Research

12 pages, 3140 KiB

Open AccessArticle

Chemical and Spectral Variations between Untreated and Heat-Treated Rubies from Mozambique and Madagascar

by Qi Lu, Xinyi Li, Lihua Sun and Binrong Qin

Minerals 2022, 12(7), 894; https://0-doi-org.brum.beds.ac.uk/10.3390/min12070894 - 16 Jul 2022

Cited by 2 | Viewed by 1893

The chemical composition and spectra of untreated and heat-treated rubies from Mozambique and Madagascar were analyzed by an electron probe microscopy analysis, laser ablation inductively coupled plasma quadrupole mass spectrometry, Fourier transform infrared spectroscopy, Raman spectroscopy and UV-visible spectroscopy. Due to the different [...] Read more.

The chemical composition and spectra of untreated and heat-treated rubies from Mozambique and Madagascar were analyzed by an electron probe microscopy analysis, laser ablation inductively coupled plasma quadrupole mass spectrometry, Fourier transform infrared spectroscopy, Raman spectroscopy and UV-visible spectroscopy. Due to the different content of Fe and different inclusions, rubies from Madagascar belong to alkaline basalt deposit, while rubies from Mozambique belong to amphibole metamorphic rock. The ruby samples were heated to 900 °C to change their color. As Fe and Ti ions can be transferred into different valences and diffused into the interior of rubies, Cr ions in cracks or cleavages entered the crystal lattice during heat treatment and the content of Fe and Ti decreased, while the content of Cr increased in heat-treated rubies. After heating, blue-purple decreased and the red hue increased, while the blue color band disappeared and yellow appeared in the cracks of the samples because of the chemical changes. Compared with untreated rubies, the infrared absorption peaks of 2123 cm⁻¹ and 1990 cm⁻¹ related to inclusions disappeared, and the existence of 3236 cm⁻¹ and 3186 cm⁻¹ absorption peaks was a typical characteristic of heat-treated ruby, which was produced by changes in its inclusion. In addition, due to the weakened charge transfer of Fe²⁺ and Ti⁴⁺ and the increasing reaction of Fe²⁺ → Fe³⁺ along with the heat treatment, the UV-visible absorption peak at 400 nm shifted to purple. Full article

(This article belongs to the Special Issue Gems and Gem Minerals)

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15 pages, 4458 KiB

Open AccessArticle

Quantitative Study on Colour and Spectral Characteristics of Beihong Agate

by Yufei Zhou, Ziyuan Liu, Zitong Zhao and Ying Guo

Minerals 2022, 12(6), 677; https://0-doi-org.brum.beds.ac.uk/10.3390/min12060677 - 27 May 2022

Cited by 2 | Viewed by 1582

Abstract

The Beihong agate from the southern section of the Xing’an Mountains in northeastern China is a kind of cryptocrystalline agate with a yellow to orange-red colour. However, it has been less studied in previous research, and there is a lack of quantitative study [...] Read more.

The Beihong agate from the southern section of the Xing’an Mountains in northeastern China is a kind of cryptocrystalline agate with a yellow to orange-red colour. However, it has been less studied in previous research, and there is a lack of quantitative study on the cause of its colour. In this study, the colour of Beihong agate was quantified by a colourimeter, and the quantitative relationships between colour and spectral characteristics of Beihong agate were studied by XRF, Raman spectra, UV-VIS spectra, and a heat treatment experiment. The results show a high correlation between the lightness and the hue angle of the Beihong agate. The change of total Fe content can significantly affect the lightness and the hue of the Beihong agate. The first derivative curve can effectively distinguish the relative contents of goethite and hematite in the Beihong agate, and the position of a primary trough is related significantly to the colour of the Beihong agate. Full article

(This article belongs to the Special Issue Gems and Gem Minerals)

