Inclusions and Spectral Characterization of Demantoid from Baluchistan, Pakistan

Abstract

Demantoid is the green variety of andradite [Ca₃Fe₂(SiO₄)₃], an exceptionally rare and precious gemstone worldwide. In recent years, a small amount of gem-quality demantoid has been found in Pakistan. This research focuses on nine demantoids sourced from Muslim Bagh, Baluchistan, Pakistan, presenting a comprehensive analysis of the spectral characteristics and inclusions of Pakistani demantoid using classical gemological methods, energy dispersive X-ray fluorescence (EDXRF) chemical analyses, Fourier transform infrared (FTIR) spectroscopy, Raman spectroscopy, and ultraviolet and visible (UV-vis) spectroscopy. The results show that the content of Cr and V in most samples is lower than the detection line of EDXRF, with only one sample containing a Cr₂O₃ content of 0.032%. The extremely low Cr content sets Pakistani demantoid apart from demantoid of the serpentinite type found in other regions. Notably, the UV-vis spectrum reveals characteristic absorption at 443 nm due to Fe³⁺, while a further contribution from Cr³⁺ would be highly likely, and weak absorption at 550 nm caused by Fe³⁺. This suggests that iron (Fe) is the primary chromogenic element of Pakistani demantoid, but the role of Cr³⁺ cannot be ignored. The FTIR spectrum of Pakistani demantoid displays the absorption peaks associated with [SiO₄]⁴⁻ groups at 937 cm⁻¹, 848 cm⁻¹, and 817 cm⁻¹, while the absorption peaks resulting from trivalent cations appear at 481 cm⁻¹ and 442 cm⁻¹, which are the characteristic FTIR spectra of demantoid. Raman spectroscopy further reveals absorption peaks are displayed near 994 cm⁻¹, 843 cm⁻¹, 818 cm⁻¹, associated with (Si–O)_Str vibrations (Si–O stretching vibration), and absorption peaks are displayed near 350 cm⁻¹ and 310 cm⁻¹, related to the rotation of SiO₄–R(SiO₄)⁴⁻, and the peaks near 514 cm⁻¹ and 494 cm⁻¹ are related to (Si–O)_bend vibrations (Si–O bending vibration). Additionally, related absorption peaks near 168 cm⁻¹ are attributed to the translation of SiO₄–T(SiO₄)⁴⁻, and absorption peaks near 234 cm⁻¹ are associated with the translation of X²⁺–T(X²⁺) (X²⁺ represents divalent ions). The common dark opaque inclusions found in Pakistani demantoid consist of a combination of magnetite and hematite. Additionally, some samples of Pakistani demantoid display inclusions of calcite. This unique combination of inclusions differentiates Pakistani demantoid from demantoids sourced from other regions. It signifies that Pakistani demantoid has a distinctive geological origin resulting from the interplay of serpentinization and skarnization processes. This geological formation distinguishes it from demantoids solely hosted in serpentinite or skarn environments in other origins. The identification of these characteristics holds significant importance for accurately determining the origin of Pakistani demantoid.

1. Introduction

The general chemical formula for garnet group minerals follows the pattern A₃B₂[SiO₄]₃, where A represents divalent ions with eightfold coordination, primarily Ca²⁺, Mg²⁺, Fe²⁺, and Mn²⁺, and B represents trivalent ions with sixfold coordination, primarily Al³⁺, Fe³⁺, V³⁺, and Cr³⁺ [1,2]. According to the definitions by GIA (Gemological Institute of America) and CIBJO (World Jewellery Confederation), demantoid is the green variety of andradite [Ca₃Fe₂(SiO₄)₃] [3,4].

