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Article

Effect of Dual-Site Co-Cultivation on Spectral Characteristics and Trace Element Enrichment in Akoya Pearls

by
Peiqi Zhou
1,
Geng Li
1,* and
Fabian Schmitz
2
1
School of Gemology, China University of Geosciences (Beijing), Beijing 100083, China
2
Rhein Main Gem Consulting, Forsterstraße 4, 55118 Mainz, Germany
*
Author to whom correspondence should be addressed.
Minerals 2025, 15(6), 654; https://doi.org/10.3390/min15060654
Submission received: 7 May 2025 / Revised: 4 June 2025 / Accepted: 16 June 2025 / Published: 18 June 2025
(This article belongs to the Section Biomineralization and Biominerals)
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Abstract

:
This study systematically investigates for the first time the effects of dual-site co-cultivation on spectral characteristics and trace element enrichment in marine-cultured Akoya pearls from Beihai, China. Akoya pearls were cultured over a one-year period, with the final 40-day stage designated as the terminal phase. During this period, two experimental groups of pearl oysters were established: Group Y remained in Beihai for continued local cultivation and harvest, while Group B was transferred to Weihai, Shandong Province, for terminal-stage farming under different thermal conditions. A series of comparative analyses were performed using Fourier-transform infrared (FTIR) spectroscopy, ultraviolet-visible (UV-Vis) spectroscopy, Raman spectroscopy, X-ray fluorescence (XRF), and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). The FTIR results revealed distinct differences between the two groups in the distribution of amide and polysaccharide functional groups, particularly around 1643 cm−1 and 1100 cm−1. The UV-Vis spectra of Group B displayed characteristic absorption bands at 430 nm and 460 nm, associated with the organic matrix of the nacre. Raman spectroscopy further indicated a higher abundance of organic-related vibrational features in Group B. Additionally, both XRF and LA-ICP-MS analyses consistently showed significant differences in the concentrations and distributions of trace elements, particularly copper (Cu), cobalt (Co), and zinc (Zn). The findings demonstrate that the dual-site co-cultivation mode significantly impacts both the organic composition and trace element enrichment patterns in seawater Akoya pearls. This research provides valuable references for optimizing environmental parameters in pearl cultivation processes.

1. Introduction

Akoya cultured pearls have long been favored in the global market, and since the Maritime Silk Road era during the Qin and Han dynasties, Akoya pearls from Beihai, China, have played an important role in international trade [1,2,3]. Until the 1980s and 1990s, Akoya pearls from Beihai were exported to more than 20 countries and regions, with an annual output reaching 8.8 tons [4]. Beihai’s Akoya pearl industry flourished during the 1980s and 1990s, with exports to over 20 countries and regions and an annual production of 8.8 tons. However, due to technological constraints and environmental challenges, production experienced a significant decline. In recent years, the introduction of advanced technologies, specialized personnel, and improved farming techniques has helped restore annual output to approximately 700 kg, with high-quality pearls continuing to enter the international market.
Previous studies conducted across various marine regions—including China, Japan, Korea, and the Middle East—have reported the influence of water temperature on the survival rate and quality of cultured pearls across different pearl oyster species and pearl types [5,6,7,8,9,10,11,12,13,14,15,16]. In addition, investigations on Akoya pearls from various regions have also explored farming procedures and gemological characteristics under diverse environmental conditions [17,18,19,20,21,22,23,24,25,26,27].
Building on this body of research, the present study focuses on Akoya pearls cultivated in Beihai, China. Following extensive field visits to local pearl farms and the collection of representative samples, we selected two groups of Akoya pearls that underwent final-stage cultivation (the terminal phase) under distinct water temperature regimes. This allowed for a targeted analysis of the environmental impacts during this critical cultivation period. We conducted a comparative study using Fourier-transform infrared (FTIR) spectroscopy, ultraviolet-visible (UV-Vis) spectroscopy, Raman spectroscopy, X-ray fluorescence (XRF), and inductively coupled plasma mass spectrometry (ICP-MS) to examine the differences in organic matter composition and trace element distribution between the two groups. The findings provide new insights into how terminal-phase water temperature influences the biochemical and elemental characteristics of seawater Akoya pearls produced in Beihai. These results are expected to contribute valuable information to the gemological evaluation of Akoya pearls, trace element research, and practical guidance for pearl farming operations.