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16 pages, 1188 KiB

Open AccessArticle

A Multi-Methodological Investigation of Natural and Synthetic Red Beryl Gemstones

by Giacomo Diego Gatta, Ilaria Adamo, Andrea Zullino, Valentina Gagliardi, Roberto Lorenzi, Nicola Rotiroti, Ludovica Faldi and Loredana Prosperi

Minerals 2022, 12(4), 439; https://0-doi-org.brum.beds.ac.uk/10.3390/min12040439 - 01 Apr 2022

Cited by 1 | Viewed by 2679

Abstract

In this study, we report the experimental findings of a multi-methodological and comparative investigation of a natural (from the Wah Wah Mountains of Beaver County, Utah) and three synthetic (hydrothermally grown) gem-quality red beryls by means of: gemmological standard testing, laser ablation inductively [...] Read more.

In this study, we report the experimental findings of a multi-methodological and comparative investigation of a natural (from the Wah Wah Mountains of Beaver County, Utah) and three synthetic (hydrothermally grown) gem-quality red beryls by means of: gemmological standard testing, laser ablation inductively coupled mass spectroscopy, infrared and Raman spectroscopy, ultraviolet–visible–near infrared absorption spectroscopy, and single-crystal X-ray diffraction. Gemmological, crystallo-chemical, and spectroscopic features of the natural and synthetic stones enabled us to unveil the causes of their color (from red, to purplish-red, or orange-red) and how different and complementary techniques can be efficiently used to discriminate between natural and synthetic materials, based on non-destructive, micro-destructive, and destructive techniques. Full article

(This article belongs to the Special Issue Gems and Gem Minerals)

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24 pages, 4657 KiB

Open AccessArticle

²⁹Si Solid-State NMR Analysis of Opal-AG, Opal-AN and Opal-CT: Single Pulse Spectroscopy and Spin-Lattice T₁ Relaxometry

by Neville J. Curtis, Jason R. Gascooke, Martin R. Johnston and Allan Pring

Minerals 2022, 12(3), 323; https://0-doi-org.brum.beds.ac.uk/10.3390/min12030323 - 04 Mar 2022

Cited by 3 | Viewed by 2644

Abstract

Single pulse, solid-state ²⁹Si nuclear magnetic resonance (NMR) spectroscopy offers an additional method of characterisation of opal-A and opal-CT through spin-lattice (T₁) relaxometry. Opal T₁ relaxation is characterised by stretched exponential (Weibull) function represented by scale (speed of [...] Read more.

Single pulse, solid-state ²⁹Si nuclear magnetic resonance (NMR) spectroscopy offers an additional method of characterisation of opal-A and opal-CT through spin-lattice (T₁) relaxometry. Opal T₁ relaxation is characterised by stretched exponential (Weibull) function represented by scale (speed of relaxation) and shape (form of the curve) parameters. Relaxation is at least an order of magnitude faster than for silica glass and quartz, with Q₃ (silanol) usually faster than Q₄ (fully substituted silicates). 95% relaxation (Q₄) is achieved for some Australian seam opals after 50 s though other samples of opal-AG may take 4000 s, while some figures for opal-AN are over 10,000 s. Enhancement is probably mostly due to the presence of water/silanol though the presence of paramagnetic metal ions and molecular motion may also contribute. Shape factors for opal-AG (0.5) and opal-AN (0.7) are significantly different, consistent with varying water and silanol environments, possibly reflecting differences in formation conditions. Opal-CT samples show a trend of shape factors from 0.45 to 0.75 correlated to relaxation rate. Peak position, scale and shape parameter, and Q₃ to Q₄ ratios offer further differentiating feature to separate opal-AG and opal-AN from other forms of opaline silica. T₁ relaxation measurement may have a role for provenance verification. In addition, definitively determined Q₃/Q₄ ratios are in the range 0.1 to 0.4 for opal-AG but considerably lower for opal-AN and opal-CT. Full article

(This article belongs to the Special Issue Gems and Gem Minerals)

