Research

21 pages, 25486 KiB

Open AccessArticle

Revealing the Secrets behind the Color and Sea-Wave Patterns of Larimar

by Hao-Ming Huang, Yu-Hong Shih, Huei-Fen Chen, Hao-Yang Lee, Jiann-Neng Fang, Chuan-Chou Shen and Bing-Sheng Yu

Minerals 2023, 13(9), 1221; https://doi.org/10.3390/min13091221 - 17 Sep 2023

Cited by 1 | Viewed by 2466

Abstract

In the last century, a blue–green colored gemstone known as Larimar with a special sea-wave pattern was discovered in the Dominican Republic. Larimar is composed of the mineral pectolite, which has a chemical composition of NaCa₂Si₃O₈(OH) and [...] Read more.

In the last century, a blue–green colored gemstone known as Larimar with a special sea-wave pattern was discovered in the Dominican Republic. Larimar is composed of the mineral pectolite, which has a chemical composition of NaCa₂Si₃O₈(OH) and is usually white in color. Cu²⁺ has always been considered to be the primary genesis of the blue color shown in Larimar, because native copper often grows together with Larimar. To clarify whether copper is the main reason for the origin of blue–green pectolite, we utilized laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) techniques to analyze trace elements in the pectolite samples and compared the relationship between elements and colors. The results show that vanadium and iron are the main origins of the sky-blue and green color of Larimar. We also discovered that it is not only the chemical elements that affect the color shades of the mineral, but the orientation of the radial fiber crystals also plays a critical role. The sea-wave pattern and the changes in the color saturation of radial pectolite are due to the transmittance of visible light through different viewed angles under changing crystal orientations. Our results reveal the chemical and physical factors behind the color and sea-wave pattern of Larimar. In addition, to our knowledge, this is the first time that the formation age of Larimar has been proven to be approximately equal to or younger than 40 ka, using the U-Th dating of calcite growth together with pectolite. Full article

(This article belongs to the Special Issue Gem Deposits: Mineralogical and Gemological Aspects)

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12 pages, 2809 KiB

Open AccessArticle

Chemical Composition and Spectroscopic Characteristics of Alexandrite Effect Apatite from the Akzhailyau Mountains of Kazakhstan

by Chuting Zhang, Chaoyang Chen, Zhibin Li and Andy H. Shen

Minerals 2023, 13(9), 1139; https://doi.org/10.3390/min13091139 - 28 Aug 2023

Viewed by 1031

Abstract

The alexandrite effect is a pivotal optical phenomenon in gemmology, evident in several notable gemstones. However, the manifestation of this effect in apatite from Kazakhstan stands out as both rare and highly sought after. This apatite displays a yellowish-green hue in daylight and [...] Read more.

The alexandrite effect is a pivotal optical phenomenon in gemmology, evident in several notable gemstones. However, the manifestation of this effect in apatite from Kazakhstan stands out as both rare and highly sought after. This apatite displays a yellowish-green hue in daylight and transitions to a pinkish-orange shade under incandescent lighting. This research involved analyzing the apatite’s chemical composition using LA-ICP-MS, in addition to obtaining its infrared, Raman, UV-Vis, and fluorescence spectra. From the elemental assessment results, the primary trace elements in the apatite were identified as Na, Mn, and Fe, along with rare-earth elements including Gd, Dy, Ce, Nd, and Sm. The infrared and Raman spectra showcased peaks corresponding to phosphate and hydroxyl groups. The apatite’s alexandrite effect predominantly stems from absorption peaks at 748, 738, 583, 578, and 526 nm in the visible spectrum, all of which are attributed to the rare-earth element, Nd. The fluorescence peaks of the apatite are primarily influenced by elements such as Ce, Eu, Nd, and Mn. Through this research, a theoretical foundation has been laid for the non-destructive identification of apatite exhibiting the alexandrite effect. Full article

(This article belongs to the Special Issue Gem Deposits: Mineralogical and Gemological Aspects)

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13 pages, 4753 KiB

Open AccessArticle

Color Origin of Greyish-Purple Tremolite Jade from Sanchahe in Qinghai Province, NW China

by Nina Gong, Chaowen Wang and Shuai Xu

Minerals 2023, 13(8), 1049; https://doi.org/10.3390/min13081049 - 07 Aug 2023

Viewed by 880

Abstract

Greyish-purple tremolite jade has become well known in the past few years, and the origin of its color has attracted the attention of gemologists. In this study, FT-IR spectra, EPMA, EPR spectra, micro-XRF, UV–Vis–NIR spectra, and LA-ICP-MS in situ mapping were analyzed to [...] Read more.

