Special Issue "Mineralogy and Geochemistry of Gems"

A special issue of Minerals (ISSN 2075-163X).

Deadline for manuscript submissions: closed (31 December 2018)

Special Issue Editors

Guest Editor
Assoc. Prof. Panagiotis Voudouris

National and Kapodistrian University of Athens, 157 84 Athens, Greece
Website | E-Mail
Interests: Ore minerals; Ore-forming Processes; Magmatic-Hydrothermal Deposits; Hydrothermal Alteration
Guest Editor
Dr. Stefanos Karampelas

Bahrain Institute for Pearls & Gemstones (DANAT), 4th floor, East Tower, Bahrain World Trade Centre, P.O. Box 17236, Manama, Kingdom of Bahrain
Website | E-Mail
Phone: +973 17201333
Interests: gemology; mineralogy; spectroscopy
Guest Editor
Assist. Prof. Vasilios Melfos

Aristotle University of Thessaloniki, 541 24 Thessaloniki, Greece
Website | E-Mail
Phone: +30 2310 998539
Interests: ore deposits; porphyry-epithermal mineralization; geochemistry; fluid inclusions
Guest Editor
Dr. Ian Graham

School of Biological, Earth and Environmental Sciences (BEES), Faculty of Science, The University of New South Wales, Sydney, NSW 2052, Australia
Website | E-Mail
Phone: +61 2 9385 8720
Fax: +61 2 9385 1558
Interests: mineralogy; petrology; ore deposits; gemstones

Special Issue Information

Dear Colleagues,

Gems have been used in the manufacture of jewelry and as ornaments since antiquity. Recent statistics have shown that about 15 billion Euros are annually at stake. The purpose of this Special Issue is to present recent advances on the study of various types of gems based on different aspects of research (e.g., geology, trace element geochemistry, inclusion studies, geochronology, spectroscopy, archeogemology), which can be used to constrain the conditions of their formation. A combination of non- and micro-destructive methods, such as UV-Vis-NIR spectroscopy, FTIR spectroscopy, Raman diffusion spectroscopy, EDXRF, LA-ICP-MS, micro-CT and others, may provide valuable information regarding the exact formation, appearance (e.g., color) and treatment of gem materials.

This Special Issue will emphasize on the recent advances in both fundamental and applied studies on gems, as well as the application of mineralogical and geochemical methods to their exploration, provenance and treatment identification from previously known or from new localities worldwide. 

Dr. Panagiotis Voudouris
Dr. Stefanos Karampelas
Dr. Vasilios Melfos
Dr. Ian Graham
Guest Editors

Manuscript Submission Information

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Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Minerals is an international peer-reviewed open access monthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 1400 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

  • gemology
  • mineralogy
  • geology
  • geochemistry
  • spectroscopy

Published Papers (11 papers)

