Gemological Characteristics of Blue-Violet Cordierite

Abstract

Cordierite is a violet-blue gem mineral primarily composed of magnesium aluminum silicate. This study employed three samples of Mg-cordierite and conducted tests on their gemological characteristics, spectroscopic features, and chemical composition using Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, ultraviolet spectrophotometry, X-ray photoelectron spectroscopy (XPS), and electron microprobe. The study also explored and analyzed the polychroism and coloration mechanisms of the samples. The results indicate that the lattice vibrations of the Mg-cordierite samples differ along the directions parallel to the a, b, and c crystallographic axes, leading to certain variations in spectral characteristics among these directions. The article provides experimental evidence for the reasons for the polychroism of cordierite in different crystal axes, which is of great significance in the quality evaluation of cordierite.

1. Introduction

Purple-series gemstones have always been widely popular in the gem world for their rarity and mystery. However, research on purple gemstones has largely focused on amethyst, purple sapphires, and others. Comparatively, less attention has been paid to the gemological study of blue-violet cordierite. Cordierite has numerous sources, with gem-quality cordierite primarily distributed in India, Sri Lanka, Madagascar, the United States (California, Idaho, and Wyoming), Canada, and Greenland [1]. Cordierite is an orthorhombic crystal system mineral with a vitreous luster, potentially containing trace elements such as Na, K, Ca, Fe, Mn, and H₂O. Among these, Mg and Fe can form a complete isomorphous series. As the Fe content increases, the color deepens, along with corresponding increases in refractive index and density. Cordierite is found in metamorphic rocks, granulites, and gneisses, and cordierite coexists with other magnesium facies, such as garnet, biotite, orthopyroxene, spinel, or hornblende. It also coexists with aluminum-containing minerals, such as sillimanite and corundum [2]. Cordierite with blue-violet color is the most valuable among cordierites, and it shares many similarities in appearance and gemological properties with amethyst [3], sapphire [4], and tanzanite [5], making them easily confusable. The consensus regarding the bulk color and strong trichroism of cordierite attributes it to the charge transfer between Fe³⁺ and Fe²⁺ [6]. However, the valence state distribution of Fe ions in cordierite and the connection between the crystallographic axis direction and polychroism have not yet been explained.

Current research on cordierite is largely focused on the fields of materials and ceramics [7,8], with fewer studies on its gem mineralogic characteristics [9,10]. Charles and others have explained the coloration mechanism of cordierite, stating that its blue color originates from the charge transfer between Fe²⁺ in the octahedral sites and Fe³⁺ in the T₁1 tetrahedral sites [11]. Dudka et al. [12] conducted precise X-ray diffraction analysis on cordierite, identifying the locations of metal cations Na⁺, H₂O, and CO₂ within the channels of cordierite. Pollak [13] explained the sites of charge transfer in cordierite by examining the differences between infrared absorption bands along different crystallographic axes, providing a deeper explanation for its polychroism. However, specific studies on the spectral differences between different crystal axes are lacking. Duncan et al. [14] measured the Mössbauer spectra of cordierite single crystals in different directions, determining the positions and occupancy rates of Fe²⁺ and Fe³⁺ ions, suggesting that the exchange interaction between Fe³⁺ ions on the T₁ sites and the six-fold sites is the source of cordierite’s polychroism. Raphaël et al. [15] evaluated the spectroscopic properties of cordierite through TD-DFT, assessing the simulation of polychroism and the variation of color with different light sources. However, studies on the spectral differences along different crystallographic axes of cordierite and the content ratio of Fe²⁺ to Fe³⁺ are still lacking.

This paper investigates the coloration mechanism and the causes of polychroism in purple cordierite through conventional gemological testing, Fourier Transform Infrared Spectroscopy (FTIR), Raman Spectroscopy, Ultraviolet-Visible Spectrophotometry (UV–vis), X-ray Photoelectron Spectroscopy (XPS), and Electron Probe Microanalysis (EPMA). It elucidates the spectral differences of cordierite along different crystallographic axes and their impact on its polychroism. Additionally, the study determines the valence state and content ratio of Fe in cordierite through XPS, enhancing the related research on cordierite and providing experimental evidence for its gemological characteristics and identification.