Greyish-purple tremolite jade has become well known in the past few years, and the origin of its color has attracted the attention of gemologists. In this study, FT-IR spectra, EPMA, EPR spectra, micro-XRF, UV–Vis–NIR spectra, and LA-ICP-MS in situ mapping were analyzed to investigate the chromophore elements. The study sample was chosen from the Sanchahe mine, Qinghai Province, NW China, which has the typical characteristics of a gradual color change. The FT-IR and EPMA results revealed that the mineral composition of the dark and light greyish-purple regions of the sample are primarily composed of tremolite. UV–Vis–NIR spectra demonstrated that the greyish-purple color is mainly due to strong absorptions at 560 nm and 700 nm and weak absorption at 745 nm in the visible range. The EPR spectra presented ~3400 G six hyperfine lines resulting from the hyperfine interactions of the unpaired electron with the Mn²⁺ nucleus in the octahedral site. The UV–Vis–NIR and EPR spectra analyses demonstrated that Mn²⁺ is the origin of the purple color. A comparison of the major elements in the light and dark regions indicated that the chromogenic elements have strong positive correlations with Mn, Cu, and Fe. LA-ICP-MS mapping used to analyze the first transition metals indicated possible positive correlations between the greyish-purple color and the trace chromogenic elements. This suggested that the Mn, Cu, and Fe contents are significantly high in the dark band region. Combining in situ LA-ICP-MS mapping of trace elements, UV–Vis spectra, and EPR analysis results, it was suggested that Mn, Cu, and Fe are the major contributors to the greyish-purple color. This study provides a reference for the specific experimental methods to determine chromophores and the origin of color in tremolite jades. Full article

(This article belongs to the Special Issue Gem Deposits: Mineralogical and Gemological Aspects)

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14 pages, 5072 KiB

Open AccessArticle

Age and Geochemistry of Zircon Megacrysts from Alluvial Deposits in the Huadian Area, Northeastern China

by Haiqi Sun, Shaokui Pan, Hongyu Qin, Yimiao Liu and Xiaolong Wu

Minerals 2023, 13(7), 882; https://doi.org/10.3390/min13070882 - 29 Jun 2023

Viewed by 840

Abstract

Megacrystic zircons have recently been found in alluvial deposits in the Huadian area of northeastern China. However, studies have rarely been conducted on these zircons. In this article, we present systematic in situ trace element, U-Pb age and Hf isotope data regarding these [...] Read more.

Megacrystic zircons have recently been found in alluvial deposits in the Huadian area of northeastern China. However, studies have rarely been conducted on these zircons. In this article, we present systematic in situ trace element, U-Pb age and Hf isotope data regarding these Huadian zircons, with the aim of investigating their source characteristics and provenance. The studied zircons, with a diameter of 0.7–1.2 cm, are dominantly irregular in shape and have a rounded termination, with a color ranging from near-colorless to reddish brown to maroon. The zircons show oscillatory zoning in CL images, with a Th/U value of 0.18–1.27, which is consistent with the typical features of magmatic zircons. The positive ε_Hf(t) value of Huadian zircons (4.8–9.2) further indicates the presence of precipitation from the mantle-derived melt, with limited contamination of the crustal components. The obtained weighted mean ²⁰⁶Pb/²³⁸U age for these zircons is 17.9 ± 0.12 Ma, which is slightly older than the eruptional ages of the associated alkali basalts (from 17.6 ± 1.09 Ma to 17.8 ± 0.69 Ma), implying a short residence time in the mantle before entrainment. Huadian zircons incorporate a wide range of trace elements, including ΣREE (117–2790 ppm), Hf (4902–11856 ppm) and Y (145–3645 ppm) contents, generating mixed-source protolith assignments. As is suggested by the moderate variations seen in the Hf isotopes, we propose that the source melts of Huadian zircons are complex in nature, which is likely the result of the chemical heterogeneity of the upper mantle. Full article

