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Article

Correlations between Garnet Species and Vibration Spectroscopy: Isomorphous Substitution Implications

by
Weiwei Li
1,2,
Jinyu Zheng
1,
Jingcheng Pei
1,
Xing Xu
1 and
Tao Chen
1,3,*
1
Institute of Gemology, China University of Geosciences, Wuhan 430074, China
2
State Key Laboratory of Mineral Deposit Research, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210023, China
3
State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan 430074, China
*
Author to whom correspondence should be addressed.
Crystals 2022, 12(1), 104; https://doi.org/10.3390/cryst12010104
Submission received: 4 December 2021 / Revised: 29 December 2021 / Accepted: 2 January 2022 / Published: 13 January 2022
(This article belongs to the Special Issue Gem Crystals)
Browse Figures
Versions Notes

Abstract

:
Garnet has many species because of its common isomorphism. In this study, a suite of 25 natural gem-quality garnets, including pyrope, almandine, spessartine, grossular, and andradite, were examined by standard gemological testing, LA-ICP-MS, FTIR, and Raman analysis. Internal stretching and bending vibrations of the SiO4-tetrahedra of garnet exhibit correlate with the type of cations in garnet’s dodecahedral position (A site) and octahedral position (B site). FTIR and Raman spectra showed that with the increase of the radius of Mg2+, Fe2+, Mn2+, and Ca2+ in A site, or the unit cell volumes of pyrope, almandine, spessartine, and grossular, the spectral peaks of Si–Ostr and Si–Obend modes shift to low wavenumber. Because of the largest cations both in A site (Ca2+) and in B site (Fe3+), andradite exhibited the lowest wavenumber of Si–Ostr and Si–Obend modes of the five garnet species. Therefore, garnet has correlations between chemical composition and vibration spectroscopy, and Raman or IR spectroscopy can be used to precisely identify garnet species.

1. Introduction

The popular garnet family is one of the most prolific mineral groups and branches into a spectrum of colors and gem types [1,2]. The garnets used in gems are the common silicate garnets with relatively simple chemical compositions [3]. They crystallize in the highly symmetric cubic structure (space group Ia3d) and are represented by the general formula A3B2[SiO4]3. The garnets belong to two subgroups and six end-members (species), known as the pyralspite subgroup (pyrope-almandine-spessartine; B = Al3+ and A = Mg2+, Fe2+, and Mn2+, respectively) and the ugrandite subgroup (grossular-andradite-uvarovite; A = Ca2+ and B = Al3+, Fe3+and Cr3+, respectively) [3,4]. The really end-member garnet is uncommon because of broad chemical substitution [5].
Garnets have been widely studied by many gemologists, such as the cause of color, localities, and the typical inclusions of different garnet types, for example, in [4,6,7,8,9,10,11,12,13,14]. Commonly, the garnet types are determined using standard gemological methods, such as color, refractive indices (RI), specific gravity (SG), and absorption lines over the visible energy range by hand spectroscope [1,9,15]. However, because of complex isomorphous substitution, the RI and SG values of some garnets deviate from the range of relative end-members. Such garnets cannot be easily determined by standard gemological methods [1].
Garnet offers an excellent system to study the vibrational spectroscopic properties of silicate structures because of its high symmetry and chemical substitution. A series of compositional different end-members gives chance to investigate changes in the spectra with changing chemistry [1,16]. The correlation between the Raman or infrared (IR) spectra and compositional changes have been investigated usually by using synthetic binary garnet samples with regularly changing compositions, such as the pyrope–grossular series [17,18], grossular-uvarovite series [19], and skiagite–andradite and skiagite–almandine series [20]. Adamo et al. discussed the correlations among gemological properties, chemical composition, and IR vibrational frequencies in a suite of nature gem-quality garnets [1]. They proposed that the IR active bands over the 1150–800 and 650–450 cm−1 ranges allow discrimination between pyralspite and ugrandite subgroups. However, identification of the end-members or species of garnets by vibrational spectra has not been proposed so far.
In the present paper, a suite of gem-quality garnets, including pyrope, almandine, spessartine, grossular, and andradite, were studied for the correlations among gemological properties, composition, and spectroscopic features. The distinction of garnet species by Raman and IR spectra was proposed. It provides a detailed set of spectroscopic data to help the precise identification of garnet species without composition analysis.