2. Materials and Methods

2.1. Sample Materials

Figure 1 displays three samples of blue and purple cordierite from unknown locations in Madagascar, which were selected and purchased from local suppliers. All samples exhibit a fragmented appearance with evident polychroism. Notable cracks are present internally, and distinct inclusions can be observed.

2.2. Experimental Methods

Conventional Gemological Testing: The refractive index, specific gravity, ultraviolet fluorescence characteristics, and microscopic features of the samples were tested using a refractometer, hydrostatic weighing method, ultraviolet light, and gemological microscope, respectively. The testing was conducted at the Gemology Experimental Teaching Center of the Gemology College, China University of Geosciences, Beijing (CUGB).

The samples O1, O2, and O3 were cut into cubic blocks along their lattice directions and polished. The three central lines of the cube pass through the center point of the squared polished surface and are perpendicular to each other, with the directions of the three central lines parallel to crystallographic axes a, b, and c, respectively. By systematically testing and analyzing the UV-visible spectroscopy, infrared spectroscopy, and Raman spectroscopy of the cordierite samples in the three crystallographic axis directions, combined with XPS analysis and electron microprobe data, the study aims to reveal the polychroism and coloration causes of cordierite.

Raman spectra were obtained using the HR-Evolution micro-Raman spectrometer, manufactured by HORIBA, Japan, at the Gem Testing Laboratory within the School of Gemmology at CUGB. A microscope equipped with ×50 objectives was used to focus on the samples under precise test conditions: aperture set to 0.865 μm; power output maintained at 100 mW; the spectral range of 200–4000 cm⁻¹; integration time of 10 s; and laser wavelength of 532 nm.

The infrared spectra of the samples were obtained using the Tensor 27 FT-IR spectrometer produced by Bruker, Berlin, Germany. The tests were carried out at the Gem Testing Laboratory of the Gemology College, CUGB, with a scanning range of 400–2000 cm⁻¹ (reflection), a resolution of 4 cm⁻¹, and a scan time of 32 s. In addition, the samples were tested using the powder transmission method. The samples were divided into O1, O2, and O3, respectively, and then refined to 200 mesh with an agate mortar after cleaning. The mineral powder is mixed with analytically pure potassium bromide powder, and the ratio is about 1:100. After being fully mixed, the tablet is pressed on the oil press, and then the test is carried out.

Ultraviolet-visible absorption spectra were collected using the UV-3600 UV-Vis-NIR spectrophotometer, produced by Shimadzu Corporation, Kyoto, Japan, at the Gem Testing Laboratory of the Gemology College, CUGB. The test conditions were as follows: transmission method, wavelength range of 300~800 nm, slit width of 20 nm, and a sampling interval of 1.0 s.

EMPA analysis JXA-8230 was used for qualitative and quantitative analysis of major elements in samples at the Geochemical Test Center of China University of Geosciences (Beijing). EMPA uses X-ray energy dispersive spectrometry (EDS) for qualitative and quantitative analysis of the major elements in the sample. Samples were prepared by mounting them on 25 × 75 mm silica glass sheets and polished to 30 μm. The accelerating voltage was 15 kV, the current was 10 nA, and the beam spot diameter was 5 μm. ZAF3 was used to correct the data, and the principal elements and standards tested were Na-Albite, Fe-Garnet, Mg-Olivine, Mn-Rhodonite, Si-Plagioclase, K-Sanidine, and Al-Plagioclase.