(This article belongs to the Special Issue Gem Deposits: Mineralogical and Gemological Aspects)

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14 pages, 2097 KiB

Open AccessArticle

The Covariation of Color and Orange Fluorescence Instabilities in Yellow Sapphires

by Yunqi Yang, Chaowen Wang, Chengsi Wang, Xibing Shen, Ke Yin, Tao Chen, Andy Hsitien Shen, Thomas J. Algeo and Hanlie Hong

Minerals 2023, 13(5), 663; https://doi.org/10.3390/min13050663 - 12 May 2023

Cited by 1 | Viewed by 1716

Abstract

In recent years, some sapphires were found to fade in sunlight and to increase their color after UV irradiation. This unstable color phenomenon is attributed to the photochromism of corundum. The photochromic effect seriously affects the grading and evaluation of sapphires, although its [...] Read more.

In recent years, some sapphires were found to fade in sunlight and to increase their color after UV irradiation. This unstable color phenomenon is attributed to the photochromism of corundum. The photochromic effect seriously affects the grading and evaluation of sapphires, although its mechanism is still uncertain. Here, we performed a set of photochromic experiments on sapphire specimens using a 254 nm shortwave UV light source and a D65 light source (which simulates sunlight) to generate different color states exhibiting characteristic absorption, emission, and excitation spectra. We observed that, for different color states, variation in the intensity of the absorption band at ~460 nm was consistent with that of orange fluorescence at 500–800 nm. This observation indicates a relationship between color instability and orange fluorescence. Peaks in excitation spectra at 320, 420, 490, 560, and 637 nm provide insight into the source(s) of excited orange fluorescence, which are related to different types of F-centers and Mg-trapped holes. We propose an explanation for the photochromic phenomenon: the color of photochromic yellow sapphire is the result of a variety of defects that release orange fluorescence simultaneously. Further, we hypothesize that the mechanism of photochromism in yellow sapphires is linked to electron transfer between F-centers and Mg-trapped holes. Full article

(This article belongs to the Special Issue Gem Deposits: Mineralogical and Gemological Aspects)

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17 pages, 12519 KiB

Open AccessArticle

Inclusions and Gemological Characteristics of Emeralds from Kamakanga, Zambia

by Yi Zhang and Xiao-Yan Yu

Minerals 2023, 13(3), 341; https://doi.org/10.3390/min13030341 - 28 Feb 2023

Cited by 2 | Viewed by 3746

Abstract

Currently, Zambia is one of the world’s major emerald-producing countries. In this study, emerald samples from Kamakanga, Zambia, were systematically analyzed by standard gemological tests, microscopic observation, Raman spectroscopy test of mineral inclusions, and fluid inclusions. The study found Kamakanga emeralds have higher [...] Read more.

Currently, Zambia is one of the world’s major emerald-producing countries. In this study, emerald samples from Kamakanga, Zambia, were systematically analyzed by standard gemological tests, microscopic observation, Raman spectroscopy test of mineral inclusions, and fluid inclusions. The study found Kamakanga emeralds have higher RI (refractive index) and SG (specific gravity) than average. The common inclusions in Kamakanga emeralds are pseudo-hexagonal, dark green, brownish, or oval platelet phlogopite; red spot or skeletal hematite; black spot, platelet, or dendritic oxide inclusions (pyrolusite, magnetite, ilmenite); or schorl. The common paragenetic mineral is schorl. Other mineral inclusions are fluorapatite, tremolite, and calcite. All Kamakanga emeralds contain at least two kinds of common mineral inclusions that are described above, and the characteristic mineral inclusions are pseudo-hexagonal dark green platelet phlogopite and a large quantity of fluorapatite. The fluorapatite inclusions with colorless transparent rims and greyish hazy interiors are reported for the first time. Most fluid inclusions in Kamakanga emeralds are rectangular two- or three-phase inclusions, containing gas phase (CO₂ + CH₄ + H₂O or CO₂), aqueous fluid, and sometimes solid phase (carbonate). A small number of hexagonal three-phase fluid inclusions can be seen in Kamakanga emeralds, containing gas phase (CO₂ + CH₄), aqueous fluid, and daughter crystals (siderite). Full article