2. Production Locations

Farming Environment

The pearl farming industry in Beihai has greatly benefited from its advantageous geographical location in the northeastern region of the Beibu Gulf. The coastal waters in this area offer an ideal environment for the growth of Pinctada fucata martensii due to the favorable balance of sediment composition, salinity, abundant algal resources, and the occurrence of seasonal southern swells. All pearl samples used in this study originated from Beihai, Guangxi Province, with a portion of them undergoing final-stage cultivation in Weihai, Shandong Province (Figure 1). Beihai is located at approximately 21.5° N latitude, whereas Weihai lies at around 37.5° N. During the cultivation period of October to November, the seawater temperature in Beihai ranges from approximately 23 °C to 28 °C, with an average salinity of about 28.9‰ and a mean pH value of 8.17. In contrast, the seawater in Weihai during the same period exhibits lower temperatures ranging from 13 °C to 22 °C, with an average salinity of around 30‰ and a mean pH value of approximately 8.1 (Figure 2) [28,29,30,31]. During the cultivation period, the variation in water temperature is the most pronounced. Although pH and salinity are also factors affecting the condition of Pinctada martensii, their differences are minimal compared to that of water temperature [32,33].

3. Materials and Methods

3.1. Materials

Eighteen light-colored Akoya pearls were selected from dozens of candidates based on their similar quality, size, and color for comparative analysis (Figure 3).
Group B pearls were initially cultivated in the Tieshangang area of Beihai and then transferred to Weihai for final-stage cultivation in October. Group Y pearls were cultured exclusively in the Tieshangang waters of Beihai throughout the entire farming process. The pearls were classified based on appearance quality, diameter, body color, and luster (Table 1). All samples were untreated, with diameters ranging from 5 to 7 mm and smooth surface textures. Their body colors included light yellow and white. The total cultivation period for these samples ranged from 12 to 15 months.

3.2. Methods

3.2.1. Fourier Transform Infrared (FTIR) Spectroscopy

The FTIR analysis was conducted at the Gem Testing Center of the China University of Geosciences (Beijing) using a BRUKER TENSOR 27 Fourier Transform Infrared Spectrometer (Ettlingen, Germany). The reflection method was applied under an operating voltage of 220 V, with a scanning range of 4000–400 cm−1 and a spectral resolution of 4 cm−1. FTIR analysis was conducted on the untreated outer surface of the pearls.

3.2.2. Ultraviolet-Visible (UV-Vis) Spectroscopy

The UV-Vis absorption spectra of the pearls were analyzed using a GEM-3000 UV-Vis Fiber Optic Spectrometer (Guangzhou, China). The measurement conditions included a wavelength range of 220–900 nm, an integration time of 180 ms, and an average of 10 scans for signal optimization. The measurements were performed directly on the untreated outer surface of the pearls.

3.2.3. Raman Spectroscopy

Raman spectral analysis was performed using a HORIBA T64000 Laser Raman Spectrometer (Villeneuve d’Ascq, France) with a 514.5 nm laser source. The instrument operated at 220 V, with a scanning range of 200–4000 cm−1 and a spectral resolution of 1 cm−1. The measurement data were recorded and processed using LabSpec 6 software. All measurements were performed directly on the untreated outer surface of the pearls, with all data confirmed to be unaffected by spontaneous fluorescence interference.

3.2.4. X-Ray Fluorescence (XRF) Spectroscopy

The elemental composition of the pearls was analyzed using an EXD-7000 X-ray Fluorescence (XRF) Spectrometer (Kyoto, Japan), operating at 45 kV and 100 µA. All measurements were performed directly on the untreated outer surface of the pearls. The samples were placed in Mylar film sample cups for testing, with a measurement diameter of 1 mm. This XRF instrument features a wide dynamic range and detection limits reaching the ppm level. Three points of each sample were taken for measuring, and the average was calculated to obtain the final result.