X-ray Photoelectron Spectroscopy (XPS) analysis was performed using a Thermo K-Alpha XPS instrument (Thermo Fisher, Waltham, MA, USA). The X-ray excitation source utilized monochromatic Al K-Alpha, with a sample test area of 500 μm and a scanning range of 0–1400 eV at a step size of 1 eV per step, with one scan repetition. The transmission rate of the energy analyzer was set to 100 eV. Data processing was carried out using the Avantage 5.99 software, and the binding energies were calibrated using the standard C1s peak at 284.8 eV.

3. Results and Discussion

3.1. Conventional Gemological Characteristics

The cordierite samples in this paper have a refractive index of 1.535–1.555, a birefringence of 0.015–0.020, and a relative density range of 2.574–2.618 (

Table 1. Common gemological characteristics of blue-violet cordierite.

Sample	Luster	Transparency	Fluorescence	Refractive	Birefringence	Relative Density
O1	Vitreous	Transparent	Inert	1.535–1.555	0.020	2.618
O2	Vitreous	Transparent	Inert	1.539–1.555	0.016	2.585
O3	Vitreous	Transparent	Inert	1.535–1.550	0.015	2.574

), which is consistent with the theoretical range of optical parameters for cordierite, showing fluorescence inertia.

Crystal inclusions are common in cordierite, such as hematite, apatite, tourmaline, etc. [16]. Upon magnified observation, various mineral inclusions can be seen in the cordierite samples (Figure 2). Figure 2 shows the inclusions and their Raman spectra in samples O1, O2, and O3. In sample O1, golden platy inclusions (Figure 2a) and irregular blocky dark inclusions (Figure 2b) are observed. By comparing with the standard Raman spectra in the RUFF database, it is identified that the golden platy inclusions are phlogopite, and the dark inclusions are apatite; Sample O2 contains numerous brownish-red inclusions of hematite and limonite, which appear as hexagonal platy crystals, and additionally, euhedral to subhedral prismatic crystalline inclusions are observed, confirmed by Raman analysis to be tourmaline (Figure 2c); In sample O3, dark striped inclusions are visible, often exposed on the surface (Figure 2d), and Raman testing determined them to be actinolite.

3.2. Main Chemical Composition

The samples O1, O2, and O3 to be measured were prepared as electron microprobe thin sections, with ten points selected on each thin section for electron microprobe measurement. The measurement results (

Table 2. Elemental composition of the sample (wt.%).

Component	O1	O2	O3
SiO2	52.17	51.38	56.66
MnO	0.06	0.53	bdl
Na2O	0.17	0.27	0.51
MgO	11.64	9.96	11.63
FeO	2.91	4.60	1.68
K2O	bdl	bdl	bdl
Al2O3	32.05	32.11	27.34
Total	99.02	98.84	97.83
Oxygen	18	18	18
Si	5.188	5.169	5.638
Al	3.757	3.807	3.206
Fe	0.242	0.387	0.140
Mn	0.005	0.045	0.001
Mg	1.726	1.494	1.725
Na	0.033	0.053	0.098
K	0.003	0.001	0.001
Total	10.950	10.954	10.808
XMg	0.877	0.794	0.925

bdl = below detection limit.

) show that the cordierite samples have an average MgO content of 9.96–11.64 wt.%, an average SiO₂ content of 51.38–56.66 wt.%, and an average FeO content of 1.68–4.60 wt.%. Since volatile substances such as H₂O and CO₂, as well as cations like Na and K, are present in the channels, the total measured chemical composition data are usually less than 100 wt.% [17]. In the chemical analysis of different samples (Table 2), the Mg content of the samples ranges from 1.494 to 1.725 a.p.f.u. The X_Mg values (X_Mg = Mg/(Mg + Fe)) of the current samples were calculated based on the a.p.f.u data, ranging between 0.794 and 0.925, indicating that the samples are Mg-cordierite.

Cordierite is a cyclic silicate mineral composed of different types of tetrahedra and octahedra, with its chemical formula expressed as (M)₂(T₁1)₂(T₂6)₂(T₂3)₂(T₂1)₂(T₁6)O₁₈,(H₂O, CO₂). The crystal structure is shown in Figure 3.