(This article belongs to the Special Issue Gem Deposits: Mineralogical and Gemological Aspects)

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16 pages, 5297 KiB

Open AccessArticle

The Correlation of Colour and Iron Oxides in Yellow Seal Stones from Northern Laos

by Jinglin Tian, Tao Chen, Jinyu Zheng, Jiaxin Wu and Yating Xu

Minerals 2023, 13(2), 291; https://doi.org/10.3390/min13020291 - 19 Feb 2023

Cited by 1 | Viewed by 1825

Abstract

The yellow seal stone from northern Laos is one possible substitute for the Tianhuang Stone, the most famous Chinese seal stone, because of its similar yellow to orange-yellow appearance and the same main mineral composition. The colour causation of the yellow seal stone [...] Read more.

The yellow seal stone from northern Laos is one possible substitute for the Tianhuang Stone, the most famous Chinese seal stone, because of its similar yellow to orange-yellow appearance and the same main mineral composition. The colour causation of the yellow seal stone from northern Laos was studied. The samples’ phase, micro-morphology and chemical components were studied by Raman spectroscopy, and scanning electron microscopy (SEM) with energy disperse spectroscopy (EDS), respectively. The yellow seal stone from northern Laos is mainly composed of dickite, occasionally with minor impurity minerals, such as hematite, anatase, barite, diaspore and pyrite. Micro- to nano-sized iron oxides/hydroxides were observed and detected by SEM and EDS in the yellow to orange-yellow part of the samples. Moreover, these iron oxides/hydroxides were suggested to cause the yellow to orange-yellow in the seal stone from northern Laos. The UV-Vis spectrum and its second derivative, the Kubelka-Munk spectra, were used to identify and quantify hematite and goethite. The samples’ colour parameters were obtained with the Commission Internationale de l’Eclairage (CIE) 1931 standard space. According to the observation of the samples and the results obtained from experiments and calculations, the colour of the yellow parts (L* = 33.56~47.99, a* = 0.35~3.65, b* = 4.55~9.89) correlated with goethite (goethite is about 0.175~0.671 g/kg, the content of hematite was too low to be figured out in the yellow parts). In contrast, the colour of the orange-yellow parts (L* = 33.99~46.27, a* = 3.98~12.39, b* = 8.04~22.14) was more closely related with the content of hematite (goethite is about 0.096~0.691 g/kg, hematite is about 0.258~2.383 g/kg). The results of correlation analysis also support that the contents of iron oxides or hydroxides influence the samples’ colour. Therefore, it is suggested that micro- to nano-scaled hematite and goethite caused the colour of yellow and orange-yellow in the studied seal stone. Hematite can strengthen the red hue and change the colour from yellow to orange-yellow. Full article

(This article belongs to the Special Issue Gem Deposits: Mineralogical and Gemological Aspects)

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17 pages, 8089 KiB

Open AccessArticle

Spectral Characteristics of Unique Species of Burmese Amber

by Zhaotong Shi, Chenxing Xin and Yamei Wang

Minerals 2023, 13(2), 151; https://doi.org/10.3390/min13020151 - 19 Jan 2023

Cited by 3 | Viewed by 1920

Abstract

Special species of Burmese amber are highly valued within the gemological market due to their fancy optical characteristics. However, some ordinary amber species are misidentified as precious species, which has disrupted consumers’ purchasing behavior and the market order. In this study, seven Burmese [...] Read more.