2. Materials and Methods

A total of 25 gem-quality faceted garnet samples, covering all the main subgroups of garnet, were selected to be studied for this study (Figure 1 and Table 1). UV fluorescence was detected by UV5000XL (Skyray, Suzhou, China). The RI was measured by refractometer (GIC, Wuhan, China, myopia method for faceted samples and hyperopia method for arc samples), and SG was determined hydrostatically (GIC, Wuhan, China). The above experiments were performed at the Gemological Institute, China University of Geosciences (Wuhan).
The chemical composition of all samples was tested by laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS, Agilent, Santa Clara, CA, UAS) at a 23.32 kV accelerating voltage performed at the Wuhan Sample Solution Analytical Technology Co. It is a micro-damaging test method and can prevent the destruction of the sample. The ablation spot size was 44 μm. The testing points were selected at the waist edge or pavilion position of faceted samples or at the bottom position of arc samples. Fe2+-Fe3+ redistribution of garnet samples from LA-ICP-MS analyses was recalculated using the general equation to estimate Fe3+ [21].
Raman spectroscopy of 21 samples was performed with a Horiba Lab RAM HR Evolution confocal micro-Raman spectrometer coupled with a Leica microscope, using a solid-state Nd-YAG laser at 532 nm (Horiba Scentific, Paris, France). Raman spectra were collected in the range of 4000–200 cm−1 and used a 6 mm raster, performed at the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences (Wuhan). Each sample selected a test position without impurities.
Infrared spectroscopy of all samples was performed using a Bruker Vertex 80 Fourier-transform infrared (FTIR) spectrometer (Bruker, Ettlingen, German) at the Gemological Institute, China University of Geosciences (Wuhan). The specular reflection method was used (Kramers–Kronig transformation) under the following conditions: 220 V scanning voltage, 6 mm raster, 10 kHz scanning rate, 32 scans, 400–4000 cm−1 range, and 4 cm−1 resolution. The Raman and FTIR spectra were recorded from the randomly oriented specimens. The nonpolarized spectra were shown in this report for the gem-quality garnet samples.

3. Results

3.1. Gem Properties of the Garnet Samples

Garnet types of the studied samples are primally determined using standard gemological methods. The tested RI and SG values of the samples are listed in Table 1. The four green samples did not appear red through the Chelsea filter. All the garnet samples are inert fluorescence. Their inclusions were not studied in detail because they are not usually be used to identify garnet types. In the studied samples, there are two kinds of garnets having special optical effects. One is star garnet. The samples are dark purple and translucence having white six-rayed (SLS-7) and four-rayed (SLS-8) stars, which belong to almandine according to RI and SG values [1,22]. The other is color-changing garnet. The samples (SLS-16~18) are purple-gray in daylight and orange-brown in incandescent light as shown in Figure 1. Rose-red garnet (SLS-21) and Mali garnets (SLS-24 and 25) also have been tested in this study. Sample SLS-22 is demantoid with a grass green color. The gemological properties of the demantoid are consistent with those reported RI and SG values [11,23]. The garnet types of some of the studied samples cannot be determined only by gem properties. The colors of the garnet samples are described in Table 1, and the UV-Vis absorption spectra of the samples are shown in Supplementary. The determination of garnet types is checked by the following chemical analysis.

3.2. Chemical Composition Analysis

The quantitative chemical compositions were calculated assuming ideal stoichiometry of 8 cations on a 12-oxygen basis and are given in Table 2. The end-member components of the garnet samples are also given in Table 2. Chemical compositional triangular plots were drawn according to the content of the end-members in the pyralspite subgroup and ugrandite subgroup (Figure 2). Most of the samples are nearly located in the boundary of the triangular plots, indicating they are nearly two binary garnet solid solutions.
Samples SLS 11~13 examined are the intermediate members of the pyrope-almandine series with typical color and gem properties of rhodolite [1]. The rose-red garnet (SLS-21) has a special rose-red color with compositions of approximately Py53Al43Gr2Sp2. However, according to RIM values (N(Fe2+)/N(Fe2+ + Mg2+)), the studied rhodolite and rose-red garnet samples belong to pyrope specie (RIM < 0.5) [24]. The studied almandine samples (SLS-7~10) contain a different amount of Mg2+ substituting Fe2+ in A site, and contain a little amount of spessartine and a grossular component.
There are three kinds of chemical substitution series in the studied spessartine species, near ideal end-member spessartine (SLS-1), spessartine-almandine series (SLS-3~6), and spessartine-pyrope series (SLS-16~18). The studied color-changing garnets belong to the spessartine-pyrope series. The color-changing phenomenon may be attributable to a little amount of Cr2O3 [12]. However, no chromium was detected in sample SLS-16. The color-changing causes will not be discussed here.
The grossular species of the studied samples contain three varieties, tsavorite, hessonite, and Mali garnet. Tsavorite samples (SLS-19 and 20) are near the ideal end-member grossular composition (above 95% wt.% grossular), having a very little amount of iron cations in the structure. Their minor V2O3 contents cause a green hue. No Cr2O3 was detected in the samples, which has no contribution to the color [11]. Hessonite is a variety of grossular that contains sufficient Fe3+ to cause the orange-yellow color [25]. Sample SLS-2 appears lighter orange-yellow color than sample SLS-14 and SLS-15 because of fewer Fe3+ cations. Two Mali garnets are greenish-yellow (SLS-24) and yellow (SLS-25) with compositions of Gr77And21Py2 and Gr75And23Py2, respectively [10].
Two andradite samples (SLS-22 and 23) are representative and have dominated Fe3+ cations in the B site. They have similar andradite (88.50 wt.% and 89.45 wt.% andradite, respectively) and grossular (10.16 wt.% and 10.05 wt.% grossular, respectively) components but different green colors. One of them (SLS-22) is demantoid, which is grass green, with minor chromium (nearly 0.3% wt.% Cr2O3) contributing the valued green color [23]. The other andradite (SLS-23) is yellow-green without the Cr component.
In order to obviously show the relationship between cations and spectra in garnet types, the calculated chemical structure formula of the studied garnets was listed in Table 3. The samples in Table 3 were classified into two groups according to the dominant cations in the B site. The aluminosilicate garnet, including pyrope, almandine, spessartine, and grossular, has Al3+ in the B site but has different divalent cations in the A site. The divalent cations have different masses and radius. However, andradite has relatively larger cations both in the A site (Ca2+) and in the B site (Fe3+) than aluminosilicate garnet has. So, andradite is expected to show more different Raman and IR spectra than other garnet species. In Table 3, the order of the samples of the same types is listed according to the increasing order of the main garnet component in the aluminosilicate garnets.