3.2.5. LA-ICP-MS Spectroscopy

Trace element analysis was carried out at the Key Laboratory of Elemental and Morphological Microanalysis, National Research Center for Geoanalysis (NRCG), Beijing, China. An NWR193UC 193 nm ArF excimer laser ablation system coupled with an Element II high-resolution sector field ICP-MS was employed for the measurements. Helium was used as the carrier gas for aerosol transport, with a small amount of nitrogen added to enhance sensitivity and suppress oxide formation, maintaining a ThO+/Th+ ratio below 0.05%. The laser beam diameter was 40 μm, with a repetition rate of 8 Hz and a fluence of approximately 6 J/cm2. Calcium oxide (CaO) was used as the internal standard for pearl trace element analysis, with its content determined by electron probe microanalysis (EPMA). NIST SRM 610 served as the external reference material. Data reduction was performed using the Iolite software package (v4.10.2). In this study, the measured concentrations of the carbonate reference material MACS-3 deviated from the recommended values by less than 5%, confirming the reliability of the method [34]. The data represent the result rounded to three decimal places, and all of these measurements were performed directly on the untreated outer surface of the pearls.

4. Results and Discussion

4.1. Spectroscopy

4.1.1. Infrared Spectra

The table presents the major FTIR absorption peaks observed in both groups and their corresponding vibrational mode assignments (Table 2). The FTIR reflectance spectra of Groups B and Y exhibit characteristic absorption bands at 698 cm−1, 711 cm−1, 877 cm−1, 1078 cm−1, 1487 cm−1, and 1776 cm−1. These bands correspond to the fundamental vibrational modes of aragonite, indicating that the dominant mineral phase in the pearls is aragonitic calcium carbonate (CaCO3) [35].
Specifically, the bands near 698 cm−1 and 711 cm−1 are assigned to the in-plane bending vibrations (ν4) of the carbonate ion (CO32−), while the 877 cm−1 band corresponds to its out-of-plane bending mode (ν2). The absorption at 1078 cm−1 is associated with symmetric stretching (ν1), and the 1487 cm−1 band is attributed to asymmetric stretching (ν3) of CO32− [36,37]. Additionally, a distinct absorption band is observed near 1643 cm−1 in both groups, corresponding to the C=O stretching vibration of Amide I (Figure 4) [38].
Beyond the shared spectral features, the organic phase of Group B exhibits additional vibrational characteristics (Figure 5a). As illustrated in Figure 5, sample B-5 presents not only the characteristic Amide I peak at 1643 cm−1, but also several prominent bands in the 1750–1600 cm−1 region and around 1100 cm−1 (Figure 5c,e). In the samples of Group Y, the same proportional graph of sample Y4 is used as a comparison (Figure 5b,d,f).
Notably, a series of absorptions appear between 1060 and 1150 cm−1, with distinct peaks at 1116 cm−1, 1101 cm−1 (observed twice), 1091 cm−1, and 1076 cm−1. Among these, the 1076 cm−1 band is attributed to the symmetric stretching mode (ν1) of the carbonate ion in aragonite, while the 1116 cm−1 peak is indicative of vibrations associated with polysaccharide groups [38,39,40]. The peaks at 1091 cm−1 and 1101 cm−1 likely represent overlapping contributions from both carbonate and organic matrix vibrations. These assignments are supported by previous studies, and together with the current experimental findings, they suggest that the organic phase in sample B-5 contributes to enhanced spectral complexity through the interplay of mineral and biopolymer signals (Figure 5e).