Six T₂ tetrahedra (including four Si-O tetrahedra and two Al-O tetrahedra) are connected at their corners to form the hexagonal ring structure of cordierite, creating a wide channel along the c-axis. This channel often contains large radius cations such as Na⁺, as well as volatile components like H₂O and CO₂. The adjacent hexagonal rings are connected by T₂ tetrahedra and are staggered by approximately 30°. Octahedra share edges with T₁ tetrahedra while also sharing O²⁻ with the T₂ tetrahedra in the hexagonal rings [18].

In cordierite, the tetrahedral T₁ and T₂ sites are typically occupied by smaller radius cations such as Be, Al³⁺, and Si⁴⁺. Among these, Al³⁺ occupies both the T₁1 and T₂6 sites, while Si⁴⁺ occupies the T₁6, T₂1, and T₂3 sites. The octahedral M site in cordierite is usually occupied by medium-sized cations, such as Mg²⁺, Fe²⁺, and Mn²⁺ [14].

Using the oxygen atom method with O=18 as the basis, the chemical formula of the cordierite sample can be calculated (

Table 3. Ideal chemical formula of cordierite.

Sample	Chemical Formula
O1	chNa0.033(Mg1.726, Fe0.242, Mn0.005)1.973Al3[Al0.757Si5.188O18]
O2	chNa0.053(Mg1.494, Fe0.387, Mn0.045)1.926Al3[Al0.807Si5.169O18]
O3	chNa0.098(Mg1.725, Fe0.140)1.865Al3[Al0.206Si5.638O18]

).

3.3. Ultraviolet-Visible Spectrum Analysis

To systematically study the color and pleochroism of cordierite, this paper processes the purple cordierite samples into nearly equal-sized cubes. The polished faces of the cubic blocks have their perpendicular lines parallel to the three crystallographic axes, respectively. The processed cordierite exhibits strong pleochroism (Figure 4), with the three samples appearing nearly colorless or light yellow in the parallel a-axis direction, light purple in the parallel c-axis direction, and uniformly dark purple in the parallel b-axis direction.

This paper conducts ultraviolet-visible light spectroscopic analysis from three crystallographic axes directions, respectively. The results show that the ultraviolet spectra (Figure 5) of the three samples are consistent with the spectral characteristics of standard cordierite samples, with certain differences in absorption intensity and band positions in different directions. We discovered that the ultraviolet spectral characteristics of cordierite are relatively consistent when measured in the parallel a-axis and c-axis directions, whereas the absorption intensity of the ultraviolet spectrum in the parallel b-axis direction differs significantly from those observed in the other two test directions.

A weak absorption band appears in the red region of the ultraviolet spectrum in the parallel b-axis direction, transmitting a small amount of red and blue light. A broad absorption band centered at 584 nm emerges in the orange-yellow region, which can absorb a large amount of yellow and orange-red light and transmit a significant amount of violet light, resulting in the cordierite exhibiting a rich blue-purple color in the parallel b-axis direction. The ultraviolet spectrum measured in the c-axis direction shows that the absorption band centered at 584 nm is much weaker than in the b-axis direction, hence presenting a light purple appearance. In contrast, the ultraviolet spectrum measured in the a-axis direction, compared to the b-axis direction, does not have a noticeable broad absorption band, thus absorbing more violet light and transmitting more yellow and red light, causing the gemstone to appear light yellow or nearly colorless when viewed with the naked eye in this direction. In summary, the ultraviolet spectrum results from the three directions are consistent with the pleochroic characteristics of the samples (

Table 4. Parameters of cordierite Fe2p peak.