3.3. Raman Spectroscopy Features

According to theoretical factor group analysis on garnet, the total number of vibrations and the number of Raman and infrared active modes were calculated [26]. The total irreducible representation at the Γ-point is given as follows:
Γ = 3A1g + 5A2g + 8Eg + 14F1g + 14F2g + 5A1u + 5A2u + 10Eu + 17F1u + 16F2u
A total of 25 modes are Raman active, which are A1g−, Eg−, and F2g− modes, and 17 F1u modes are active in the infrared [16].
Raman spectra of the studied samples are shown in Figure 3, and the observed Raman vibrational modes of the samples are described in Table 4. The zone-center Raman active vibrational modes are considered to be relative to the SiO4-tetrahedra and A site cations in garnet structure. The B site cations do not produce Raman active vibrations [16].
The range of Raman spectra of garnets can be grouped in five regions: the highest frequency modes (1100~850 cm−1) are dominated by stretching motions of Si–O (Si–Ostr), the frequency modes at around 550 cm−1 are dominated by bending motions of Si–O (Si–Obend), the frequency modes at around 420 cm−1 are dominated by rotation motions of SiO4 (R(SiO4)4−), and the modes below 300 cm−1 are due to translational motions of SiO4 (T(SiO4)4−) and A2+(x, y)-translations (T(A2+)) [16,18].
The nonpolarized spectra of the aluminosilicate garnets show similar patterns of Si–Ostr and Si–Obend modes (Figure 3a–d). However, the frequencies of these modes are different in different garnet species (Table 4), which will be discussed in detail below. The weak Si–Ostr Eg mode of all the tested aluminosilicate garnets was not detected herein, but the mode of some garnet species can be observed by polarized spectra when detecting perpendicular to the special crystal plane of the garnet single crystals [16,27]. Tsavorite samples (SLS-19 and 20) have strong fluorescence background and only show weak Si–Ostr A1g and R(SiO4)4− Eg modes. So, they show different Raman spectra in the grossular group, which may result from minor V2O3 contents in the crystals.
The patterns and frequencies of Si–Ostr and Si–Obend modes of the studied andradite samples are obviously different from those of the aluminosilicate garnets (Figure 3). The highest frequency of Si–Ostr F2g of andradite is lower than 1000 cm−1, while those of the aluminosilicate garnets are higher than 1000 cm−1. The Si–Ostr A1g mode of andradite was not detected, while Si–Ostr Eg mode (870 cm−1) existed. These phenomena are the same as the reported polarized Raman spectra [16,27].
As shown in Figure 3 the patterns and frequencies of modes below 300 cm−1 [T(SiO4)4− and T(A2+) modes] have much difference among the five garnet species, which could be used to discriminate between the garnet species’ minerals. However, because it is still difficult to describe the external vibrations as originating from certain atoms or polyhedral units [18], low-frequency modes are not suitable to establish correlations among chemical composition, structure, and Raman spectra. So, they are not described in detail and discussed herein

3.4. Infrared Spectroscopy Features

Infrared (IR) spectra of garnet end-members consist of 17 vibrational modes (F1u modes) whose assignment has been proposed [26,27]. However, incomplete sets (less than 17 modes) have been reported in many experimental IR studies, suggesting that some of the modes are characterized by low intensity [1,20,28]. In theory, there are 10 bands in the mid-infrared region (MIR, 4000–400 cm−1) with concentrations in the 1100–400 cm−1 range. The observed IR bands and average frequencies in the MIR range for all the garnets species examined here are shown in Figure 4 and listed in Table 5.
Bands B, C, and D arising from the higher energy vibrations appear in the 1100–700 cm−1 region and are assigned as asymmetric Si–O stretching modes [ν3(SiO4)] [17,19]. The shoulder which frequency is higher than band B, named band A [27], was not visible in all of the examined garnet samples. The origin of band A is not clear [17]. It was considered as an overtone by some researchers [20].
Bands E, F, and G appearing in the 700–500 cm−1 region are assigned to the symmetric Si–O bending mode [ν4(SiO4)], and the band I (about 470–450 cm−1) is assigned to the asymmetric Si–O bending mode [ν2(SiO4)]. In previous studies, the intensity of examined band E of the pyrope samples was too low to be observed, which is different from other garnets [17,28]. Bands H and J arising from the lower energy vibrations in the 500–400 cm−1 region are assigned to translations of the B cations, T(B), in the octahedral site [17,27].
The same as Raman spectra, the patterns of IR spectra of aluminosilicate garnets are different from those of andradite samples. The patterns of IR spectra of aluminosilicate garnets are similar, especially in the higher wavenumber region. In IR spectra, bands in the fingerprint region (lower wavenumber region, 680–380 cm−1) are often used to discriminate minerals, as well as the species of garnet minerals [27]. However, the behavior of bands below 500 cm−1 does not allow elucidation of the structural features [19], these bands will not be discussed in terms of the correlations with garnet species. In grossular species, there is a little amount of wavenumber shift in IR bands for samples SLS-2 (hessonite), SLS-20 (tsavorite), and SLS-24 (Mali garnet) relative to samples SLS-14 (hessonite), SLS-15 (hessonite), SLS-19 (tsavorite), and SLS-25 (Mali garnet) as shown in Figure 4. The phenomenon may be caused by Transverse Optical-Longitudinal Optical (TO-LO) splitting [27,28].