4.1.2. Ultraviolet-Visible Reflectance Spectrum

In Group Y, absorption bands were observed at 280 nm, 360 nm, and 370 nm, along with weak and broad absorption features around 600 nm and 850 nm. No distinct absorption was detected in the region near the visible light spectrum (Figure 6a). Among these samples, Y1, Y3, and Y5 lacked the 360 nm absorption feature. In previous studies, Beihai Akoya pearls were categorized into type I and type II based on the presence or absence of the 360 nm absorption band observed in Y4 and Y6 (Figure 6b). Building upon this classification, samples Y7, Y8, and Y9, which exhibited a distinct double absorption peak at 360 nm and 370 nm, are here proposed as a third type, designated as type III (Figure 6c).
Previous studies have demonstrated that the absorption at 280 nm is primarily associated with organic molecules—particularly proteins—and is unrelated to the structural components of the pearl (e.g., nacreous layering), but instead originates from the organic matrix itself [41,42,43]. Notable differences in absorption intensity at 280 nm were observed among samples Y1, Y6, and Y8. Sample Y8 showed the strongest absorption at 280 nm and simultaneously exhibited dual peaks at 360 nm and 370 nm, characteristic of type III. In comparison, sample Y6 displayed a broader peak at 280 nm along with weak absorption near 360 nm, consistent with type I characteristics. Sample Y1 showed only a weak peak at 280 nm and lacked any other absorption features, thus classified as type II. A negative correlation was observed between the absorption intensities at 280 nm and 360 nm across the three pearl types (Figure 6d).
In the UV spectra of Group B, distinct absorption peaks were observed at approximately 280 nm, 370 nm, 430 nm, and 460 nm, as well as around 650 nm and 830 nm (Figure 7a). The major absorption features in Group B were concentrated within the visible light range of 400–460 nm, whereas those in Group Y were primarily located in the near-ultraviolet region between 300 and 400 nm. In the 600–900 nm range, Group B exhibited a more pronounced absorption trend compared to Group Y, which is likely caused by subtle differences in pigment composition and nacre structure (Figure 7a,c) [44]. Notably, the samples exhibited distinct dual absorption features at 430 nm and 460 nm (Figure 7b). These absorption bands have been previously attributed to the presence of an organic matrix on the outer surface of the pearl, which is considered to contribute significantly to the high luster observed in Akoya pearls [45].

4.1.3. Raman Spectra Analysis

Characteristic vibrational modes of the carbonate ion (CO32−) were observed at 1084 cm−1 and 701/704 cm−1. The band at 1084 cm−1 corresponds to the ν1 symmetric stretching mode of CO32−, while the doublet at 701 cm−1 and 704 cm−1 is assigned to the ν4 in-plane bending mode [46]. These diagnostic peaks confirm that the pearls are predominantly composed of aragonite, rather than calcite or vaterite, consistent with the typical mineralogical composition of natural saltwater pearls. In group B samples, the band at 1275 cm−1 is attributed to the Amide III vibrational mode, which mainly arises from the coupling of N–H bending and C–N stretching vibrations in protein backbones (e.g., conchiolin) [47,48,49,50]. Two additional weak peaks at 1130 cm−1 and 1520 cm−1 correspond to natural organic pigments—specifically, the main Raman resonance features of polyene compounds—assigned to the C–C and C=C stretching modes, respectively [44,51,52,53,54]. A minor peak near 1462 cm−1 is attributed to the ν3 asymmetric stretching mode of CO32− within the aragonite lattice. A minor peak near 1462 cm−1 is attributed to the ν3 asymmetric stretching mode of CO32− within the aragonite lattice (Figure 8). In Group Y, the 1565 cm−1 peak is attributed to the C=C stretching vibration of porphyrins (Figure 9) [53].

4.2. Trace Element Analysis

4.2.1. X-Ray Fluorescence (XRF)

Energy-dispersive X-ray fluorescence spectroscopy (EDXRF) is commonly used in gemological studies to distinguish between freshwater and seawater cultured pearls [55,56,57]. X-ray fluorescence (XRF) analysis was performed at two locations on each pearl sample, resulting in a total of 36 measurement points. To highlight the differences in trace element abundance, concentrations were plotted using a logarithmic scale. The comparison clearly shows variations in the concentrations of Fe, Mn, Zn, Sr, Cu, Si, K, Sc, Co, S, and Cr between the two groups, with Group B generally exhibiting lower trace element levels than Group Y (Figure 10).
A binary scatter plot was generated to compare Mn and Sr concentrations (Figure 11). The plot shows that both groups maintain the typical Mn/Sr signature of seawater pearls, with Group B displaying more stable Sr/Mn values.
Additionally, a ternary diagram was constructed to compare the proportions of Co, Cu, and Zn in both groups (Figure 12). The ternary plot reveals a distinct compositional difference between Groups B and Y, with Group B showing notably lower proportions of Co and Zn.