The name “Demantoid” is derived from its resemblance to diamonds, which arises from the demantoid’s refractive index, an impressive 1.88, coupled with a high dispersion of 0.057. Demantoid is renowned for its stunning color, exceptional luster, and rarity, which make it one of the most highly prized gemstones within the garnet group. Gem-quality demantoids typically have diminutive dimensions, with polished stones usually achieving a weight between 1 and 2 carats. The values assigned to demantoid garnets fluctuate significantly, contingent upon the gemological properties mentioned above. Demantoid is found in limited quantities around the world, with recognized gem-quality sources in Russia, Iran, Namibia, Italy, and Madagascar [3,4,5,6,7,8,9]. In recent years, gem-quality demantoid crystals originating from serpentinite have been discovered in the Muslim Bagh region of Baluchistan, Pakistan.

From the perspective of geological genesis, demantoid garnet can be broadly divided into two petrogenetic groups: Skarn-hosted and serpentinite-hosted, with the majority originating from serpentinite environments [10]. At present, there are three methodologies for identifying the origin of demantoid. One type is based on the characteristics of inclusions: Demantoid originating from serpentinite usually contains fibrous inclusions (as known as “horsetail”), with the composition usually being chrysotile, and minerals associated with the metamorphism of mafic and ultramafic rocks such as antigorite, talc, asbestos, Cr-magnetite, and chromite [1,6,7,11]. The general distinctive inclusions of skarn-host demantoid are apatite, calcite, quartz, and Ca–Si ± Mg minerals such as wollastonite, diopside, and vesuvianite [4,5,12,13]. Another method: Some researchers systematically analyze the minor and trace elements of demantoids from different provenances (focus on Ural, Russia; Erongo area, Namibia; Antetezambato, Madagascar; Val Malenco, Italy; Khuzdar, Baluchistan, Pakistan) and differentiate the chemical fields by applying modern chemometrics [14], which helps establish a database and determine the origin of demantoid. Thirdly, the mid-infrared spectrum can reveal the differences between skarn-type and serpentinite-type demantoids: The spectra of samples originating from serpentinites have a notable absorption band around 3560 cm⁻¹, accompanied by an occasional weak band at approximately 3604 cm⁻¹; the spectra of demantoids originating from skarns display a more complex absorption pattern, featuring four peaks at approximately 3560, 3580, 3610, and 3630 cm⁻¹ [3,15]. In addition, the morphological features of rough demantoids can serve as a secondary reference for identifying their origins [13].

The Muslim Bagh area is characterized by a geological formation known as an ophiolite sequence, which is nearly complete in this region. The predominant rock types within this sequence are partially to completely serpentinized peridotite, specifically harzburgite and dunite. Notably, there are numerous outcrops of dunite that contain chromite deposits [1]. This geological composition provides the foundation for the occurrence of demantoid in the Muslim Bagh area and highlights the significance of the interplay between serpentinization and chromite mineralization in the formation of this unique gemstone.

Previous studies have provided preliminary insights into the inclusions and composition of demantoid from Pakistan, noting the common presence of magnetite inclusions [10]. Despite these initial findings, there remains a lack of comprehensive research on the spectroscopy, chromogenic mechanisms, and geological origin of Pakistani demantoid. Furthermore, the appearance and inclusions observed in the demantoid samples of this study exhibit notable differences compared to those typically associated with serpentinite. Given the potential market value of these samples and the limited existing studies [1,2,10,14], further investigation into Pakistani demantoid is warranted. Such research holds significance for understanding the unique characteristics and market potential of this gemstone.

2. Regional Geological Setting

The Saplai Tor Ghar Cr Mine in Muslim Bagh, where demantoids are found, is situated in the plate junction zone between the Indian plate and the Eurasian continent within the Muslim Bagh ophiolite (Figure 1). The ophiolite stretches approximately 100 km in a northeast-to-east direction and is around 30 km wide. Within this region, several bodies of basic ultrabasic rocks are intermittently exposed from east to west [16].

The ophiolite was thrust southward against the sedimentary layer of the continental margin on the northern edge of the Indian plate. It is unconformably covered by carbonate rocks and clastic rocks that range from the Eocene to Pliocene epochs [16,17].