Sample		Peak Position	Peak Area	Half-Height Width (eV)
O1	Fe2p3	709.6	458.09	1.88
	Fe2p3	711.4	1953.23	3.37
	Fe2p1	723.4	200.96	1.88
	Fe2p1	725.2	856.84	3.37
O2	Fe2p3	709.13	1517.27	3.34
	Fe2p3	711.89	3074.75	3.37
	Fe2p1	722.93	665.59	3.34
	Fe2p1	725.48	1348.83	3.37
O3	Fe2p3	709.37	414.43	3.37
	Fe2p3	711.40	1265.96	3.37
	Fe2p1	723.17	181.80	3.37
	Fe2p1	724.73	555.35	3.37

).

Through ultraviolet spectroscopic analysis, it can be observed that the broad absorption band centered at 584 nm in cordierite samples varies in intensity among different crystallographic axis directions. This variation is a key reason for the strong pleochroism exhibited by the cordierite samples. The intensity of this absorption band is closely related to the pronounced degree of purple hues in the samples; the stronger the absorption band, the more pronounced the purple hues presented in the sample. This broad absorption band results from the charge transfer between Fe²⁺ on the octahedral sites and Fe³⁺ on the tetrahedral T₁1 sites in cordierite [19]. This is also the main reason cordierite exhibits a blue-purple appearance.

The absorption near 494 nm in the blue-green region is related to the spin-forbidden transition of Fe²⁺. In the 300–450 nm range, three weak absorption bands are present in all three blue-purple samples. Among them, the absorption band at 441 nm is caused by the d-orbital electronic transition of Fe³⁺ from the ground state ⁶A_1g level to the excited states ⁴A_1g and ⁴E_g. The absorption at 390 nm corresponds to the spin-forbidden d–d-orbital transition of Fe³⁺ (⁶A_1g→⁴T_2g) [20].

In summary, the blue-purple body color and strong pleochroism of cordierite are primarily caused by the charge transfer between Fe²⁺ on the octahedral sites and Fe³⁺ on the tetrahedral sites. This charge transfer leads to d-orbital overlap, resulting in the variation of intensity in the broad absorption band at 584 nm across different directions, which in turn causes the strong pleochroism of cordierite [19]. Moreover, the blue-purple body color is more popular in the market compared to colorless or light yellow hues. Therefore, when cutting cordierite, the table should be made as parallel as possible to the (010) plane to maximize its rich blue-purple color. However, since cordierite has a medium cleavage along the (010) set, the table needs to be cut at a 5° angle to the (010) plane to prevent cleavage planes from flaking off.

3.4. X-ray Photoelectron Spectroscopy

The results of electron probe and ultraviolet-visible spectroscopy analyses indicate that the main coloring element in purple cordierite is (Fe), but the valence distribution of Fe in cordierite and the content of different valence states are unknown. This paper uses X-ray photoelectron spectroscopy to characterize the valence states and content of Fe. A wide spectrum scan was performed on the sample to determine the types of elements present in the sample, followed by a narrow spectrum detailed scan to determine the chemical states of transition metal elements in the sample.

The wide spectrum of the sample’s X-ray photoelectron spectroscopy is shown in Figure 6. The spectrum indicates that the cordierite sample contains elements such as O, Al, Si, Mg, Fe, and Be. It is noteworthy that the characteristic peak of the coloring element Fe has a relatively weak intensity in the wide spectrum scan, suggesting that the Fe content in the sample is low.

Under the principle of XPS, the Fe2p layer electrons in cordierite undergo energy level splitting due to spin-orbit coupling when excited by X-rays, resulting in the appearance of double peaks as 2p3/2 and 2p1/2 in the XPS spectrum (Figure 7). The 2p3/2 refers to the binding energy of Fe²⁺ at 709.2–709.6 eV, while the 2p1/2 refers to Fe³⁺, which has a higher binding energy of 711.4–711.9 eV.