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12 pages, 3324 KiB

Open AccessFeature PaperArticle

Crystallographic Characteristics of Inorganic Mineral in Mammoth Ivory and Ivory

by Xueying Sun, Mingyue He and Jinlin Wu

Minerals 2022, 12(2), 117; https://0-doi-org.brum.beds.ac.uk/10.3390/min12020117 - 20 Jan 2022

Cited by 4 | Viewed by 2409

Abstract

In this paper, the chemical composition and crystalline properties of mammoth ivory and ivory were systematically analyzed. The results revealed that the microscopic crystalline hydroxyapatites are the major component of minerals in mammoth ivory and ivory. The Ca/P ratios of the samples studied [...] Read more.

In this paper, the chemical composition and crystalline properties of mammoth ivory and ivory were systematically analyzed. The results revealed that the microscopic crystalline hydroxyapatites are the major component of minerals in mammoth ivory and ivory. The Ca/P ratios of the samples studied are obviously lower than apatite. Refined cell parameters showed a similar value between mammoth ivory (a = 9.4148 Å and b = 6.8821 Å) and ivory (a = 9.4166 Å and c = 6.8841 Å). Individual crystal sizes in mammoth ivory and ivory are estimated to be 20.41–23.16 and 18.92–21.50 nm, respectively. The calculated crystallinity of two kinds of ivory (mammoth ivory: 1.55; ivory: 1.4) is far lower than geological mineral fluorapatite due to the impact of tissue function and organic matrix. Full article

(This article belongs to the Special Issue Gems and Gem Minerals)

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15 pages, 5431 KiB

Open AccessFeature PaperArticle

Effects of Transition Metal Ions on the Colour of Blue-Green Beryl

by Hui Wang, Qiyun Guan, Yuchong Liu and Ying Guo

Minerals 2022, 12(1), 86; https://0-doi-org.brum.beds.ac.uk/10.3390/min12010086 - 12 Jan 2022

Cited by 8 | Viewed by 2054

Abstract

This study reports the effects of transition metal ions on the colour of blue-green beryl. Industrial cameras were used to measure colour in the CIELAB colour space. X-ray fluorescence (XRF), X-ray diffraction (XRD), infrared spectroscopy (IR), and ultraviolet-visible (UV–vis) spectroscopy were used for [...] Read more.

This study reports the effects of transition metal ions on the colour of blue-green beryl. Industrial cameras were used to measure colour in the CIELAB colour space. X-ray fluorescence (XRF), X-ray diffraction (XRD), infrared spectroscopy (IR), and ultraviolet-visible (UV–vis) spectroscopy were used for characterization. The d–d transition of Fe³⁺ with sixfold coordination, the O²⁻→Fe³⁺ charge transfer, and the charge transition of binuclear metal M–M complexes formed by [Fe₂(OH)₄]²⁺ in the channel caused a yellow tone, whereas the charge transfer of Fe²⁺/Fe³⁺ with sixfold coordination caused a blue-green tone. The chroma of blue-green beryl was negatively correlated with the ratio of Cs+Mn to Fe contents. The lightness of blue-green beryl was negatively correlated with the total content of transition metal ions. Full article

(This article belongs to the Special Issue Gems and Gem Minerals)

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12 pages, 5054 KiB

Open AccessArticle

Mineralogy and Magnetic Behavior of Yellow to Red Xuanhua-Type Agate and Its Indication to the Forming Condition

by Danyi Zhou, Guanghai Shi, Suzhen Liu and Bailing Wu

Minerals 2021, 11(8), 877; https://0-doi-org.brum.beds.ac.uk/10.3390/min11080877 - 13 Aug 2021

Cited by 3 | Viewed by 2049

Abstract

Iron oxides/hydroxides are important magnetic minerals to provide information about changes in the forming environment. However, the magnetic behavior in agate has been rarely investigated. In this study, the magnetic behavior of the Xuanhua-type agate with intense yellow to red colors from the [...] Read more.