Special species of Burmese amber are highly valued within the gemological market due to their fancy optical characteristics. However, some ordinary amber species are misidentified as precious species, which has disrupted consumers’ purchasing behavior and the market order. In this study, seven Burmese amber species (golden, golden-blue, blood-tea, black-tea, green-tea, brownish-red, and ‘chameleon’ amber) were collected and investigated. By using conventional gemological tests, Fourier transform infrared (FTIR), three-dimensional (3D) fluorescence, and photoluminescence (PL) spectrometers, detailed analyses were performed on unique species. The FTIR spectra identified that there are three groups of peaks that can distinguish Burmese amber from any other origin. Additionally, the ‘Chameleon’ amber exhibited special patterns in the third group, which might be due to its internal aromatic hydrocarbons structures that are different from any other species. The 3D fluorescence spectra displayed that all seven species presented similar fluorescence behavior—the 334 or 347 nm emission wavelength could be optimally excited by 240 or 294 nm excitation wavelength in the ultraviolet region and the 380 ± 10 nm or 400 ± 10 nm excitation wavelength optimally excited the 430 nm emission wavelength in the violet region. In the red region, green-tea amber, black-tea amber, and brownish-red amber presented totally different fluorescence behavior, which could be regarded as a reference feature for differentiation. Obvious pink fluorescence on the surface of the tea amber was efficiently found under PL spectra, and we firstly suggest this test could be used as an effective way to distinguish black-tea amber from green-tea amber and some ordinary species (such as blood-tea amber). Both the PL and 3D fluorescence measurements demonstrated the different luminescence behavior of tea amber in the red region, which might be related to the type and content of red fluorescent substances in the tea amber. Full article

(This article belongs to the Special Issue Gem Deposits: Mineralogical and Gemological Aspects)

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13 pages, 3858 KiB

Open AccessArticle

Crystal-Chemical and Spectroscopic Study of Gem Sphalerite from Banská Štiavnica, Slovakia

by Peter Bačík, Jana Fridrichová, Olena Rybnikova, Ján Štubňa, Ľudmila Illášová, Radek Škoda, Tomáš Vaculovič, Zuzana Pulišová and Peter Sečkár

Minerals 2023, 13(1), 109; https://doi.org/10.3390/min13010109 - 10 Jan 2023

Viewed by 2103

Abstract

A complex crystal-chemical investigation based on spectroscopic methods, Electron MicroProbe Analysis (EMPA), and Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) was made on sphalerite samples from the Terézia and Rozália veins in the Banská Štiavnica ore district. The yellow (sample A) and [...] Read more.

A complex crystal-chemical investigation based on spectroscopic methods, Electron MicroProbe Analysis (EMPA), and Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) was made on sphalerite samples from the Terézia and Rozália veins in the Banská Štiavnica ore district. The yellow (sample A) and olive-green sphalerite (C) are in association only with quartz; orange sphalerite (B) is associated with quartz and chalcopyrite; and black sphalerite (D) is associated with galena, chalcopyrite, quartz, and baryte. EMPA revealed that Cd and Fe are substituting for Zn with variable proportions; the Cd/Fe ratio decreases from 2.82–2.85 in the A sample to 0.42 in the D sample. LA-ICP-MS showed that, except Cd and Fe, only Mn has content above 20; Co and Cu vary between 2 and 17 ppm. The optical absorption spectra exhibit absorption between 644 and 740 nm with three smaller humps at 669–671, 698–702, and 732–743 nm, and weaker absorption bands at 858–894 nm in the NIR region, which can be all assigned to crystal-field transitions of Fe²⁺. The absorption edge starts at about 600 nm to the UV region. Minimal absorption is in the yellow-red part of the visible spectrum giving rise to yellowish-orange and orange-red colors. Absorption in the red region for olive-green sphalerite is more pronounced, explaining the shift to greenish hues. In black sphalerite, the absorption pattern is similar to the olive-green sphalerite, but the bands in the 644 to 740 nm region are less defined. The black color could be caused by slightly higher concentrations of Fe, the smaller size of individual crystals in the aggregate reducing macroscopic transparency, and/or the presence of submicroscopic inclusions. Full article