The coloration of cordierite is primarily due to the charge transfer between Fe²⁺ and Fe³⁺. The characteristic energy spectrum line analysis results of the Fe ion are shown in Figure 5. The peak shape of the Fe2p energy spectrum line exhibits obvious asymmetry, with a significant shoulder peak appearing on the high binding energy side of Fe2p3/2, indicating that the Fe2p3/2 energy spectrum peak is a composite. Through peak fitting, it was found that this peak is a combination of peaks at 709.4 and 711.5 eV, corresponding to FeO and Fe₂O₃ compounds, respectively. This indicates that two valence states of Fe exist in the cordierite sample, with both Fe²⁺ and Fe³⁺ being coordinated with O to form bonds. Among them, the peak area ratio of Fe³⁺ to Fe²⁺ for sample O1 is 4.26:1, for sample O2, it is 2.02:1, and for sample O3, it is 3.05:1. Based on the calculated areas of the fitted peaks, the contents of Fe²⁺ and Fe³⁺ are approximately 25.55% and 74.45%, respectively.

3.5. Infrared Spectrum Analysis

The infrared spectrum of cordierite exhibits a distinctive strong band near 1193 cm⁻¹, accompanied by a weak reflection band at 1145 cm⁻¹. The reflection band near 1193 cm⁻¹ is related to the stretching mode of SiO₄, while the satellite band at 1140 cm⁻¹ is caused by the asymmetric stretching vibration of Si-O-Si in the structure. In the parallel b-axis direction, this strong reflection band differs from the other two directions, with a higher wavenumber (located at 1199 cm⁻¹) and a higher reflectance.

There is a strong reflection band at 961 cm⁻¹ accompanied by multiple weak reflection bands. Among them, the strong band at 904 cm⁻¹ and weak reflection bands such as 961 cm⁻¹ and 1049 cm⁻¹ are attributed to the stretching vibration of the Al-O tetrahedron [21]. The band intensity of the satellite band at 904 cm⁻¹ is significantly weaker in the parallel b-axis direction compared to the other two directions.

The reflection band near 770 cm⁻¹ is a characteristic band resulting from the vibration of the cordierite’s six-membered ring structure, and the band intensity in the parallel b-axis direction is significantly higher than in the other two crystal axes. Distinct reflection bands can be observed near 671 and 617 cm⁻¹ in the infrared spectrum, both attributed to the vibration of the octahedron. The former is weak to an almost invisible parallel to the a-direction, while the latter is an invisible parallel to the b-direction. The reflection band at 584 cm⁻¹ is attributed to the vibration of the octahedron.

In the infrared spectrum, several reflection bands between 400 and 500 cm⁻¹ exhibit a finger-like band shape, specifically at 484, 446, and 421 cm⁻¹. Notably, the reflection band near 482 cm⁻¹ is associated with the bending vibration of the Si-O tetrahedron. Additionally, the band intensities of multiple reflection bands below 500 cm⁻¹ are significantly stronger in the parallel b-axis direction compared to the other two directions (Figure 8).

The sample particles were selected and tested using powder transmission methods, revealing distinct water-related spectral bands in the range of 3300–3900 cm⁻¹. Comparing the results with the studies conducted by Goldman et al. [22], the spectral band at 3687 cm⁻¹ is attributed to the type I stretching vibration mode of H₂O. The spectral bands observed at 3627 cm⁻¹ in sample O1 and 3579 cm⁻¹ in samples O2 and O3 are assigned to the type II stretching vibration of H₂O [23,24]. Through infrared spectroscopic analysis, it is confirmed that both Type I H₂O and Type II H₂O coexist in the cordierite samples [25].

3.6. Raman Spectrum Analysis

The Raman spectral results of cordierite in three crystallographic directions show that there is a significant difference in Raman intensity among different crystallization directions within the same sample.

The Raman spectra (Figure 9) of the samples largely correspond to the characteristic Raman spectra of standard cordierite. The Raman spectra of the three samples are similar in different crystallization directions but are distinct in the parallel b-axis direction compared to the parallel a-axis and c-axis directions. Bands at 1011, 973, 670, 556, 577, 260, and 238 cm⁻¹ are present in all three directions of the three samples, yet the spectral band intensities obtained from different crystallization directions vary.