The Muslim Bagh ophiolite primarily comprises three sets of tectonic schists. The lower part is known as the Bagh melange, which includes Triassic–Jurassic sedimentary rocks, Jurassic–Cretaceous basic volcanic rocks, and Cretaceous radiolarian siliceous rocks. These rocks are thrust and covered by serpentinized peridotite, gabbro, basalt, radiolarian siliceous rocks, and other melanges. The geochemical characteristics of the basalt within the Bagh melange resemble those of mid-ocean ridge basalt [16].

The middle part consists of a series of greenschist and amphibolite facies metamorphic rocks. These rocks are intermittently exposed between the upper and lower parts of the ophiolite suite, forming imbricated rock slices. The original sequence of these metamorphic rocks is challenging to identify due to strong shear transformation. The formation of this metamorphic rock set may be associated with the obduction of oceanic crust [16].

The upper part of the ophiolite is referred to as the Muslim Bagh ophiolite, primarily composed of serpentinized harzburgite, dunite, alloperidotite, and other ultrabasic rocks. The dunite occurs in strip-like formations within the oblique harzburgite [16,17].

Figure 1. Geological map of Muslim Bagh area (modified after Siddiqui et al. 1996 [16]; Kakar 2011 [18]; Mohammad et al. [19], 2014; Kakar et al., 2015) [20].

Figure 1. Geological map of Muslim Bagh area (modified after Siddiqui et al. 1996 [16]; Kakar 2011 [18]; Mohammad et al. [19], 2014; Kakar et al., 2015) [20].

All the origins here investigated are listed in

Table 1. Localities and geological environments of several areas (modified from Rosangela Bocchio, et al., 2010) [2].

Locality	Geology
Muslim Bagh, Baluchistan, Pakistan	serpentinite
Erongo Mountains, Damaraland, Namibia	skarn
Chelyabinsk Oblast, south Urals, Russia	serpentinite
Val Malenco, Sondrio, Italy	serpentinite
Kerman Province, Iran	serpentinite
Ambanja, Antetezambato, Madagascar	skarn

, with the details of localities and geological environments.

3. Materials and Methods

3.1. Materials

Nine original demantoid crystals mined from Saplai Tor Ghar Cr Mine, Muslim Bagh, Baluchistan, Pakistan, were used in this study (Figure 2), measuring 7.3 mm × 5.1 mm or less, all from the same mine. The total specimens were bought from a merchant in the Peshawar gem market. All crystals have not undergone any treatment, such as cutting, grinding, or polishing. These rough stones possess smooth facets and do not necessitate further polishing.

3.2. Methods

3.2.1. Standard Gemological Methods

The samples underwent examination utilizing standard gemological methods to characterize their refractive indices, densities, ultraviolet fluorescence, and microscopic characters. The refractive indices (RI) were measured with a Fable refractometer using sodium light (589 nm) and CH₂I₂ saturated with sulphur as a contact liquid (RI = 1.81). A hydrostatic balance assembled from a Mettler electronic balance was used to determine the densities of the samples. The samples were investigated for ultraviolet fluorescence under long (365 nm) and short (254 nm) wavelength ultraviolet light. A thorough petrographic study was conducted using a ZEISS Temi2000-C gemological microscope (made in Guangzhou, China), equipped with a Nikon DS-Ri1 (made in Ayuthaya, Thailand) high-resolution microscope camera under a cold light source.

3.2.2. Energy Dispersive X-ray Fluorescence (EDXRF)

X-ray fluorescence (EDXRF) was performed at the China University of Geosciences (Beijing, China) using an EDX-7000 made in Shimadzu, Kyoto, Japan, with a 3.15 A current, 230 V, and 6.6 A vacuum pump. Accuracy of 10 ppm. The test elements range from Al to U. Three points of each sample were taken for measuring, and the average was calculated to obtain the final result.