(This article belongs to the Special Issue Gem Deposits: Mineralogical and Gemological Aspects)

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15 pages, 3883 KiB

Open AccessArticle

Stratigraphic Section and Geochronological Studies of the Shoushan Basin, Fujian Province, South China, and Its Implications for the Mineralization of Shoushan Stone

by Yu-Juan Li, Lü-Yun Zhu, Wen Xu, Wei Meng, Min Lin, Zhong Yang and Run-Sheng Chen

Minerals 2022, 12(12), 1542; https://doi.org/10.3390/min12121542 - 30 Nov 2022

Viewed by 1480

Abstract

As one of the most famous craft-carving stones in China, Shoushan stone commonly consists of clay minerals, including the kaolinite, pyrophyllite, or illite group, which is the product of hydrothermal alteration. In Fujian Province, the Xiaoxi Formation of the Early Cretaceous is a [...] Read more.

As one of the most famous craft-carving stones in China, Shoushan stone commonly consists of clay minerals, including the kaolinite, pyrophyllite, or illite group, which is the product of hydrothermal alteration. In Fujian Province, the Xiaoxi Formation of the Early Cretaceous is a critical formation containing pyrophyllite deposits (including Shoushan stone). Here, we carry out a geological field investigation of a typical section in the Shoushan basin of southeastern China to identify lithology and volcanic sequences of the Xiaoxi Formation. The section included four lithofacies: eruption facies, flood lava facies, sedimentary facies, and volcanic channel facies. The petrogenesis of these lithofacies demonstrates the evolution of volcanism, which is critical for understanding the formation of the Shoushan-stone-associated hydrothermal system. For the geochronological study, the samples of unaltered rhyolitic tuff are collected from the layers topping and bottoming a pyrophyllite orebody. The zircon U-Pb dating results constrain the age of pyrophyllite alteration during the episodic eruption. Shoushan stone is formed in an epithermal hydrothermal environment, so we suggest that high-quality Shoushan stone is formed by the hydrothermal alterations in the interval time of the volcanic episode (135–131 Ma) and after volcanic activity (<131 Ma). Furthermore, the Shoushan basin’s stratigraphic section suggests that there have been large-scale hydrothermal systems in the volcanic basin during the Early Cretaceous volcanism. The stratigraphic correlation and geochemical results indicate that the Mesozoic basins in the Fu’an-Yongtai volcanic eruption belt have the potential for pyrophyllite deposit exploration. Full article

(This article belongs to the Special Issue Gem Deposits: Mineralogical and Gemological Aspects)

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14 pages, 2031 KiB

Open AccessArticle

Combining Rare Earth Element Analysis and Chemometric Method to Determine the Geographical Origin of Nephrite

by Yue Su and Mingxing Yang

Minerals 2022, 12(11), 1399; https://doi.org/10.3390/min12111399 - 31 Oct 2022

Cited by 3 | Viewed by 1501

Abstract

Nephrite is a high-valued gem material, whose geographical origin determination is a topic of interest to both consumers and producers since the geographic origin determines its price and reputation. In the present study, we suggest a two-step method for discriminating geographical origins of [...] Read more.