4. Discussion

Garnets present a relatively simple vibrational spectrum because of their high crystal symmetry [27]. Cation substitution on both the dodecahedral and octahedral sites of garnet is known to cause rotation of the tetrahedral site about its symmetry axis, variations of the Si–O bond distances, changes in the unit cell volume dominates, and so on [16,20,29]. Therefore, the various garnet types caused by isotropic substitution permit the study of the correlations between the changing chemistry and vibrational spectra. It provides the opportunity to distinguish garnet varieties in Raman and IR spectra according to chemical changes without composition analysis. On the other hand, some gem properties of garnet, such as the values of SG and RI, also change because of chemical composition. Many studies had discussed the correlation between the IR or Raman spectra and cation substitutions in binary garnet samples [17,18,19,20]. However, the correlations among gemological properties, chemical composition, and infrared spectroscopy of the pyralspite and ugrandite series are only discussed by Adamo et al. (2007) [1].
In previous studies, it had been proposed that the type of cations, the cation radii, and mass of the A-site cations do not greatly affect the external mode frequencies of garnets but obviously affect internal stretching and bending vibrations of the SiO4-tetrahedra [1,17,20]. Therefore, in this IR and Raman spectra study, these vibrations of garnet are used to discuss the relationship between chemical changes and relative gem properties.
Figure 5 shows correlations among values of SG and RI, wavenumber shifts of stretching and bending vibrations, and five garnet species. The tested values of SG and RI of different garnet species exhibit mutually covered ranges and show no correlations between the type of cations in the A or B sites and gem properties. That is why they are not suitable for the precise identification of some garnet species. However, the frequencies of Raman (Si-O)str F2g−, (Si-O)str A1g− and (Si-O)bend A1g− modes and wavenumber of IR bands B, E, and F exhibit correlations with chemical change.
As well known, the order of the divalent cation radii in A site is Mg2+ < Fe2+ < Mn2+ < Ca2+, and the order of the trivalent cation radii in B site is Al3+ < Fe3+. The unit cell volumes of the end-member garnets increase in the following order: pyrope (1502.9 Å3, [30]), almandine (1530.8 Å3, [31]), spessartine (1565.7 Å3, [32]), grossular (1640.9 Å3, [33]), andradite (1757.5 Å3, [34]). The relative atomic mass (Ar) of the constituent metals increases as follows: Ar(Mg) = 24.31, Ar(Al) = 26.98, Ar(Ca) = 40.08, Ar(Mn) = 54.94 and Ar(Fe) = 55.85.
For the five garnet species (Figure 5), the frequencies of Raman (Si-O)str F2g-, (Si-O)str A1g- and (Si-O)bend A1g- modes and wavenumber of IR bands B, E, and F decrease with increasing unit cell volumes of various garnets. Pyrope samples (rhodolite: SLS-11~13, rose-red garnet: SLS-21) have the highest frequencies and wavenumber of those modes and bands, while andradite samples (demantoid: SLS-22 and andradite: SLS-23) have the lowest frequencies and wavenumber. As in Raman spectra, the highest frequencies of (Si-O)str F2g mode is about 1046 cm−1 for pyrope, downshifted about 7 cm−1 for almandine, downshifted about 20 cm−1 for spessartine, and downshifted about 43 cm−1 for grossular, but just about 990 cm−1 for andradite (Figure 5 and Table 4). In IR spectra, the highest wavenumber of (Si-O)str band B is about 980 cm−1 for pyrope, downshifted about 5 cm−1 for almandine, downshifted about 12 cm−1 for spessartine, and downshifted about 40 cm−1 for grossular but just about 928 cm−1 for andradite (Figure 5 and Table 5).
For aluminosilicate garnets, the frequencies and wavenumber of Si–Ostr modes exhibit a close relationship with the dominant cations in A site. As shown in Figure 5 and Table 4 and Table 5, the order of decreasing frequencies and wavenumber of those modes is the same as the increasing order of divalent cation radii and unit cell volumes but the peaks exhibit little relation with the relative atomic mass in a certain species. Such as in pyrope, correlation cannot be found between chemical compositions (e.g., pyr%, or the mass of Mg cations, or the value of Mg/(Ca + Fe)) and the frequencies of (Si-O)str A1g mode or the wavenumber of (Si-O)str band B (Table 2, Table 3, Table 4 and Table 5 and Figure 3 and Figure 4).
Andradite has the largest cations both in A site (Ca2+) and in B site (Fe3+) relative to those in aluminosilicate garnets and has relative the largest unit cell volume. It results in a sharp drop of frequencies and wavenumber of those modes and bands in Raman and IR spectra (Figure 5 and Table 4 and Table 5). Therefore, it is easy to identify andradite according to the peak positions of Si–Ostr modes in Raman or in IR spectra. However, it needs to be noticed that the peak position at about 870 cm−1 in andradite belongs to Si–Ostr Eg mode not Si–Ostr A1g mode (Table 4).
When the combination of the Roman spectra and IR spectra can be used to identify the garnet species, the 1000–800 cm−1 region is assigned, as asymmetric Si–O stretching modes can provide rapid and precise identification of the garnet species. The garnet species show obvious differences in the frequencies of the strongest peak ((Si-O)str A1g mode) and the following weak peak ((Si-O)str F2g mode) in Roman spectra, and the bands B and C in IR spectra. For pyrope, the peaks are about 913 cm−1 and 860 cm−1 in Roman spectra, the bands are about 980 cm−1 and 908 cm−1 in IR spectra. For almandine, the peaks are about 916 cm−1 and 863 cm−1 in Roman spectra, the bands are about 975 cm−1 and 908 cm−1 in IR spectra. For spessartine, the peaks are about 906 cm−1 and 850 cm−1 in Roman spectra, the bands are about 968 cm−1 and 900 cm−1 in IR spectra. For grossular, the peaks are about 876 cm−1 and 820 cm−1 in Roman spectra, the bands are about 940 cm−1 and 873 cm−1 in IR spectra. For andradite, the strongest peak is (Si-O)str Eg mode (about 870 cm−1) followed by two weak (Si-O)str F2g modes (about 839 cm−1and 812 cm−1) in Roman spectra. Finally, in IR spectra, its bands B and C reduce to 916~928 cm−1 and 848 cm−1, respectively.