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Research

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Open AccessArticle Fingerprinting Paranesti Rubies through Oxygen Isotopes
Minerals 2019, 9(2), 91; https://doi.org/10.3390/min9020091
Received: 1 December 2018 / Revised: 23 January 2019 / Accepted: 30 January 2019 / Published: 3 February 2019
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Abstract
In this study, the oxygen isotope (δ18O) composition of pink to red gem-quality rubies from Paranesti, Greece was investigated using in-situ secondary ionization mass spectrometry (SIMS) and laser-fluorination techniques. Paranesti rubies have a narrow range of δ18O values between [...] Read more.
In this study, the oxygen isotope (δ18O) composition of pink to red gem-quality rubies from Paranesti, Greece was investigated using in-situ secondary ionization mass spectrometry (SIMS) and laser-fluorination techniques. Paranesti rubies have a narrow range of δ18O values between ~0 and +1‰ and represent one of only a few cases worldwide where δ18O signatures can be used to distinguish them from other localities. SIMS analyses from this study and previous work by the authors suggests that the rubies formed under metamorphic/metasomatic conditions involving deeply penetrating meteoric waters along major crustal structures associated with the Nestos Shear Zone. SIMS analyses also revealed slight variations in δ18O composition for two outcrops located just ~500 m apart: PAR-1 with a mean value of 1.0‰ ± 0.42‰ and PAR-5 with a mean value of 0.14‰ ± 0.24‰. This work adds to the growing use of in-situ methods to determine the origin of gem-quality corundum and re-confirms its usefulness in geographic “fingerprinting”. Full article
(This article belongs to the Special Issue Mineralogy and Geochemistry of Gems)
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Open AccessArticle Gem-Quality Zircon Megacrysts from Placer Deposits in the Central Highlands, Vietnam—Potential Source and Links to Cenozoic Alkali Basalts
Minerals 2019, 9(2), 89; https://doi.org/10.3390/min9020089
Received: 30 December 2018 / Revised: 28 January 2019 / Accepted: 29 January 2019 / Published: 1 February 2019
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Abstract
Gem-quality zircon megacrysts occur in placer deposits in the Central Highlands, Vietnam, and have euhedral to anhedral crystal shapes with dimensions of ~3 cm in length. These zircons have primary inclusions of calcite, olivine, and corundum. Secondary quartz, baddeleyite, hematite, and CO2 [...] Read more.
Gem-quality zircon megacrysts occur in placer deposits in the Central Highlands, Vietnam, and have euhedral to anhedral crystal shapes with dimensions of ~3 cm in length. These zircons have primary inclusions of calcite, olivine, and corundum. Secondary quartz, baddeleyite, hematite, and CO2 fluid inclusions were found in close proximity to cracks and tubular channels. LA-ICP-MS U-Pb ages of analyzed zircon samples yielded two age populations of ca. 1.0 Ma and ca. 6.5 Ma, that were consistent with the ages of alkali basalt eruptions in the Central Highlands at Buon Ma Thuot (5.80–1.67 Ma), Pleiku (4.30–0.80 Ma), and Xuan Loc (0.83–0.44 Ma). The zircon geochemical signatures and primary inclusions suggested a genesis from carbonatite-dominant melts as a result of partial melting of a metasomatized lithospheric mantle source, but not from the host alkali basalt. Chondrite-normalized rare earth element patterns showed a pronounced positive Ce, but negligible Eu anomalies. Detailed hyperspectral Dy3+ photoluminescence images of zircon megacrysts revealed resorption and re-growth processes. Full article
(This article belongs to the Special Issue Mineralogy and Geochemistry of Gems)
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Open AccessArticle Gem Corundum Deposits of Greece: Geology, Mineralogy and Genesis
Minerals 2019, 9(1), 49; https://doi.org/10.3390/min9010049
Received: 13 December 2018 / Revised: 10 January 2019 / Accepted: 14 January 2019 / Published: 15 January 2019
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Abstract
Greece contains several gem corundum deposits set within diverse geological settings, mostly within the Rhodope (Xanthi and Drama areas) and Attico-Cycladic (Naxos and Ikaria islands) tectono-metamorphic units. In the Xanthi area, the sapphire (pink, blue to purple) deposits are stratiform, occurring within marble [...] Read more.