4.2.2. LA-ICP-MS

Based on the semi-quantitative results obtained from EDXRF analysis, further investigation of element distribution was conducted using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). For laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), six ablation spots were selected per group, yielding 12 analytical points across the two groups. The results revealed a significant enrichment in Co (20.8-fold), Zn (18.7-fold), and Cu (2.5-fold) in Group Y. In addition, the strontium (Sr) concentration in Group Y (1440.549 ppmw) was substantially higher than that in Group B (1063.707 ppmw). Similarly, magnesium (Mg, 317.826 ppmw) and potassium (K, 218.568 ppmw) were also notably more abundant in Group Y than in Group B. Further analysis showed that the titanium (Ti) concentration in Group Y (1.843 ppmw) was considerably higher than that in Group B (0.074 ppmw), and barium (Ba) was also enriched in Group Y (3.21 ppmw) (Table 3). A comparative bar chart of Co, Cu, and Zn concentrations across all measured points clearly indicated that the contents of these elements were substantially higher in the pearls from Group Y than those from Group B (Table 3).
With respect to elemental combinations, ICP-MS results showed that Group Y exhibited higher levels of Mg, Fe, Zn, Mn, and P compared to Group B. These findings are consistent with previous studies, which reported that lower-quality pearls tend to have higher concentrations of trace elements [58]. The LA-ICP-MS results indicate that the overall concentrations of trace elements in Group B are lower than those in Group Y, with particularly significant differences observed in Cu, Co, and Zn. These findings are consistent with the trends revealed by EDXRF analysis (Figure 13).

5. Conclusions

This study conducted a comparative analysis of Akoya seawater pearls under dual-site co-cultivation conditions, focusing on samples from Beihai. Spectroscopic techniques, including Fourier-transform infrared (FTIR), ultraviolet-visible (UV-Vis), Raman spectroscopy, as well as a compositional analysis through X-ray fluorescence (XRF) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), were employed to investigate differences in organic matrix and trace elemental composition.
Fourier-transform infrared (FTIR) spectroscopy revealed the presence of amide absorption bands in both groups of Akoya pearl samples, with additional polysaccharide-associated features observed in Group B. This suggests that variations in water temperature during the late stage of cultivation may have influenced the distribution and role of these organic matrices within the pearls. These organic matrices are actively involved in the biomineralization process, facilitating the ordered deposition of aragonite crystals and thereby contributing to the formation of a layered, dense nacre. Raman spectroscopy further confirmed the organic compositional differences between the two groups. In the 1000–2000 cm−1 region, additional peaks associated with organic or potentially chromophoric substances—such as amide and polyene compounds—were observed in Group B. In contrast, Group Y displayed characteristic peaks related to porphyrins. The integration of FTIR and Raman spectral data revealed notable differences in organic composition between the two groups, including amide compounds, polyenes, porphyrin-type pigments, and polysaccharides. These substances include both pigment molecules and structural organic matrices. Organic matrix proteins play a regulatory role in nacre deposition and formation, and their expression and distribution are species-specific. In this study, seawater Akoya pearls produced by Pinctada fucata martensii from Beihai exhibited significant differences in the content and types of organic matter under varying water temperature conditions. These variations may further affect the spatial distribution and functional roles of organic substances in the pearl nacre. Based on the spectral data and environmental parameters, we conclude that water temperature during the final stage of cultivation has a significant impact on the form and distribution of organic substances in Akoya pearls. However, the precise molecular mechanisms and their effects on pearl biomineralization require further investigation.