3.2.3. UV-Vis Spectrophotometry

UV-vis spectrophotometry was performed at the School of Gemology of the China University of Geosciences (Beijing, China). The instrument is a Shimadzu UV-3600 ultraviolet-visible spectrophotometer from Kyoto, Japan, equipped with an integrating sphere. Test conditions: reflection method, wavelength range of 200–900 nm, resolution of 0.1 nm, sampling interval of 0.5 s, data interval of 0.5 nm, scan rate of 60 nm/min.

3.2.4. Fourier Transform Infrared (FTIR) Spectroscopy

Fourier transform infrared spectroscopy was performed at the School of Gemology of the China University of Geosciences (Beijing, China). The instrument is a BrukerTensor27, a Fourier transform infrared spectrometer from Hamburg, German. Test conditions: reflected light, the scan time of the background was 8 scans, the resolution was 4 cm⁻¹, the grating set to 6 mm, 10 kHz, the spectral range set to 400~2000 cm⁻¹, and the number of collections was 64.

3.2.5. Raman Spectroscopy

Raman spectroscopy was performed at the School of Gemology of China University of Geosciences, Beijing. The instrument model is a LabSpec 6 laser Raman spectrometer produced by Horiba Scientific, Osaka, Japan. The experimental conditions are as follows: the exposure time was 1 s, the wavelength of the excitation laser was 532 nm, and the scanning wave number range was 100~2000 cm⁻¹.

4. Results and Discussion

4.1. Standard Gemological Properties

The gemological properties of samples of Pakistani demantoid are summarized in

Table 2. Gemological properties of the demantoid samples.

Sample	P1	P2	P3	P4	P5	P6	P7	P8	P9
Color	Green	Brownish green	Yellowish green	Yellowish green	Green	Deep green	Green	Brownish green	Green
Luster	Bright glassy luster	Bright glassy luster	Bright glassy luster	Bright glassy luster	Bright glassy luster	Glassy luster	Bright glassy luster	Glassy luster	Bright glassy luster
Diaphaneity	Transparent	Transparent	Transparent	Transparent	Transparent	Transparent	Transparent	Transparent	Transparent
Refractive index	>1.81	>1.81	>1.81	>1.81	>1.81	>1.81	>1.81	>1.81	>1.81
Density	3.84	3.86	3.83	3.83	3.83	3.86	3.84	3.86	3.84
UV fluorescence	Inert	Inert	Inert	Inert	Inert	Inert	Inert	Inert	Inert

. The samples vary from yellowish green to deep or brownish green, from glassy luster to bright glassy luster with refractive indices >1.81, and from translucent to transparent. The measured density ranges from 3.83 to 3.86 g/cm³ and the samples are inert to both short- and long-wavelength ultraviolet radiation. The appearances of the samples are mostly the poly-shaped rhombic dodecahedron and tetragonal trisoctahedron (Figure 3a). There are corrosion marks and pits on the surface of some samples, and there is no cleavage (Figure 3b,c). A conchoidal fracture can be seen at the damaged part (Figure 3d); multiple growths can be seen in some parts, which range from 2 to 6 mm in size (Figure 3d,e); most of the black inclusions can be seen by the naked eye. Under the spectroscope, a strong absorption band at 443 nm of Fe³⁺ can be seen in each demantoid sample.

4.2. Inclusions

Black, opaque, cubic, solid inclusions with a diameter of 0.3–0.5 mm are typically found in demantoids from Pakistan, and the inclusions have relatively complete crystal shapes (Figure 4). This characteristic sets it apart from demantoid found in other areas. Raman testing has confirmed that the black solid inclusions are composed of magnetite.