Greece contains several gem corundum deposits set within diverse geological settings, mostly within the Rhodope (Xanthi and Drama areas) and Attico-Cycladic (Naxos and Ikaria islands) tectono-metamorphic units. In the Xanthi area, the sapphire (pink, blue to purple) deposits are stratiform, occurring within marble layers alternating with amphibolites. Deep red rubies in the Paranesti-Drama area are restricted to boudinaged lenses of Al-rich metapyroxenites alternating with amphibolites and gneisses. Both occurrences are oriented parallel to the ultra-high pressure/high pressure (UHP/HP) Nestos suture zone. On central Naxos Island, colored sapphires are associated with desilicated granite pegmatites intruding ultramafic lithologies (plumasites), occurring either within the pegmatites themselves or associated metasomatic reaction zones. In contrast, on southern Naxos and Ikaria Islands, blue sapphires occur in extensional fissures within Mesozoic metabauxites hosted in marbles. Mineral inclusions in corundums are in equilibrium and/or postdate corundum crystallization and comprise: spinel and pargasite (Paranesti), spinel, zircon (Xanthi), margarite, zircon, apatite, diaspore, phlogopite and chlorite (Naxos) and chloritoid, ilmenite, hematite, ulvospinel, rutile and zircon (Ikaria). The main chromophore elements within the Greek corundums show a wide range in concentration: the Fe contents vary from (average values) 1099 ppm in the blue sapphires of Xanthi, 424 ppm in the pink sapphires of Xanthi, 2654 ppm for Paranesti rubies, 4326 ppm for the Ikaria sapphires, 3706 for southern Naxos blue sapphires, 4777 for purple and 3301 for pink sapphire from Naxos plumasite, and finally 4677 to 1532 for blue to colorless sapphires from Naxos plumasites, respectively. The Ti concentrations (average values) are very low in rubies from Paranesti (41 ppm), with values of 2871 ppm and 509 in the blue and pink sapphires of Xanthi, respectively, of 1263 ppm for the Ikaria blue sapphires, and 520 ppm, 181 ppm in Naxos purple, pink sapphires, respectively. The blue to colorless sapphires from Naxos plumasites contain 1944 to 264 ppm Ti, respectively. The very high Ti contents of the Xanthi blue sapphires may reflect submicroscopic rutile inclusions. The Cr (average values) ranges from 4 to 691 ppm in the blue, purple and pink colored corundums from Naxos plumasite, is quite fixed (222 ppm) for Ikaria sapphires, ranges from 90 to 297 ppm in the blue and pink sapphires from Xanthi, reaches 9142 ppm in the corundums of Paranesti, with highest values of 15,347 ppm in deep red colored varieties. Each occurrence has both unique mineral assemblage and trace element chemistry (with variable Fe/Mg, Ga/Mg, Ga/Cr and Fe/Ti ratios). Additionally, oxygen isotope compositions confirm their geological typology, i.e., with, respectively δ18O of 4.9 ± 0.2‰ for sapphire in plumasite, 20.5‰ for sapphire in marble and 1‰ for ruby in mafics. The fluid inclusions study evidenced water free CO2 dominant fluids with traces of CH4 or N2, and low CO2 densities (0.46 and 0.67 g/cm3), which were probably trapped after the metamorphic peak. The Paranesti, Xanthi and central Naxos corundum deposits can be classified as metamorphic sensu stricto (s.s.) and metasomatic, respectively, those from southern Naxos and Ikaria display atypical magmatic signature indicating a hydrothermal origin. Greek corundums are characterized by wide color variation, homogeneity of the color hues, and transparency, and can be considered as potential gemstones. Full article
(This article belongs to the Special Issue Mineralogy and Geochemistry of Gems)
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Open AccessArticle Origin of Blue Sapphire in Newly Discovered Spinel–Chlorite–Muscovite Rocks within Meta-Ultramafites of Ilmen Mountains, South Urals of Russia: Evidence from Mineralogy, Geochemistry, Rb-Sr and Sm-Nd Isotopic Data
Minerals 2019, 9(1), 36; https://doi.org/10.3390/min9010036
Received: 19 November 2018 / Revised: 3 January 2019 / Accepted: 4 January 2019 / Published: 11 January 2019
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Abstract
Blue sapphire of gem quality was recently discovered in spinel–chlorite–muscovite rock within meta-ultramafites near the Ilmenogorsky alkaline complex in the Ilmen Mountains of the South Urals. More than 20 minerals were found in the assemblage with the blue sapphire. These sapphire-bearing rocks are [...] Read more.