5. Conclusions

Chemical composition tests, such as electron microprobe analysis and LA-ICP-MS analysis, can provide precise specifications of types and series according to a full compositional characterization. However, it is less often used than IR and Raman spectroscopy in routine identification work in gem labs. Internal stretching vibrations of the SiO4-tetrahedra of garnet provide more precise identification of species than external modes because they have a correlation with the type of cations in the A or the B site and are closely related to the cation radii and the unit cell volumes. Therefore, a set of frequencies and wavenumber of Si–Ostr modes obtained from Raman and IR spectroscopy can be used to quickly identify the garnet species without damaging the samples.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/cryst12010104/s1, Figure S1: The UV-Vis spectra of gem garnet sample.

Author Contributions

T.C. conceived the idea of the project, gave data interpretation, and drafted the manuscript. W.L. carried out experiments, wrote the original draft and gave data interpretation. J.Z. and X.X. carried out IR and Roman experiments. J.P. provided garnets samples. W.L. and T.C. contribute equally. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China, gran numbers 42072252 and the Fundamental Research Funds of Gemology Institute, China University of Geosciences, Wuhan, grant numbers CIGTXM-02-202002.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

All supporting data and computational details are available on written request. These data are stored by the main author of this article.

Acknowledgments

We thank Zhaochu Hu’s group for obtaining LA-ICP-MS data of garnet samples. We thank two anonymous reviewers for comments that improved our manuscript.

Conflicts of Interest

The authors declare no conflict of interest.