As shown in Figure 10. In the parallel b-axis direction, the Raman bands at 1181 and 368 cm⁻¹ are sharp, while in the other two directions, they are weak to the point of being almost invisible. Both Raman bands at 973 and 1011 cm⁻¹ are present in all three directions of the sample, but the band at 1011 cm⁻¹ is very weak in the parallel b-axis direction, whereas in the parallel a-axis and c directions, the band at 1011 cm⁻¹ is high and significantly stronger than the band at 973 cm⁻¹. The Raman bands at 260 cm⁻¹ and its companion band at 238 cm⁻¹ are sharper in the parallel b-axis, with the companion band at 238 cm⁻¹ being very weak in the other directions. The band at 1381 cm⁻¹ position is noticeably sharp in the parallel b-axis, while it appears somewhat duller in the other directions. The Raman band at 327 cm⁻¹ is duller in the parallel b-axis direction and sharper in the parallel a-axis and c directions.

This study found that within the range of 1100–200 cm⁻¹, the diffraction intensity parallel to crystallographic axes a and c is stronger than the parallel b-axis direction. This indicates that the Si-O symmetric stretching vibration diffraction is weakest along the parallel b-axis direction. The density of silicon-oxygen groups per unit area can explain this phenomenon. It is inferred that cordierite is most difficult to compress along the b-axis direction due to anisotropic compression, and thus, compared with other crystallographic directions, there are more SiO₄ tetrahedra or Si₅O₁₈ groups per unit length in the parallel b-axis direction. In other words, on the parallel b-axis plane, the number of SiO₄ or Si₅O₁₈ per unit area is the lowest, which is why the Si-O vibration diffraction obtained along the parallel b-axis direction is the weakest.

The band at 1383 cm⁻¹ may be attributed to the symmetric stretching vibration of CO₂ molecules, which often align parallel to the b-axis within the channel cavities. Hence, the Raman band parallel to the b-axis is stronger. This may relate to the relatively longer cavity length parallel to the b-axis. Both 1181 and 973 cm⁻¹ are attributed to the stretching vibrations of SiO₄ [23].

The band at 1011 cm⁻¹ is caused by the stretching of the T₂1 and T₂3 tetrahedra in the hexagonal ring and the T16 tetrahedron connected to the M site, while 973 cm⁻¹ may also be related to the stretching of the T₂1 and T₁6 sites [24]. The band position at 670 cm⁻¹ is caused by the Al-O tetrahedral stretching vibration at the T₁1 site. The strong Raman bands between 530 and 600 cm⁻¹ split into 556 and 577 cm⁻¹, related to the bending vibrations of the T₂6, T₂1, and T₂3 sites, as well as the stretching of the M octahedron and the T₂6 site. The Raman band at 482 cm⁻¹ may be the result of a combination of bending vibrations and other modes, sharp in the parallel c-axis direction and duller in the parallel b-axis and directions. The weak band at 459 cm⁻¹ is caused by the bending modes of the T₂1 and T₂3 sites. The 430 cm⁻¹ is attributed to the bending vibration of the T₂6, and 368 cm⁻¹ is due to the bending vibration of the M octahedron, sharp in the parallel b-axis direction, and weakening to almost disappearance in the parallel a-axis and c-axis directions [26]. The band positions at 295, 260, and 238 cm⁻¹ are attributed to the bending vibrations of the Si-O tetrahedra, Al-O tetrahedra, and M-O octahedra.

In the hexagonal ring structure of cordierite, the interaction between the alkali metal ions located in the [000] direction and H₂O molecules determines the type of H₂O. H₂O that does not interact with alkali metal ions is classified as Type I H₂O, while H₂O that interacts with alkali metal ions belongs to Type II H₂O [24,27].