According to previous studies on the classification of UV-reflectance spectra of Akoya pearls, double absorption features at 360 nm and 370 nm were detected in Group Y. Comparative analysis suggests that the intensity of the peak near 360 nm may be related to the absorption strength at 280 nm. For instance, pearls with weaker luster often show less pronounced absorption at 280 nm and similarly weak or absent features at 360 nm. This trend may indicate that absorptions at 280 nm and 360 nm are related to pearl luster. A comparative analysis of the absorption trends between the two groups of samples reveals that, in the 600–900 nm range, the absorption intensity of Group Y pearl samples is weaker than that of Group B. Considering that numerous studies have identified the 280 nm absorption feature as being associated with conchiolin, and that characteristic peaks corresponding to polysaccharide were detected in the FTIR spectra of Group B, it is plausible that polysaccharides—components of the conchiolin matrix in both pearls and shells—may represent one of the potential factors influencing the absorption behavior in this spectral region. Furthermore, lower water temperatures during pearl formation slow the growth of crystal platelets, producing thinner, more uniform layers that are considered more desirable than thicker layers formed under higher temperatures, particularly during the late cultivation phase. In the UV-reflectance spectra of Group B, distinct absorption features at 400 nm and 460 nm were observed. These features are associated with the organic matrix present on the pearl surface, which are thought to contribute to improved luster. The seawater temperatures in Weihai, where Group B pearls were cultivated, ranged from approximately 13 °C to 22 °C. Such lower temperatures may have facilitated the accumulation of organic matrices on the pearl surface.
Both X-ray fluorescence (XRF) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) were employed to analyze trace element concentrations in the two groups. The Sr/Mn ratios were consistent with those typically found in seawater pearls. Distinct differences in trace element concentrations and Cu-Co-Zn ratios were observed between the two groups. LA-ICP-MS results showed that Group B samples had slightly higher average concentrations of Be, Na, Si, Sc, Mn, Fe, and Ni compared to Group Y samples. Conversely, Group Y samples had significantly higher levels of Li, B, Mg, Al, P, K, Ti, Co, Cu, Zn, Ga, Sr, and Ba. Collectively, Group B samples exhibited lower overall trace element concentrations, particularly for Mg, Fe, Zn, Mn, and P. Previous studies have indicated that differences in trace element composition may result from variations in developmental stages, biomineralization processes, and environmental factors such as salinity and temperature. Given the environmental conditions during the final stage of cultivation, Group B pearls—cultivated in the cooler waters of Weihai—experienced reduced seawater temperatures, similar salinity, and pH levels. Lower temperatures likely led to decreased nutrient uptake by the oysters, affecting the absorption, deposition, and the crystallization of trace metals such as Sr and Mg. In particular, Cu, Co, and Zn exhibited a notable decreasing trend and changes in proportional distribution, which appeared to be influenced by differences in aquaculture environments and water temperatures during the same final cultivation stage. The changes in elemental concentrations in Group B may reflect a slowdown under low-temperature conditions, particularly in the involvement of trace elements such as Cu, Co, and Zn in the biomineralization of pearls. Additionally, these changes suggest a reduced impact from the historically higher water temperatures and inherent environmental factors of the cultivation site. In conclusion, the observed variations in spectral signatures and trace element enrichment profiles of Beihai Akoya pearls are attributable to the synergistic effects of terminal-phase thermal fluctuations and aquaculture environment modulations under dual-site co-cultivation regimes.