The Raman spectrum of inclusions shows that the black opaque inclusions of Pakistani demantoids are magnetite, which is consistent with the results obtained by Palke et al. from LA-ICP-MS (laser ablation (microprobe) inductively coupled plasma mass spectrometry, an instrument for detecting chemical composition.) testing on black inclusions [10]. Taking the Raman spectrum of sample P3 (Figure 5) as an example. The Raman spectrum shows the characteristic absorption peaks at 310 cm⁻¹, 553 cm⁻¹, and 679 cm⁻¹ of magnetite, which agree with magnetite from the RRUFF online database. In addition, hematite and calcite were also detected among the inclusions. Hematite inclusions: Characteristic absorption peaks at 114 cm⁻¹, 283 cm⁻¹, 396 cm⁻¹, 594 cm⁻¹, and 1303 cm⁻¹ can be seen. Calcite inclusions: characteristic absorption peaks at 153 cm⁻¹, 280 cm⁻¹, 710 cm⁻¹, 1086 cm⁻¹, and 1435 cm⁻¹ can be seen.

The characteristics of demantoids from different origins indicate variations in their inclusions and geological environments. The most significant internal features in worldwide demantoids are summarized in

Table 3. Distinctive inclusions of demantoids from different areas.

Locality	Distinctive Inclusions
Muslim Bagh, Baluchistan, Pakistan	Magnetite, hematite, caltite, fibrous chrysotile
Erongo Mountains, Damaraland, Namibia	Diopside, wollastonite, calcite, sphalerite
Chelyabinsk Oblast, south Urals, Russia	“Horsetail” of fibrous chrysotile, serpentine-group mineral, magnetite, chromospinel
Val Malenco, Sondrio, Italy	“Horsetail” of fibrous chrysotile, serpentine-group mineral
Kerman Province, Iran	Straight-to-curved fibrous chrysotile
Ambanja, Antetezambato, Madagascar	Diopside, dolomite, calcite, apatite

and described below. Russian demantoid found in serpentine rocks is known for its “horsetail” of fibrous serpentine (chrysotile), and in a few cases, it also contains chromospinel and magnetite [9,21]. Diopside and Calcite, common minerals, are found in the demantoids from skarn-type mining areas of Madagascar and Namibia [4,5,12]. These inclusions differ significantly from those commonly found in Pakistani demantoids.

The geological environment of demantoid-producing areas in Xinjiang, China, is serpentinized ultramafic rock, characterized by mineral assemblages such as olivine, hematite, serpentine, and late-stage carbonate minerals [22]. Val Malenco in Italy, another demantoid-producing area, is also associated with ultramafic rocks consisting of magnetite, diopside, and antigorite-bearing serpentinites; these serpentines formed through the metamorphism of mantle rocks during the Alpine orogeny [2,3,6].

In the case of demantoids from Saplai Tor Ghar Cr Mine, it is distinct in that its host rock is completely serpentinized. Such typical serpentinite minerals such as chrysotile, Pakistani samples investigated do not contain these minerals as inclusions, but previous researchers found fibrous chrysotile in demanoids from Baluchistan, Pakistan [1,11]. Additionally, inclusions such as calcite are present, indicating a combination of serpentinization and skarnization processes in the formation of Pakistani demantoids. This unique combination suggests a distinctive origin for Pakistani demantoids.

4.3. Chemical Composition

The chemical compositions established by EDXRF of all Pakistani crystals here investigated are reported in

Table 4. EDXRF data of Pakistan demantoid (weight%).

Sample	P1	P2	P3	P4	P5	P6	P7	P8	P9	Average	Standard Deviation
SiO2	38.385	34.005	35.276	38.573	38.148	38.442	34.9905	37.088	37.959	36.985	1.656
CaO	34.212	37.129	34.192	34.057	33.873	32.858	33.501	33.799	33.298	34.102	1.149
Fe2O3	25.951	27.589	28.7	26.411	26.702	26.778	27.397	28.1575	27.626	27.257	0.824
K2O	0.129	0.223	0.164	0.025	0.226	0.049	0.054	0.032	0.066	0.108	0.076
MnO	0.035	0.031	0.023	0.025	0.013	0.008	0.0215	0.031	0.035	0.025	0.009
Al2O3	0.03	-	0.08	0.06	0.13	-	0.06	0.05	0.09
Bi2O3	-	-	0.009	0.006	-	0.012	-	-	0.015
NbO	-	-	-	0.005	-	-	-	0.01	0.007
CuO	-	-	-	-	-	0.011	-	-	-
ZnO	-	-	-	-	0.006	-	-	0.007	0.005
NiO	-	-	0.014	-	0.01	-	-	-	-
Er2O3	-	-	0.019	0.032	0.01	-	-	-	-
SrO	-	-	-	-	-	0.016	-	-	0.005
ThO2	-	-	-	-	-	0.029	-	-	-
V2O5	-	-	-	-	-	0.019	-	-	-
Cr2O3	-	-	-	-	-	-	0.029	-	-

-: below detection limit.