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Crystals 12 00104 g001 550
Figure 1. The studied gem garnet samples including spessartine (SLS-1, 3 to 6 and 16 to 18), grossular (SLS-2, 14, 15, 19, 20, 24 and 25), almandine (SLS-7 to 10), pyrope (SLS-11 to 13 and 21), andradite (SLS-22 and 23). Samples SLS-16~18 are Color-change garnets, were also observed in incandescent light and shown in the left bottom. The others were observed in daylight. Photos by T. Chen.
Figure 1. The studied gem garnet samples including spessartine (SLS-1, 3 to 6 and 16 to 18), grossular (SLS-2, 14, 15, 19, 20, 24 and 25), almandine (SLS-7 to 10), pyrope (SLS-11 to 13 and 21), andradite (SLS-22 and 23). Samples SLS-16~18 are Color-change garnets, were also observed in incandescent light and shown in the left bottom. The others were observed in daylight. Photos by T. Chen.
Crystals 12 00104 g001
Crystals 12 00104 g002 550
Figure 2. The chemical compositional triangular plot of gem garnet samples. (a) The Pyrope-spessartine-almandine triangular plot of the sixteen samples belonging to the pyralspite subgroup. (b) The andradite-grossular-uvarovite triangular plot of the nine samples belonging to the ugrandite subgroup. Chemical composition details see Table 2.
Figure 2. The chemical compositional triangular plot of gem garnet samples. (a) The Pyrope-spessartine-almandine triangular plot of the sixteen samples belonging to the pyralspite subgroup. (b) The andradite-grossular-uvarovite triangular plot of the nine samples belonging to the ugrandite subgroup. Chemical composition details see Table 2.
Crystals 12 00104 g002
Crystals 12 00104 g003 550
Figure 3. The Raman spectra of gem garnet samples. (a) pyrope, (b) almandine, (c) spessartine, (d) grossular, and (e) andradite.
Figure 3. The Raman spectra of gem garnet samples. (a) pyrope, (b) almandine, (c) spessartine, (d) grossular, and (e) andradite.
Crystals 12 00104 g003
Crystals 12 00104 g004 550
Figure 4. The Infrared spectra of gem garnet samples. (a) pyrope, (b) almandine, (c) spessartine, (d) grossular, and (e) andradite.
Figure 4. The Infrared spectra of gem garnet samples. (a) pyrope, (b) almandine, (c) spessartine, (d) grossular, and (e) andradite.
Crystals 12 00104 g004
Crystals 12 00104 g005 550
Figure 5. Correlations of SG values (yellowish-green circles), RI values (brown squares), Raman (Si-O)str F2g−, (Si-O)str A1g− and (Si-O)bend A1g− modes frequencies (bluish-green squares, orange circles, and pink triangles) and IR B, E and F bands wavenumber (black squares, red circles, and blue triangles) with different garnet species. Pyr = pyrope, Alm = almandine, Spe = spessartine, Gro = grossular, and = andradite.
Figure 5. Correlations of SG values (yellowish-green circles), RI values (brown squares), Raman (Si-O)str F2g−, (Si-O)str A1g− and (Si-O)bend A1g− modes frequencies (bluish-green squares, orange circles, and pink triangles) and IR B, E and F bands wavenumber (black squares, red circles, and blue triangles) with different garnet species. Pyr = pyrope, Alm = almandine, Spe = spessartine, Gro = grossular, and = andradite.
Crystals 12 00104 g005
Table
Table 1. General gemological properties of the twenty-five garnet samples.
Table 1. General gemological properties of the twenty-five garnet samples.
Sample No.Weight (ct)SGRIColorUVF *Properties
SLS-10.7553.872NR*light-orangeIF *spessartine
SLS-20.7104.0571.745yellow-orangeIFgrossular
SLS-30.3354.188NRred-orangeIFspessartine
SLS-40.5504.074NRorangeIFspessartine
SLS-50.7904.158NRbrown-redIFspessartine
SLS-60.4704.273NRorande-redIFspessartine
SLS-72.3554.4431.78 (hm *)dark purple-redIFstar almandine
SLS-82.1703.8751.78 (hm)dark purple-redIFstar almandine
SLS-91.5604.1601.780purple-redIFalmandine
SLS-101.4854.3681.780purple-redIFalmandine
SLS-111.4503.7661.760purple-redIFrhodolite
SLS-121.3603.9421.760purple-redIFrhodolite
SLS-131.0004.0821.765brown-redIFrhodolite
SLS-141.0403.5861.750orande-redIFgrossular
SLS-150.9153.5881.750orande-redIFgrossular
SLS-160.7453.9211.775purple-gray (daylight), orange-brown (incandescent light)IFCcg *
SLS-170.6104.0671.776IFCcg *
SLS-180.7503.9471.773IFCcg *
SLS-190.4903.6301.738greenIFtsavorite
SLS-200.4203.3601.733greenIFtsavorite