Author Contributions

Writing—original draft preparation, P.Z.; visualization, P.Z.; software, P.Z.; writing—review and editing, G.L.; methodology, G.L.; resources, G.L.; data curation, G.L.; validation, F.S.; conceptualization, F.S.; investigation, F.S. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

Data are contained within the article; further inquiries can be directed to the corresponding authors.

Acknowledgments

The authors are grateful to the Gemological Institute, China University of Geosciences (Beijing), for their help with the preparation of data in this paper.

Conflicts of Interest

Author Fabian Schmitz was employed by the company Main Gem Consulting. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Figure 1. The geographic locations of the pearl farming sites in Guangxi Beihai and Shandong Weihai, along with in situ harvesting photographs. (a) Geographic locations of the two cultivation sites, Beihai (21.5° N) and Weihai (37.5° N), representing distinct latitudinal and environmental conditions. (b) On-site images of pearl farming environments.
Figure 1. The geographic locations of the pearl farming sites in Guangxi Beihai and Shandong Weihai, along with in situ harvesting photographs. (a) Geographic locations of the two cultivation sites, Beihai (21.5° N) and Weihai (37.5° N), representing distinct latitudinal and environmental conditions. (b) On-site images of pearl farming environments.
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Figure 2. Comparison of seawater temperatures between Beihai and Weihai during the final cultivation stage from October to November.
Figure 2. Comparison of seawater temperatures between Beihai and Weihai during the final cultivation stage from October to November.
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Figure 3. Beihai Akoya pearl samples from Group B and Group Y.
Figure 3. Beihai Akoya pearl samples from Group B and Group Y.
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Figure 4. (a) FTIR reflectance spectrum of a sample from Group B; (b) FTIR reflectance spectrum of a sample from Group Y; (c) peak position at 1643 cm−1 in a Group B sample; (d) peak position at 1643 cm−1 in a Group Y sample.
Figure 4. (a) FTIR reflectance spectrum of a sample from Group B; (b) FTIR reflectance spectrum of a sample from Group Y; (c) peak position at 1643 cm−1 in a Group B sample; (d) peak position at 1643 cm−1 in a Group Y sample.
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Figure 5. (a) FTIR reflectance spectra of sample B5; (c) FTIR reflectance spectra of sample B5 in the range of 1750–1600 cm−1; (e) FTIR reflectance spectra of sample B5 in the range of 1150–1060 cm−1; (b) FTIR reflectance spectra of sample Y4; (d) FTIR reflectance spectra of sample Y4 in the range of 1750–1600 cm−1; (f) FTIR reflectance spectra of sample Y4 in the range of 1150–1060 cm−1.
Figure 5. (a) FTIR reflectance spectra of sample B5; (c) FTIR reflectance spectra of sample B5 in the range of 1750–1600 cm−1; (e) FTIR reflectance spectra of sample B5 in the range of 1150–1060 cm−1; (b) FTIR reflectance spectra of sample Y4; (d) FTIR reflectance spectra of sample Y4 in the range of 1750–1600 cm−1; (f) FTIR reflectance spectra of sample Y4 in the range of 1150–1060 cm−1.
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Figure 6. (a) UV-Vis reflectance spectra of pearl samples from Group Y; (b) absorption features of type II Beihai Akoya pearls; (c) absorption features of type III Beihai Akoya pearls; (d) absorption features of samples Y1, Y6, and Y8.
Figure 6. (a) UV-Vis reflectance spectra of pearl samples from Group Y; (b) absorption features of type II Beihai Akoya pearls; (c) absorption features of type III Beihai Akoya pearls; (d) absorption features of samples Y1, Y6, and Y8.
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Figure 7. (a) UV-Vis reflectance spectra of pearl samples from Group B. (b) Absorption features of Group B samples at 430 nm and 460 nm. (c) Comparison of absorption for Group Y samples between the 430–460 nm and the 600–900 nm ranges.
Figure 7. (a) UV-Vis reflectance spectra of pearl samples from Group B. (b) Absorption features of Group B samples at 430 nm and 460 nm. (c) Comparison of absorption for Group Y samples between the 430–460 nm and the 600–900 nm ranges.
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Figure 8. Raman spectra of pearl samples from Group B.
Figure 8. Raman spectra of pearl samples from Group B.
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Figure 9. Raman spectra of pearl samples from Group Y.
Figure 9. Raman spectra of pearl samples from Group Y.
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Figure 10. Logarithmic spider plot of selected trace element concentrations (wt%) in B and Y pearl groups.
Figure 10. Logarithmic spider plot of selected trace element concentrations (wt%) in B and Y pearl groups.
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Figure 11. Binary plot of Sr/Mn ratios in pearl samples from Groups B and Y.
Figure 11. Binary plot of Sr/Mn ratios in pearl samples from Groups B and Y.