. Each element is presented in the form of oxide, and the total Fe element is calculated as Fe₂O₃. Ca, Fe, Si, and O are the main constituent elements of the Pakistani demantoid. Calcium oxide and iron oxide end members account for more than 90%, which belongs to complete isomorphism (Table 4).

The samples are almost pure andradite with a very low content of Al. Fe is the main component of demantoid and plays a crucial role in its coloration as an end member [23]. Mn was detected in each sample, Mn and V were rather constant, and Mn did not show any correlation with color in demantoid [6]. Cr and V were only detected in one sample each.

Regarding the Cr content in demantoids from different locations, previous studies have provided valuable insights. Iranian demantoids exhibited a higher Cr₂O₃ content of 1.33% [5]. In contrast, demantoids from Kaghan Valley, Pakistan showed a lower average Cr₂O₃ content of 0.15% [24]; the Cr₂O₃ content in the demantoids from Baluchistan, Pakistan ranged from below the microprobe’s detection limit (0.01 wt.%) up to approximately 1 wt.%, and this content decreased with increasing distance from the Cr-bearing magnetite inclusion [1]. LA-ICP-MS analysis conducted in a previous study on Pakistani demantoids also confirmed very low Cr concentrations, ranging from below the detection limit to approximately 10 ppm (0.001%) [10]; in the study by Bindereif et al., the chromium concentrations in demantoid samples from Balochistan, Pakistan, ranged from below the detection limit to 1000 ppm (0.1%) [14]. Adamo et al. revealed chromium concentrations in demantoids from Balochistan, Pakistan, ranging from 3.71 to 23.6 ppm [1]. In the current study, most of the samples had Cr content below the detection limit, with only one sample showing a Cr₂O₃ content of 0.032%.

To investigate the variations in chemical composition between demantoids from Baluchistan, Pakistan, and other origins, data from previous studies on demantoids from Russia, Namibia (measured by authors), Italy [6], Kerman in Iran (Iran 1) [2,7], Khorasan Razavi in Iran (Iran 2, measured by authors) and Madagascar [2] were selected and analyzed, and only data that exceeded the detection limit were analyzed in the binary diagram. The article appropriately references and cites these data obtained from other researchers’ studies.

The analysis of major elements (Figure 6a) reveals distinct chemical fields for demantoids from different origins, with the Ca and Fe content of demantoids from Pakistan occupying an intermediate position among various locations.

Fe and Cr are common chromogenic elements in demantoid, so the contents of these two elements are compared in a binary diagram (Figure 6b). There are significant regional differences in elemental content, especially in the content of Cr. Demantoids from Pakistan and Namibia generally have low Cr content and show variable iron content, which is the result of Fe being replaced by Al (grossular) [2]. Russian and Italian demantoids have variable and higher Cr content compared to other origins. Iranian demantoids exhibit a wide range of Cr content, from almost none to the highest among the analyzed data, with one exception being a sample with Cr₂O₃ accounting for about 1%, indicating variation in the uvarovite component due to internal diffusion of chromium from Cr-bearing magnetite inclusions [2]. Demantoids from Madagascar have higher Fe content and lower Cr content, but there is a lack of data for them.

As Fe is both the chromogenic element and the major element in demantoid, the correlation between Fe and Cr content is not evident in the binary diagram. Cr-rich Russian and Italian demantoids exhibit an “emerald green” color, while lower Cr content gives Pakistani and Namibian demantoids a more pronounced yellow hue.