Blue sapphire of gem quality was recently discovered in spinel–chlorite–muscovite rock within meta-ultramafites near the Ilmenogorsky alkaline complex in the Ilmen Mountains of the South Urals. More than 20 minerals were found in the assemblage with the blue sapphire. These sapphire-bearing rocks are enriched in LREE and depleted in HREE (with the negative Eu anomalies) with REE distribution similar to those in miascites (nepheline syenite) of the Ilmenogorsky alkaline complex. 87Sr/86Sr ratios in the sapphire-bearing rocks varied from 0.7088 ± 0.000004 (2σ) to 0.7106 ± 0.000006 (2σ): epsilon notation εNd is −7.8. The Rb-Sr isochrone age of 289 ± 9 Ma was yielded for the sapphire-bearing rocks and associated muscovite. The blue sapphires are translucent to transparent and they have substantial colorless zones. They occur in a matrix of clinochlore-muscovite as concentric aggregates within spinel-gahnite coronas. Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) analyses showed values with trace elements typical for “metamorphic” blue sapphires, with Ga/Mg < 2.7, Fe/Mg < 74, Cr/Ga > 1.5 (when Cr is detectable), and Fe/Ti < 9. Sapphires overlap “metasomatic” at “sapphires in alkali basalts” field on the FeO–Cr2O3–MgO–V2O3 versus FeO + TiO2 + Ga2O3 discriminant diagram. The sapphires formed together with the spinel-chlorite-muscovite rock during metasomatism at a contact of orthopyroxenites. Metasomatic fluids were enriched with Al, HSFE, and LILE and genetically linked to the miascite intrusions of Ilmenogorsky complex. The temperature required for the formation of sapphire–spinel–chlorite–muscovite rock was 700–750 °C and a pressure of 1.8–3.5 kbar. Full article
(This article belongs to the Special Issue Mineralogy and Geochemistry of Gems)
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Open AccessFeature PaperArticle Diversity in Ruby Geochemistry and Its Inclusions: Intra- and Inter- Continental Comparisons from Myanmar and Eastern Australia
Minerals 2019, 9(1), 28; https://doi.org/10.3390/min9010028
Received: 28 November 2018 / Revised: 22 December 2018 / Accepted: 25 December 2018 / Published: 5 January 2019
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Abstract
Ruby in diverse geological settings leaves petrogenetic clues, in its zoning, inclusions, trace elements and oxygen isotope values. Rock-hosted and isolated crystals are compared from Myanmar, SE Asia, and New South Wales, East Australia. Myanmar ruby typifies metasomatized and metamorphic settings, while East [...] Read more.
Ruby in diverse geological settings leaves petrogenetic clues, in its zoning, inclusions, trace elements and oxygen isotope values. Rock-hosted and isolated crystals are compared from Myanmar, SE Asia, and New South Wales, East Australia. Myanmar ruby typifies metasomatized and metamorphic settings, while East Australian ruby xenocrysts are derived from basalts that tapped underlying fold belts. The respective suites include homogeneous ruby; bi-colored inner (violet blue) and outer (red) zoned ruby; ruby-sapphirine-spinel composites; pink to red grains and multi-zoned crystals of red-pink-white-violet (core to rim). Ruby ages were determined by using U-Pb isotopes in titanite inclusions (Thurein Taung; 32.4 Ma) and zircon inclusions (Mong Hsu; 23.9 Ma) and basalt dating in NSW, >60–40 Ma. Trace element oxide plots suggest marble sources for Thurein Taung and Mong Hsu ruby and ultramafic-mafic sources for Mong Hsu (dark cores). NSW rubies suggest metasomatic (Barrington Tops), ultramafic to mafic (Macquarie River) and metasomatic-magmatic (New England) sources. A previous study showed that Cr/Ga vs. Fe/(V + Ti) plots separate Mong Hsu ruby from other ruby fields, but did not test Mogok ruby. Thurein Taung ruby, tested here, plotted separately to Mong Hsu ruby. A Fe-Ga/Mg diagram splits ruby suites into various fields (Ga/Mg < 3), except for magmatic input into rare Mogok and Australian ruby (Ga/Mg > 6). The diverse results emphasize ruby’s potential for geographic typing. Full article
(This article belongs to the Special Issue Mineralogy and Geochemistry of Gems)
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Open AccessArticle Gem-Quality Tourmaline from LCT Pegmatite in Adamello Massif, Central Southern Alps, Italy: An Investigation of Its Mineralogy, Crystallography and 3D Inclusions
Minerals 2018, 8(12), 593; https://doi.org/10.3390/min8120593
Received: 12 November 2018 / Revised: 1 December 2018 / Accepted: 7 December 2018 / Published: 13 December 2018
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Abstract
In the early 2000s, an exceptional discovery of gem-quality multi-coloured tourmalines, hosted in Litium-Cesium-Tantalum (LCT) pegmatites, was made in the Adamello Massif, Italy. Gem-quality tourmalines had never been found before in the Alps, and this new pegmatitic deposit is of particular interest and [...] Read more.
