, and it is the domain band for its coloration [20]. After our predecessors’ effort, we assigned it to Mn³⁺.

From the UV–Vis spectra, it can be inferred that impurities Cr³⁺/V³⁺, Mn²⁺ and Mn³⁺ are related to chromophore and may lead to luminescence. V²⁺ was supposed to cause the asymmetric 716 nm band in the former PL spectra, but its existence needs further discussion. V²⁺ belongs to 3d³ ions and is isoelectronic with Cr³⁺, so the V²⁺ and Cr³⁺ emission spectra can overlap. In this work, we checked it by EPR, a useful tool for studying unpaired electrons in transition metals and other defects [21]. If V²⁺ or V⁴⁺ exist, there should be a significant signal in EPR.

The sample BH2 was tested by EPR. As in Figure 6, the EPR signals centered at g-values of 3.788 are probably due to isolated Cr³⁺ ions at octahedral sites, while the peak at 1.975 is related to the dual contribution from both Cr³⁺–Cr³⁺ ion exchange coupled pairs and isolated Cr³⁺ ions [22]. The g = 4.262 singlet is assigned to Fe³⁺ and a typical six-line spectrum of Mn²⁺ is also shown [23]. However, the sample BH2 showed no EPR signal of V²⁺ [24] and V⁴⁺ [25], implying that V may occur as V³⁺. Therefore, we prefer to assign the 716 nm band to Cr³⁺ rather than to V²⁺. This means that only Cr³⁺ contributes to the emission in the red region.

3.4. Suppression Influence on the Vanadium-Rich Sample

According to the EPR spectrum, the correlation between V²⁺ and/or V⁴⁺ and the emissions in the red region was excluded. However, there is a more interesting phenomenon related to V.

Chemical and spectroscopic analyses were performed as described above. Given that we had ascribed the emissions in the yellow-orange region to Mn²⁺ and the emissions in the red region to Cr³⁺, some would expect these elements to be more concentrated in samples with stronger luminescence; however, the fact is reversed. Samples BH6–7 have relatively high Cr and Mn contents and emit the weakest luminescence. It seems that there should be some other unknown mechanism behind it. Recently, Ma and Guo studied thirty-three tsavorites from Tanzania and Kenya, with colors ranging from pale to vivid green, which can produce orange-red luminescence under the LWUV, similar to the luminescence phenomena in this work. They speculated that trivalent chromium in grossular is the activator of luminescence; however, luminescence gradually weakened and disappeared as vanadium increased, which means vanadium is a potent quencher [26]. In addition, lately, some rumors about this case have shown up in the gem market as well.

In this work, we found an identical correlation. The grossular garnet with a relatively high chromium content and abundant vanadium (e.g., 318 ppmw Cr and 2470 ppmw V in BH5) does not display strong luminescence. The discrepancy, in a multitude of orders, in vanadium content (e.g., 121 ppmw in BH1 vs. 2470 ppmw in BH5) indicates that vanadium inhibits luminescence. Therefore, lighter green grossular with lower vanadium concentration exhibits stronger luminescence.

4. Conclusions

In this work, seven faceted grossular garnets from light yellowish-green to intense green have been investigated by PL, 3D fluorescence spectroscopy, EPMA, LA-ICP-MS, UV–Vis spectroscopy and EPR. The chemical analysis revealed a composition consistent with that of grossular, with calcium, aluminum and silicon as major elements, and manganese, titanium, chromium, vanadium, magnesium and iron as trace elements.

The PL spectra show a broad band near 600 nm and a series of sharp peaks centered at 697, 702 and 716 nm. Combined with the EPR spectra, the UV–Vis spectra show the characteristic absorption peaks related to Cr³⁺ and V³⁺ for the green grossular garnets. Mn²⁺ and Mn³⁺ absorption peaks also appear, which are more evident in the lighter green samples. It was determined that Mn²⁺ and Cr³⁺ are the causes of the emission in the yellow-orange and red region, respectively.

According to the luminescence intensity and trace element data from LA-ICP-MS, the much higher concentration of vanadium found in intense green grossular garnets has a weak luminescence. Therefore, vanadium is identified as a quencher for grossular luminescence.