SLS-211.1803.7461.758rose-redIFRrg *
SLS-220.2454.083NRgrass greenIFdemantoid
SLS-230.5253.621NRyellow-greenIFandradite
SLS-240.9203.6801.765green-yellowIFMlg*
SLS-250.8753.6461.767green-yellowIFMlg*
* Abbreviation: NR = negative reading, hm = hyperopia method. UVF = Ultraviolet fluorescence, IF = inert fluorescence. Ccg = Color-change garnet, Rrg = Rose-red garnet, Mlg = Mali garnet.
Table
Table 2. Chemical composition of garnet samples obtained by LA-ICP-MS.
Table 2. Chemical composition of garnet samples obtained by LA-ICP-MS.
Sample No.SLS-1SLS-2SLS-3SLS-4SSLS-5SLS-6SSLS-7SSLS-8SSLS-9
Oxides(wt.%)
SiO236.32239.17936.12136.57336.64536.48138.18438.54137.807
TiO20.0850.2020.0240.0980.1090.2740.0800.0260.006
Al2O319.29521.21719.55319.99419.55319.55220.38020.34020.397
Cr2O3000000000
FeOTa0.3482.3524.5542.1008.5574.93935.15234.01035.777
MnO43.2700.09638.99240.41634.15137.9070.5100.4981.788
MgO0.0080.1180.0050.0080.0170.0023.9723.4402.592
CaO0.17836.7130.2850.3960.4040.2630.4001.9140.434
V2O3000000000
Na2O0.0050.0020.0050.00500.0120.0420.0200.021
K2O0.0010.00100.0010.0050000.001
P2O50.2460.0190.2250.1960.1910.2540.2590.0400.098
Total99.75899.89999.76499.78799.63299.68498.97998.82998.921
Ions based on 12 oxygens
Si3.0242.9683.0093.0213.0343.0233.0803.1013.077
Ti0.0050.0160.0020.0060.0070.0150.0050.0020.001
Al1.8931.8941.9181.9461.9081.9101.9391.9291.957
Gr000000000
Fe3+0.0220.1340.046000000
Fe2+000.2660.1450.5930.3422.3732.2892.435
Mn3.0510.0062.7482.8282.3952.6610.0350.0340.123
Mg0.0010.0130.0010.0010.0020.0010.4780.4130.315
Ca0.0162.9800.0250.0350.0360.02340.0350.1650.038
V000000000
Mol% of end members
Uvarovite000000000
Andradite1.076.612.271.803.313.290.250.140
Pyrope0.030.440.020.030.070.0116.3914.2410.72
Spessartine99.450.2090.4095.1180.9189.841.201.174.20
Grossular091.3300000.945.551.29
Almandine008.743.6817.819.3781.2278.8982.98
Other01.420000000.82
Sample No.SLS-10SLS-11SLS-12SLS-13SLS-14SLS-15SLS-16SLS-17SLS-18
Oxides(wt.%)
SiO238.17941.08741.20341.10839.47538.81238.78939.59339.153
TiO20.0170.0160.0130.0140.1630.1990.0760.0430.105
Al2O320.56122.53122.28621.66019.41119.43320.52920.70520.620
Cr2O300000000.0930.106
FeOTa35.33417.62218.09020.5944.0004.5751.9871.8922.498
MnO1.4450.4870.5860.7670.2570.26129.00027.40727.401
MgO2.76113.69813.35711.8190.1410.3757.0738.2007.964
CaO0.4693.9603.8183.35736.40036.1691.8091.8131.465
V2O3000000000
Na2O0.0290.0050.0040.0060.0030.0040.0200.0110.014
K2O00.0010.001000.00100.0010.001
P2O50.1450.0500.0570.0520.0130.0200.0550.0610.087
Total98.94099.45799.41599.37799.86399.84999.33899.81999.414
Ions based on 12 oxygens
Si3.0943.0463.0623.0873.0092.9673.0393.0653.047
Ti0.0010.0010.0010.0010.0090.0110.0050.0030.006
Al1.9641.9691.9521.9171.7441.7511.8961.8891.891
Gr00000000.0060.007
Fe3+00000.2290.263000
Fe2+2.3951.0931.1241.294000.1300.1230.163
Mn0.0990.0310.0370.0490.0170.0171.9241.7741.806
Mg0.3281.5141.4801.3230.0160.0430.8260.9460.924
Ca0.0410.3150.3040.2702.9722.9630.1520.1500.122
V000000000
Mol% of end members
Uvarovite00000000.290.33
Andradite000011.4513.063.843.083.42
Pyrope11.1451.2750.7146.680.531.4127.9432.2731.35
Spessartine3.371.041.261.680.550.5665.0860.4861.28
Grossular1.3810.6510.008.0487.4684.961.301.760.39
Almandine81.3037.0038.0343.60001.842.123.23
Other2.820.050000000
Sample No.SLS-19SLS-20SLS-21SLS-22SLS-23SLS-24SLS-25
Oxides(wt.%)
SiO239.07238.93641.25834.98234.67838.76638.654
TiO20.3540.5380.0290.04100.2430.432
Al2O321.71921.83321.8260.0720.00217.41916.788
Cr2O30000.296000
FeOT a0.0730.07219.88029.05429.6296.5047.158
MnO1.3281.0190.7110.0100.0050.1570.102
MgO2.5350.56113.9600.0940.1240.5440.471
CaO36.27536.5791.42734.44934.72036.12236.139
V2O30.2940.20100000
Na2O0.0040.0040.0200.00100.0020.002
K2O000.001000.0010.001
P2O50.0170.0140.1760.0100.0130.0220.018
Total101.67199.75799.28899.00999.17199.78099.765
Ions based on 12 oxygens
Si2.9692.9533.0772.9912.9682.9732.971
Ti0.0200.0310.0020.00300.0140.025
Al1.9451.9511.9180.00701.5741.521
Gr0000.020000
Fe3+0.0040.00401.8691.9090.4170.460
Fe2+001.2400000
Mn0.0860.0660.0450.0010.0010.0100.007
Mg0.0610.0631.5520.0120.0160.0620.054
Ca2.9532.9720.1143.1563.1842.9682.976
V0.0340.02400000
Mol % of end-members
Uvarovite0000.95000