Iron oxides/hydroxides are important magnetic minerals to provide information about changes in the forming environment. However, the magnetic behavior in agate has been rarely investigated. In this study, the magnetic behavior of the Xuanhua-type agate with intense yellow to red colors from the Xuanhua District (China) was investigated by temperature dependence of magnetic susceptibility, hysteresis loop, isothermal remanent magnetization and the analysis of remanent coercivity components from the gradient acquisition plot. Yellow goethite and red hematite can be quantitatively identified by XRD and Raman spectroscopy due to their relatively higher content. Results showed that the red, yellow and orange Xuanhua-type agate had different magnetic behavior, and magnetite existed in the yellow and orange ones. Fluid inclusions in such agate had the homogenization temperature of ~168 °C to 264 °C. All results suggested that the dehydration of goethite to form hematite was the main reason for the high remnant coercivity (above 1000 mT) of hematite in the red agate. The co-existence of magnetite and goethite in the yellow and orange agate reflects the transformation from Fe²⁺ to Fe³⁺, indicating the change in the redox property of the environment. Unique patterns mainly formed by hematite and goethite make it a popular gem-material with high research value. Full article

(This article belongs to the Special Issue Gems and Gem Minerals)

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23 pages, 10725 KiB

Open AccessArticle

An Attractive Blue Diopside from Sissone Valley, Western Alps, Italy

by Franca Caucia, Maurizio Scacchetti, Luigi Marinoni, Mattia Gilio, Antonio Langone, Omar Bartoli, Marco Vanotti and Ivano Foianini

Minerals 2021, 11(8), 837; https://0-doi-org.brum.beds.ac.uk/10.3390/min11080837 - 01 Aug 2021

Cited by 1 | Viewed by 2669

Abstract

In this work a rare and attractive blue diopside present in Sissone valley in the Western Alps was investigated through different methodologies: geological survey; standard gemological methods; X-Ray Powder Diffraction; SEM observations; Raman spectrometry; EMP analyses of major elements; and LA-ICP-MS analyses for [...] Read more.

In this work a rare and attractive blue diopside present in Sissone valley in the Western Alps was investigated through different methodologies: geological survey; standard gemological methods; X-Ray Powder Diffraction; SEM observations; Raman spectrometry; EMP analyses of major elements; and LA-ICP-MS analyses for minor and trace elements. The host rock of investigated gems is represented by a Mg-calcite bearing marble, belonging to the Suretta nappe and composed of blue diopside, lizardite, phlogopite, forsterite, Ca-Mg-amphibole, and thomsonite; the rock was metamorphosed by the intrusion of Masino-Bregaglia pluton. The diopside is generally found in the core of veins in contact with green–blue tremolite and, more externally, with green–yellowish lizardite. The diopside samples show opaque diaphaneity, are inert to long and short-waves UV radiation, and their specific density varies between 3.24 and 3.30 g/cm³ while medium refraction between 1.680–1.683. The diopside shows a polycrystalline texture with interstitial Mg-calcite which acts as binder. The characteristic blue–turquoise color is mainly determined by traces of V and subordinately of Fe, Mn, Cr and Ti. The contents of V and Ti show a good positive correlation. The minerals associated with diopside in the lenticular veins also show enrichments in V. The blue diopside of the Sissone valley could certainly present a good commercial value, but unfortunately it is difficult to reach the outcrop sites. Full article

(This article belongs to the Special Issue Gems and Gem Minerals)

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13 pages, 4218 KiB

Open AccessArticle

“Horsetail” Inclusions in the Ural Demantoids: Growth Formations

by Aleksander Yurevich Kissin, Valery Vasilevich Murzin and Elizaveta Sergeevna Karaseva

Minerals 2021, 11(8), 825; https://0-doi-org.brum.beds.ac.uk/10.3390/min11080825 - 29 Jul 2021

Cited by 4 | Viewed by 3165

Abstract

The term “demantoid”, first proposed in 1856 by the famous Finnish mineralogist Nils von Nordensheld, refers to a highly dispersed yellow-green mineral from the Central Urals placers. In 1874, it was found to be a gem variety of andradite garnet. “Horsetail” inclusions are [...] Read more.