In the early 2000s, an exceptional discovery of gem-quality multi-coloured tourmalines, hosted in Litium-Cesium-Tantalum (LCT) pegmatites, was made in the Adamello Massif, Italy. Gem-quality tourmalines had never been found before in the Alps, and this new pegmatitic deposit is of particular interest and worthy of a detailed characterization. We studied a suite of faceted samples by classical gemmological methods, and fragments were studied with Synchrotron X-ray computed micro-tomography, which evidenced the occurrence of inclusions, cracks and voids. Electron Microprobe combined with Laser Ablation analyses were performed to determine major, minor and trace element contents. Selected samples were analysed by single crystal X-ray diffraction method. The specimens range in colour from colourless to yellow, pink, orange, light blue, green, amber, brownish-pink, purple and black. Chemically, the tourmalines range from fluor-elbaite to fluor-liddicoatite and rossmanite: these chemical changes occur in the same sample and affect the colour. Rare Earth Elements (REE) vary from 30 to 130 ppm with steep Light Rare Earth Elemts (LREE)-enriched patterns and a negative Eu-anomaly. Structural data confirmed the elbaitic composition and showed that high manganese content may induce the local static disorder at the O(1) anion site, coordinating the Y cation sites occupied, on average, by Li, Al and Mn2+ in equal proportions, confirming previous findings. In addition to the gemmological value, the crystal-chemical studies of tourmalines are unanimously considered to be a sensitive recorder of the geological processes leading to their formation, and therefore, this study may contribute to understanding the evolution of the pegmatites related to the intrusion of the Adamello pluton. Full article
(This article belongs to the Special Issue Mineralogy and Geochemistry of Gems)
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Open AccessArticle Zircon Xenocrysts from Cenozoic Alkaline Basalts of the Ratanakiri Volcanic Province (Cambodia), Southeast Asia—Trace Element Geochemistry, O-Hf Isotopic Composition, U-Pb and (U-Th)/He Geochronology—Revelations into the Underlying Lithospheric Mantle
Minerals 2018, 8(12), 556; https://doi.org/10.3390/min8120556
Received: 26 October 2018 / Revised: 16 November 2018 / Accepted: 19 November 2018 / Published: 30 November 2018
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Abstract
Zircon xenocrysts from alkali basalts in Ratanakiri Province, Cambodia represent a unique low-Hf zircon within a 12,000 km long Indo-Pacific megacryst zone. Colorless, yellow, brown, and red crystals ({100}, {101}, subordinate {211}, {1103}), with hopper growth and corrosion features range up to 20 [...] Read more.
Zircon xenocrysts from alkali basalts in Ratanakiri Province, Cambodia represent a unique low-Hf zircon within a 12,000 km long Indo-Pacific megacryst zone. Colorless, yellow, brown, and red crystals ({100}, {101}, subordinate {211}, {1103}), with hopper growth and corrosion features range up to 20 cm in size. Zircon chemistry indicates juvenile, Zr-saturated, mantle-derived alkaline melt (Hf 0.6–0.7 wt %, Y <0.2 wt %, U + Th + REE (Rare-Earth Elements) < 600 ppm, Zr/Hf 66–92, Eu/Eu*N ~1, positive Ce/Ce*N, HREE (Heavy REE) enrichment). Incompatible element depletion with increasing Yb/SmN from core to rim at ~ constant Hf suggests single stage growth. Ti-in-zircon temperatures (~570–740 °C) are lower than predicted by crystal morphology (800–900 °C) and decrease from core to rim (ΔT = 10–50 °C). The δ18O values (4.88 to 5.01‰ VSMOW (Vienna Standard Mean Ocean Water)) are relatively low for xenocrysts from the zircon Indo-Pacific zone (ZIP). The 176Hf/177Hf values (+ εHf 4.5–10.2) give TDepleted Mantle model source ages of 260–462 Ma and TCrustal ages of 391–754 Ma. The source magmas reflect variably depleted lithospheric mantle with little supracrustal input. Zircon U-Pb (0.88–1.56 Ma) and (U-Th)/He (0.86–1.02 Ma) ages are older than host basalt ages (~0.7 Ma), which suggests limited residence before transport. Zircon genesis suggests Zr-saturated, Al-undersaturated, carbonatitic-influenced, low-degree partial melting (<1%) of peridotitic mantle at ~60 km beneath the Indochina terrane. Full article
(This article belongs to the Special Issue Mineralogy and Geochemistry of Gems)
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Open AccessArticle On the Color and Genesis of Prase (Green Quartz) and Amethyst from the Island of Serifos, Cyclades, Greece
Minerals 2018, 8(11), 487; https://doi.org/10.3390/min8110487