Andradite0.200.20088.5089.4520.5822.73
Pyrope1.962.0553.120.380.492.051.78
Spessartine2.762.111.540.020.010.340.22
Grossular95.0995.652.4010.1610.0577.0475.28
Almandine0042.950000
Other0000000
a T = Total.
Table
Table 3. List of chemical structure formulas of all the studied garnets.
Table 3. List of chemical structure formulas of all the studied garnets.
ClassificationsSpeciesSample No.chemical Structure Formula
B3+ = Al3+PyropeSLS-13(Mg1.32Fe2+1.29Ca0.27Mn0.05)2.93Al1.92[Si3.19O12]
SLS-12(Mg1.48Fe2+1.12Ca0.30Mn0.04)2.94Al1.95[Si3.06O12]
SLS-11(Mg1.51Fe2+1.09Ca0.31Mn0.03)2.94Al1.97[Si3.05O12]
SLS-21(Mg1.55Fe2+1.24Ca0.11Mn0.04)2.94Al1.92[Si3.08O12]
AlmandineSLS-8(Fe2+2.29Mg0.41Ca0.17Mn0.03)2.90Al1.93[Si3.10O12]
SLS-7(Fe2+2.37Mg0.48Mn0.03Ca0.04)2.92Al1.94[Si3.08O12]
SLS-10(Fe2+2.40Mg0.33Mn0.10Ca0.04)2.87Al1.96[Si3.09O12]
SLS-9(Fe2+2.44Mg0.31Mn0.12Ca0.04)2.91Al1.96[Si3.08O12]
SpessartineSLS-17(Mn1.77Mg0.95Ca0.15Fe2+0.12)2.99(Al1.89Cr0.01)1.90[Si3.07O12]
SLS-18(Mn1.81Mg0.92Fe2+0.16Ca0.12)3.01(Al1.89Cr0.01Ti0.01)1.91[Si3.05O12]
SLS-16(Mn1.92Mg0.83Ca0.15Fe2+0.13)3.03Al1.90[Si3.04O12]
SLS-5(Mn2.39Fe2+0.59Ca0.04)3.02(Al1.91Ti0.01)1.92[Si3.04O12]
SLS-6(Mn2.66Fe2+0.34Ca0.02)3.02(Al1.91Ti0.02)1.93[Si3.02O12]
SLS-3(Mn2.75Fe2+0.27Ca0.03)3.05(Al1.92Fe3+0.05)1.97[Si3.01O12]
SLS-4(Mn2.83Fe2+0.15Ca0.04)3.02(Al1.95Ti0.01)1.96[Si3.02O12]
SLS-1(Mn3.05Ca0.02)3.07(Al1.89Fe3+0.02Ti0.01)1.92[Si3.02O12]
GrossularSLS-25(Ca2.98Mg0.05Mn0.01)3.04(Al1.52Fe3+0.46Ti0.03)2.01[Si2.97O12]
SLS-24(Ca2.97Mg0.06Mn0.01)3.04(Al1.57Fe3+0.42Ti0.01)2.00[Si2.97O12]
SLS-15(Ca2.96Mg0.04Mn0.02)3.02(Al1.75Fe3+0.26Ti0.01)2.02[Si2.97O12]
SLS-14(Ca2.97Mn0.02Mg0.02)3.01(Al1.74Fe3+0.23Ti0.01)1.98[Si3.01O12]
SLS-2(Ca2.98Mg0.01Mn0.01)3.00(Al1.89Fe3+0.13Ti0.01)2.03[Si2.97O12]
SLS-19(Ca2.95Mn0.09Mg0.06)3.10(Al1.94V0.03Ti0.02)1.99[Si2.97O12]
SLS-20(Ca2.97Mn0.07Mg0.06)3.10(Al1.95Ti0.03V0.02)2.00[Si2.95O12]
B3+ = Fe3+AndraditeSLS-22(Ca3.16Mg0.01)3.17(Fe3+1.87Cr0.02Al0.01)1.90[Si2.99O12]
SLS-23(Ca3.18Mg0.02)3.20Fe3+1.91[Si2.97O12]
Table
Table 4. Raman activity and assignment of the studied garnets (cm−1).
Table 4. Raman activity and assignment of the studied garnets (cm−1).
SpeciesSi-O Stretching VibrationSi-O Bending Vibration[SiO4]4− Rotational Vibration[SiO4]4- Translation VibrationA2+ Translation Vibration
F2gEgA1gF2gEgA1gF2gEgA1gF2gEgF2gEg
Pyrope1046, 860 913635, 503, 485 555317 354 204
Almandine1039, 863 916631, 583, 500, 478596, 372555 315347 167211
Spessartine1026, 850 906628, 496, 471590, 369548 318347 163214
Grossular1003, 820 876625, 502, 474411544 325372 178243, 275
Andradite990, 839, 812870 548, 446572, 488513322, 308347365 170232, 261293
Table
Table 5. Infrared absorbance band frequencies (in cm−1) for the studied garnets.
Table 5. Infrared absorbance band frequencies (in cm−1) for the studied garnets.
SpeciesAsymmetric Stretching Vibration of Si-OSymmetric Bending
Vibration of Si-O		Asymmetric Bending Vibration of Si-OLattice Vibration
BCDEFGIHJ
Pyrope980908883-580529465490417
Almandine975908884637575529456483415
Spessartine968 ± 5900 ± 5874632570522457486 ± 10417
Grossular940 ± 3873 ± 3847621554508463490 ± 5415
Andradite928 ± 10848820590520482-447409
“-” peak or shoulder was not detected herein.
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Li, W.; Zheng, J.; Pei, J.; Xu, X.; Chen, T. Correlations between Garnet Species and Vibration Spectroscopy: Isomorphous Substitution Implications. Crystals 2022, 12, 104. https://doi.org/10.3390/cryst12010104

AMA Style

Li W, Zheng J, Pei J, Xu X, Chen T. Correlations between Garnet Species and Vibration Spectroscopy: Isomorphous Substitution Implications. Crystals. 2022; 12(1):104. https://doi.org/10.3390/cryst12010104

Chicago/Turabian Style

Li, Weiwei, Jinyu Zheng, Jingcheng Pei, Xing Xu, and Tao Chen. 2022. "Correlations between Garnet Species and Vibration Spectroscopy: Isomorphous Substitution Implications" Crystals 12, no. 1: 104. https://doi.org/10.3390/cryst12010104

APA Style

Li, W., Zheng, J., Pei, J., Xu, X., & Chen, T. (2022). Correlations between Garnet Species and Vibration Spectroscopy: Isomorphous Substitution Implications. Crystals, 12(1), 104. https://doi.org/10.3390/cryst12010104

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