The term “demantoid”, first proposed in 1856 by the famous Finnish mineralogist Nils von Nordensheld, refers to a highly dispersed yellow-green mineral from the Central Urals placers. In 1874, it was found to be a gem variety of andradite garnet. “Horsetail” inclusions are considered a sign of the Ural type demantoid. Although these inclusions are large (visible to the naked eye), their diagnostics remains debatable: some researchers attribute them to byssolite (amphibole-asbestos), others consider them chrysotile. We investigated the horsetail inclusions in the Ural demantoids through various methods: optical microscopy, scanning electron microscopy (SEM), Raman spectrometry, X-ray powder diffraction, and thermal analysis. In most cases, “horsetail” inclusions in the Ural demantoid were represented by hollow channels and only the outcrops, on the demantoid surface, were occasionally filled with serpentine (established by SEM); in one case, magnetite was observed. Hollow canals were usually collected not in bundles, such as a “horsetail”, but in fans, sometimes curved into cones. The structure of the grains was spheroidal, sectorial, and sometimes had induction surfaces, which, to the periphery of the grain, were replaced by tubular channels assembled in a fan. The specifics of the growth of the “horsetail” inclusions of the demantoid grains can be explained by the decompression conditions that arose when the ultrabasites (a crust-mantle mixture) were squeezed upwards during collision. Full article

(This article belongs to the Special Issue Gems and Gem Minerals)

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16 pages, 4819 KiB

Open AccessArticle

Spectroscopy and Microscopy of Corundum from Primary Deposits Found in Greece

by Vilelmini Karantoni, Stefanos Karampelas, Panagiotis Voudouris, Vasilios Melfos, Lambrini Papadopoulou, Triantafyllos Soldatos and Constantinos Mavrogonatos

Minerals 2021, 11(7), 750; https://0-doi-org.brum.beds.ac.uk/10.3390/min11070750 - 10 Jul 2021

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Abstract

Corundum primary deposits in Greece occur in four locations: Paranesti in Drama and Gorgona in Xanthi, both belonging to the wider Rhodope Massif, as well as, Ikaria island and Kinidaros in Naxos island, both belonging to Attic-Cycladic Massif. Eight samples were examined with [...] Read more.

Corundum primary deposits in Greece occur in four locations: Paranesti in Drama and Gorgona in Xanthi, both belonging to the wider Rhodope Massif, as well as, Ikaria island and Kinidaros in Naxos island, both belonging to Attic-Cycladic Massif. Eight samples were examined with spectroscopic methods (FTIR, UV-Vis, EDXRF) in order to better characterize these four primary deposits: two pink sapphires from Paranesti, a pink and a blue sapphire from Gorgona, two blue sapphires from Ikaria and three blue sapphires from Kinidaros. Under the microscope, all samples present characteristics linked to post-crystallization deformation, decreasing their gem quality. The FTIR absorption spectra of all samples present in different relative intensities, bands of boehmite, diaspore, goethite, mica and/or chlorite inclusions and CO₂ in fluid inclusions. Boehmite and diaspore inclusions are most likely epigenetic. In the UV-Vis spectra, the pink color of the samples is linked with Cr³⁺ absorptions and the blue color with absorptions due to Fe²⁺-Ti⁴⁺ intervalence charge transfer. EDXRF analyses in the studied samples show relatively high titanium and iron concentrations that are related with mineral inclusions. Gallium is slightly variable in samples from different regions; also, different colored samples from Gorgona present diverse gallium content. Full article

(This article belongs to the Special Issue Gems and Gem Minerals)

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