Nephrite is a high-valued gem material, whose geographical origin determination is a topic of interest to both consumers and producers since the geographic origin determines its price and reputation. In the present study, we suggest a two-step method for discriminating geographical origins of nephrite based on the rare earth element (REE) contents combined with chemometrics. In the first step, the REE contents of nephrite samples were determined by laser ablation-inductively coupled plasma mass spectrometry (LA-ICP-MS), combined with previously reported data—the chondrite-normalized REE distribution pattern; the REE parameters of nephrite samples from six origins, namely Xinjiang, Qinghai, Russia, Guangxi, Guizhou, and Liaoning were then compared. In the second step, origin discriminant models were established by linear discriminant analysis (LDA), and the accuracy of the model was evaluated by leave-one-out cross-validation (LOOCV). The results showed that the REE contents were significantly different among the six nephrite origins with regional characteristics, which makes it possible to trace the origin. Using chondrite-normalized REE distribution patterns, the six nephrite origins can be divided into three separate groups: Xinjiang−Qinghai−Russia, Luodian−Dahua, and Xiuyan. Xiuyan nephrite can be directly distinguished from the other origins due to its unique REE distribution pattern. In the second step, the LDA discrimination models were performed on the remaining two groups. For the Luodian−Dahua group, the accuracy of the original classification and LOOCV were 97.9% and 85.4%, which indicated REE combined with LDA could effectively identify Luodian nephrite and Dahua nephrite. For the Xinjiang−Qinghai−Russia group, the accuracy of the original classification and LOOCV was 74.1% and 63.9%, respectively. Overall, this work proves that a combination of REE analysis and chemometrics has a certain feasibility and broad application prospects for geographical origin, and the same methodology can be applied to study the origin of other gem materials. Full article

(This article belongs to the Special Issue Gem Deposits: Mineralogical and Gemological Aspects)

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11 pages, 8781 KiB

Open AccessArticle

Study on Spectral Characteristics and Color Origin of Scheelite from Xuebaoding, Pingwu County, Sichuan Province, P.R. China

by Xianyu Liu, Jiuchang Yang and Quanli Chen

Minerals 2022, 12(11), 1344; https://doi.org/10.3390/min12111344 - 24 Oct 2022

Cited by 1 | Viewed by 1388

Abstract

The Xuebaoding deposit, which is located 14.5 km northwest of Huya Town, Pingwu County, Mianyang City, Sichuan Province, China, produces a kind of yellow-orange-hued scheelite with ideal crystal shapes, large-grain crystals, and high market value which is favored by gem and mineral crystal [...] Read more.

The Xuebaoding deposit, which is located 14.5 km northwest of Huya Town, Pingwu County, Mianyang City, Sichuan Province, China, produces a kind of yellow-orange-hued scheelite with ideal crystal shapes, large-grain crystals, and high market value which is favored by gem and mineral crystal collectors. In this article, five Xuebaoding scheelite samples are used as research objects. The infrared absorption in the fingerprint region (2000 to 400 cm⁻¹) of scheelite is at 440 cm⁻¹ and 800 to 900 cm⁻¹, which shows the out-of-plane bending vibration and asymmetric stretching vibration attributed to the [WO₄]²⁻ tetrahedral group, respectively. The Raman shift at 911 cm⁻¹ is assigned to the ν₁ symmetric stretching vibration of [WO₄]²⁻; the Raman spectra scattering peak at 797 cm⁻¹ belongs to the ν₃ asymmetric stretching vibration of [WO₄]²⁻; the Raman shift at 332 cm⁻¹ and the low-intensity Raman scattering peak near 400 cm⁻¹ belong to the ν₂ out-of-plane bending vibration of [WO₄]²⁻. Furthermore, the low-intensity Raman shift around 211 cm⁻¹ is caused by the transitional mode of (Ca–O). The UV-Vis-NIR absorption is attributed to the existence of “didymium”, a mixture of the rare earth elements Pr and Nd, and the absorption at 584 and 803 nm is assigned to Nd, which may be related to origin of the color of scheelite. The 3D fluorescence spectra show that the colorless and colored scheelite samples produce the same number of main fluorescence peaks with similar positions. Those 3D fluorescence peaks are located near λ_ex235 nm/λ_em455 nm, λ_ex250 nm/λ_em490 nm, and λ_ex265 nm/λ_em523 nm. In addition to the above-mentioned main fluorescence peaks, the pale-yellow-colored samples also produced fluorescence peaks near λ_ex250 nm/λ_em425 nm, which may be associated with the rare earth elements in scheelite. Combined with the test results of LA-ICP-MS, the yellow-orange hue of Xuebaoding scheelite is caused by the isomorphic rare earth elements, such as La, Ce, Pr and Nd ions that replace Ca²⁺. Full article

(This article belongs to the Special Issue Gem Deposits: Mineralogical and Gemological Aspects)

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