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Figure 12. Ternary plot of Cu-Co-Zn composition in pearl samples from Groups B and Y.
Figure 12. Ternary plot of Cu-Co-Zn composition in pearl samples from Groups B and Y.
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Figure 13. Comparison of mean Cu, Co, and Zn concentrations in B and Y Groups (ICP-MS analysis).
Figure 13. Comparison of mean Cu, Co, and Zn concentrations in B and Y Groups (ICP-MS analysis).
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Table 1. Diameter, weight, body color, luster, and characteristic features of Akoya pearl samples from Group B and Group Y.
Table 1. Diameter, weight, body color, luster, and characteristic features of Akoya pearl samples from Group B and Group Y.
Pearl Samples from Beihai, Guangxi
PhotoSample IDDiameter (mm)Weight (ct)Body ColorLusterSurface Characteristics
Minerals 15 00654 i001B15.931.7WhiteGoodSlight nacre overgrowth, smooth surface.
Minerals 15 00654 i002B25.991.4WhiteGoodPresence of mixed-color overlay, smooth surface.
Minerals 15 00654 i003B35.771.35YellowGoodSlight nacre overgrowth, smooth surface.
Minerals 15 00654 i004B45.711.4WhiteGoodPresence of mixed-color overlay, smooth surface.
Minerals 15 00654 i005B55.991.6YellowGoodPresence of mixed-color overlay, slight nacre overgrowth, smooth surface.
Minerals 15 00654 i006B661.55YellowGoodPresence of mixed-color overlay, smooth surface.
Minerals 15 00654 i007B75.821.65WhiteGoodSlight nacre overgrowth, smooth surface.
Minerals 15 00654 i008B85.551.3YellowGoodPresence of mixed-color overlay, smooth surface.
Minerals 15 00654 i009B96.11.62YellowGoodPresence of mixed-color overlay, smooth surface.
Minerals 15 00654 i010Y15.411.25WhitepoorWhite spots present, mixed-color overlay, smooth surface.
Minerals 15 00654 i011Y25.331.2WhitepoorPresence of mixed-color overlay, smooth surface.
Minerals 15 00654 i012Y35.150.9WhiteMediumPresence of mixed-color overlay.
Minerals 15 00654 i013Y47.092.75WhiteGoodPresence of mixed-color overlay, slight nacre overgrowth, smooth surface.
Minerals 15 00654 i014Y55.301.15WhiteMediumWhite spots present, presence of mixed-color overlay, slight nacre overgrowth.
Minerals 15 00654 i015Y65.321.1WhiteMediumA few white spots present.
Minerals 15 00654 i016Y76.571.84GrayGoodPresence of mixed-color overlay, smooth surface.
Minerals 15 00654 i017Y86.251.7GrayGoodSlight nacre overgrowth, smooth surface.
Minerals 15 00654 i018Y94.631.1YellowGoodSlight nacre overgrowth, smooth surface.
Table 2. Comparison of FTIR peak positions between pearl samples from Group B and Group Y. Absorption peak data are based on sample averages; the maximum error for individual samples is ±5 cm−1.
Table 2. Comparison of FTIR peak positions between pearl samples from Group B and Group Y. Absorption peak data are based on sample averages; the maximum error for individual samples is ±5 cm−1.
SamplePeak 1Peak 2Peak 3Peak 4Peak 5Peak 6
GourpB696711877108014851776
GourpY698711879108014871776
Table 3. Trace element compositions (unit: ppmw) of Beihai Akoya pearl samples from Groups B and Y, analyzed by LA-ICP-MS.
Table 3. Trace element compositions (unit: ppmw) of Beihai Akoya pearl samples from Groups B and Y, analyzed by LA-ICP-MS.
BY
ElementMaximumMinimumAverageMaximumMinimumAverage
Li7bdlbdlbdl3.389bdl0.977
Be90.208bdl0.0650.231bdl0.046
B118.6034.7167.05511.9575.2038.787
Na236480.0065523.2726129.255477.3464847.8725097.639
Mg24281.892119.848174.64461.67228.371317.826
Al271.311bdl0.3274.491bdl1.292
Si291687.9721192.1621422.7591422.2931185.471276.714
P31106.73156.07675.066133.55169.409101.49
K39103.14954.16478.986413.18962.713218.568
Sc450.234bdl0.1080.234bdl0.06
Ti490.441bdl0.0742.8780.3871.843
Mn5515.6824.198.90828.8991.8388.427
Fe579.276bdl7.24716.529bdl7.197
Co590.166bdl0.0393.2650.1910.811
Ni602.247bdl0.9922.402bdl0.775
Cu651.2010.1140.5232.0880.7371.323
Zn662.990.1261.25245.2373.52423.387
Ga690.119bdl0.020.076bdl0.023
Sr881232.423984.6781063.7071586.391116.6811440.549
Ba1372.251.2231.7424.4151.5463.21
Abbreviation: bdl = below detection limit.
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Zhou, P.; Li, G.; Schmitz, F. Effect of Dual-Site Co-Cultivation on Spectral Characteristics and Trace Element Enrichment in Akoya Pearls. Minerals 2025, 15, 654. https://doi.org/10.3390/min15060654

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Zhou P, Li G, Schmitz F. Effect of Dual-Site Co-Cultivation on Spectral Characteristics and Trace Element Enrichment in Akoya Pearls. Minerals. 2025; 15(6):654. https://doi.org/10.3390/min15060654

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Zhou, Peiqi, Geng Li, and Fabian Schmitz. 2025. "Effect of Dual-Site Co-Cultivation on Spectral Characteristics and Trace Element Enrichment in Akoya Pearls" Minerals 15, no. 6: 654. https://doi.org/10.3390/min15060654

APA Style

Zhou, P., Li, G., & Schmitz, F. (2025). Effect of Dual-Site Co-Cultivation on Spectral Characteristics and Trace Element Enrichment in Akoya Pearls. Minerals, 15(6), 654. https://doi.org/10.3390/min15060654

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