It is noteworthy that the content of alkali metal ions in cordierite is related to the spectral band intensity caused by Type II H₂O. As the Na content in cordierite increases, the spectral band intensity of Type II H₂O in both infrared and Raman spectra also correspondingly strengthens [22,28]. The infrared and Raman spectra of the sample correspond to the Na content of the sample; the Na content of O1 is lower than that of O2 and O3, and its spectral band intensity related to Type II H₂O is also lower. Raman analysis confirms that the sample contains a certain amount of Type I H₂O and a small amount of Type II H₂O, which is consistent with the results of infrared spectroscopy analysis (Figure 11).

To determine the species of the cordierite samples, the sample’s Raman bands were compared with the Raman spectrum of Mg-cordierite, calculated by Kaindl et al. (2011) [23] to be closest to the pure endmember composition.

Previous studies have shown that with the increase of Fe, most of the Raman modes of cordierite will shift to lower wavenumbers [29,30]. The Raman band positions of the cordierite samples in this study are slightly offset to lower wavenumbers compared to pure Mg-cordierite (

Table 5. Raman spectra of cordierite samples and Kaindl et al. (2011) calculated the vibration mode deviation of pure Mg-cordierite and its attribution.

Mg-Crd	O1	Dev.	O2	Dev.	O3	Dev.	Assignment
244	238	−6	238	−6	238	−6	b(M)
260	260	0	258	−2	258	−2	b(T11,T26)
297	295	−2	307	10	294	−3	b(T16,T21,T23)
330	327	−3	325	−5	325	−5	b(M)
370	368	−2	366	−4	366	−4	b(M)
424	428	4	426	2	428	4	b(T26)
556	556	0	556	0	554	−2	s(T26,M),b(T16,T26)
574	577	3	577	3	577	3	s(M),b(T21,T23,T26)
672	670	−2	670	−2	670	−2	s(T11)
964	951	−13	951	−13	949	−15	s(T21,T16)
978	973	−5	971	−7	971	−7	s(T21,T23)
1015	1011	−4	1011	−4	1011	−4	s(T16,T21,T23)
1176	1181	5	1181	5	1181	5	s(T21,T23)

Note: Mg-Crd = calculated Raman band for pure Mg-cordierite by Kaindl et al. 2011; Dev. = deviation of the observed band from the calculated band for Mg-cordierite (Mg-Crd); s = stretching; b = bending.

). Combined with the results of the electron probe and XPS analysis, it is inferred that the samples in this study are Mg-cordierite.

4. Conclusions

This article presents the gemological characteristics, spectroscopic characteristics, and chemical composition of cordierite through fundamental gemological testing, ultraviolet spectroscopy, infrared spectroscopy, electron probe analysis, and Raman spectroscopy. The sample’s refractive index and relative density are lower compared to iron cordierite, with an X_Mg range between 0.79 and 0.96. The Raman spectral characteristics are closer to those of pure Mg-cordierite, indicating that the samples are Mg-cordierite. The cordierite samples contain both Fe²⁺ and Fe³⁺ valence states, with an average ratio of 25.55% to 74.45%. The ultraviolet spectral characteristics of cordierite indicate that the difference in the broad absorption band at 584 nm is the key to cordierite’s strong pleochroism (deep purple/light purple/light yellow), caused by charge transfer between Fe²⁺ in the octahedron and Fe³⁺ in the tetrahedron. Infrared and Raman spectroscopy results reveal that differences in the Si-O group density in different directions cause variations in the spectral characteristics among the three optical principal axis directions of cordierite, with the greatest difference along the parallel b-axis, while the spectral characteristics of the other two directions are similar. Both infrared and Raman spectroscopy also prove the presence of a certain amount of Type I H₂O and a small amount of Type II H₂O in Mg-cordierite, with the band intensity of Type II H₂O being directly proportional to the sample’s Na content. This preliminary study focuses on the basic gemological characteristics of cordierite, explores spectral differences along various crystallographic directions, and undertakes a preliminary investigation into the coloration mechanism and causes of pleochroism in cordierite. Such insights not only enhance our understanding of cordierite but also offer a new approach to its identification and classification.