Figure 6c,d and the color of the samples suggest that low Cr content results in a yellow-green or dark green appearance, while high Cr content contributes to an emerald green color, but the contribution of V content to emerald green is relatively weak.

Based on the above study and Table 4, the Cr content of samples from Pakistan is significantly lower compared to demantoids from other serpentinite-type origins.

The Cr–V–Mn ternary diagram (Figure 7) shows that the chemical fields of demantoids from various areas are located in different regions. The proportion of Cr content in demantoids from Russia, Italy, and Iran is higher compared to Mn and V. The demantoids from Pakistan have a higher content of Mn compared to Cr and V. The trace element composition in demantoids from Namibia varies. There is no V in demantoids from Italy. The Cr, Mn, and V contents of demantoids mined from two areas in Iran are different. Demantoids from Madagascar do not contain Mn, but the content of Cr and V is relatively consistent.

4.4. Spectroscopy

4.4.1. UV-Vis Spectrophotometer

According to the UV-vis (Figure 8), Pakistani demantoids mainly exhibit absorption at 443 nm and 575 nm with weak intensity. There are concave broad bands around 620 nm, but they are not strong enough to form distinct absorption peaks.

There is still ongoing debate regarding the interpretation and attribution of certain absorption peaks.

Some researchers attribute the absorption peaks near 443 nm to ⁶A_1g → ⁴E₁⁴A_g of Fe³⁺, while a further contribution from chromium would be highly likely [3,25,26,27,28]. Some scholars suggest that the presence of ferric iron (Fe³⁺) inherent in andradite enhances the 443 nm chromium peak in demantoids [6]. Russian demantoids, which contain Cr, typically show UV-vis absorption peaks at 620 nm and 443 nm [21,29]. However, when studying the spectra of demantoids from Madagascar and Italy, strong absorption bands at 443 nm were observed in the spectra, collected at low temperatures (At low temperatures, the absorption intensity will increase) of demantoid samples with almost no Cr content [3,6,30].

The bands observed at 575 nm with varying intensities indicate the presence of Fe³⁺. While the wider bands at 620 nm are primarily attributed to ⁴A₂ → ⁴T₂ of Cr³⁺ [23,31,32], it cannot be completely ruled out that Fe³⁺ may have a minor contribution to this absorption [3,6,25,26,27,28]. The absorption of these two locations will impact each other due to their positional relationship. Notably, the Fe³⁺ 575 nm band is dominant in specimens with low Cr content but is masked by the 620 nm band in those with higher Cr³⁺ content [3,6,29]. As such, the presence or absence of the 620 nm band, for instance, is essential in predicting the levels of Cr³⁺ present in the sample [3,6,25,26,27,28,31,32]. These findings signify that the absorption’s behavior can serve as a reliable indicator of Cr³⁺ content in samples. However, regardless of the presence of chromium, previous studies on demantoids from Italy and Pakistan have demonstrated low absorption at 575 nm within the UV spectra, akin to the findings of this study. The low level of absorption observed at 575 nm and the slight broad band effect at 620 nm suggest the presence of Fe³⁺ and trace Cr³⁺ within the samples.

The doublet at 695–700 nm is attributed to Cr³⁺ [3,28,33]. It is noteworthy that a subtle absorption at 700 nm is detectable in P3. Prior research on demantoids with high and low concentrations of Cr³⁺ has revealed similarly feeble 695–700 nm peaks, even though the data were detected at low temperatures [3].

In general, demantoids produced from serpentinite typically exhibit distinct absorption bands of Cr³⁺ in their spectra, contributing to their characteristic spectral features in the 500–700 nm range [3]. The Cr content in the samples of this study is low, generally below the detection line of EDXRF. Nevertheless, the feature absorption spectrum of Cr remains visible, which may be attributed to the inherent limitations of EDXRF in achieving high detection accuracy. In addition, the intensity of some characteristic absorption is weaker compared to the strong absorption that should be exhibited by the element content in this study, such as the absorption at 443 nm, which should have been sharp and intense, and it is the most characteristic and diagnostic feature in the demantoid spectrum. The relatively smooth surface of the sample may affect the scattering during the testing process, leading to these differences.

4.4.2. Fourier Transform Infrared (FTIR) Spectroscopy

The infrared spectrum of demantoid consists of vibration of [SiO₄]⁴⁻, lattice vibration mode, and other group vibration modes, with lattice vibration generally located below 450 cm⁻¹ [31,34]. The absorption with a wavenumber below 500 cm⁻¹ is attributed to lattice vibrations aroused by cations other than Si ions [35].

The infrared spectrum results are presented in Figure 9. The absorption peaks observed at 937 cm⁻¹, 848 cm⁻¹, 817 cm⁻¹, and 517 cm⁻¹ in the sample correspond to vibrations related to [SiO₄]⁴⁻ units in the demantoid [31,34]. Additionally, the absorption within the range of 400–500 cm⁻¹ is formed by trivalent cations entering the [BO₆] lattice (octahedral coordination formed by B³⁺) [35].

4.4.3. Raman Spectroscopy

Regarding Raman spectroscopy, Figure 10 illustrates the spectral characteristics of the samples. In the spectra, distinct peaks are observed at 873 cm⁻¹, 843 cm⁻¹, 818 cm⁻¹, 514 cm⁻¹, 372 cm⁻¹, and 350 cm⁻¹. Weak peaks are also present at 994 cm⁻¹, 310 cm⁻¹, 234 cm⁻¹, and 168 cm⁻¹. Specifically, the absorption peaks near 994 cm⁻¹, 843 cm⁻¹, and 818 cm⁻¹ are related to (Si–O)_Str vibrations (Si–O stretching vibration), while the peaks near 514 cm⁻¹ and 494 cm⁻¹ are related to (Si–O)_bend vibrations (Si–O bending vibration) [36]. The absorption peaks near 350 cm⁻¹ and 310 cm⁻¹ are associated with the rotation of SiO₄–R(SiO₄)⁴⁻ vibrations, the peak near 168 cm⁻¹ is related to the translation of SiO₄–T(SiO₄)⁴⁻ vibrations, and the peak near 234 cm⁻¹ corresponds to the translation of X²⁺–T(X²⁺) vibrations (X²⁺ represents divalent ions) [29].

5. Conclusions

Pakistani demantoid is a relatively pure andradite, containing a small amount of Mn (average 0.019 wt.%) and trace amounts of Cr and V. In addition, Fe is the main chromogenic element of Pakistani demantoid.

The crystal form of Pakistani demantoid originating from serpentinite is mostly a combination of rhombic dodecahedron and tetragonal trisoctahedron, with corrosion marks and pits on the surface. The inclusions in demantoid include magnetite, hematite, and calcite, which can be regarded as its distinguishing characteristics, and reflect the complex and special geology of serpentinization and skarnization.

Compared to serpentinite-hosted demantoids from other locations, Pakistani demantoid is a kind of Cr-poor garnet. The results of the UV-vis absorption spectrum confirm that the absorption peak at 443 nm is attributed to Fe³⁺, while a further contribution from chromium would be highly likely. Although the low Cr content of Pakistani demantoid makes it difficult to detect it by EDXRF, the characteristic absorption band at 620 nm can serve as a basis for the presence of Cr in UV-vis spectra.

The results of FTIR present absorption peaks at 937 cm⁻¹, 848 cm⁻¹, 817 cm⁻¹, and 517 cm⁻¹, which are attributed to vibrations associated with [SiO₄]⁴⁻ units. Additionally, peaks at 481 cm⁻¹ and 442 cm⁻¹ are observed, corresponding to B³⁺ vibrations. Spectral characterization of Raman and FTIR can serve as bases for identifying demantoid, but their roles in identifying the source of origin are limited.