Received: 10 September 2018 / Revised: 8 October 2018 / Accepted: 24 October 2018 / Published: 26 October 2018
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Abstract
The color of quartz and other minerals can be either caused by defects in the crystal structure or by finely dispersed inclusions of other minerals within the crystals. In order to investigate the mineral chemistry and genesis of the famous prase (green quartz) [...] Read more.
The color of quartz and other minerals can be either caused by defects in the crystal structure or by finely dispersed inclusions of other minerals within the crystals. In order to investigate the mineral chemistry and genesis of the famous prase (green quartz) and amethyst association from Serifos Island, Greece, we used electron microprobe analyses and oxygen isotope measurements of quartz. We show that the color of these green quartz crystals is caused by small and acicular amphibole inclusions. Our data also shows that there are two generations of amphibole inclusions within the green quartz crystals, which indicate that the fluid, from which both amphiboles and quartz have crystallized, must have had a change in its chemical composition during the crystallization process. The electron microprobe data also suggests that traces of iron may be responsible for the amethyst coloration. Both quartz varieties are characterized by isotopic compositions that suggest mixing of magmatic and meteoric/marine fluids. The contribution of meteoric fluid is more significant in the final stages and reflects amethyst precipitation under more oxidizing conditions. Full article
(This article belongs to the Special Issue Mineralogy and Geochemistry of Gems)
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Open AccessArticle Gems and Placers—A Genetic Relationship Par Excellence
Minerals 2018, 8(10), 470; https://doi.org/10.3390/min8100470
Received: 30 August 2018 / Revised: 8 October 2018 / Accepted: 15 October 2018 / Published: 19 October 2018
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Abstract
Gemstones form in metamorphic, magmatic, and sedimentary rocks. In sedimentary units, these minerals were emplaced by organic and inorganic chemical processes and also found in clastic deposits as a result of weathering, erosion, transport, and deposition leading to what is called the formation [...] Read more.
Gemstones form in metamorphic, magmatic, and sedimentary rocks. In sedimentary units, these minerals were emplaced by organic and inorganic chemical processes and also found in clastic deposits as a result of weathering, erosion, transport, and deposition leading to what is called the formation of placer deposits. Of the approximately 150 gemstones, roughly 40 can be recovered from placer deposits for a profit after having passed through the “natural processing plant” encompassing the aforementioned stages in an aquatic and aeolian regime. It is mainly the group of heavy minerals that plays the major part among the placer-type gemstones (almandine, apatite, (chrome) diopside, (chrome) tourmaline, chrysoberyl, demantoid, diamond, enstatite, hessonite, hiddenite, kornerupine, kunzite, kyanite, peridote, pyrope, rhodolite, spessartine, (chrome) titanite, spinel, ruby, sapphire, padparaja, tanzanite, zoisite, topaz, tsavorite, and zircon). Silica and beryl, both light minerals by definition (minerals with a density less than 2.8–2.9 g/cm3, minerals with a density greater than this are called heavy minerals, also sometimes abbreviated to “heavies”. This technical term has no connotation as to the presence or absence of heavy metals), can also appear in some placers and won for a profit (agate, amethyst, citrine, emerald, quartz, rose quartz, smoky quartz, morganite, and aquamarine, beryl). This is also true for the fossilized tree resin, which has a density similar to the light minerals. Going downhill from the source area to the basin means in effect separating the wheat from the chaff, showcase from the jeweler quality, because only the flawless and strongest contenders among the gemstones survive it all. On the other way round, gem minerals can also be used as pathfinder minerals for primary or secondary gemstone deposits of their own together with a series of other non-gemmy material that is genetically linked to these gemstones in magmatic and metamorphic gem deposits. All placer types known to be relevant for the accumulation of non-gemmy material are also found as trap-site of gemstones (residual, eluvial, colluvial, alluvial, deltaic, aeolian, and marine shelf deposits). Running water and wind can separate minerals according to their physical-chemical features, whereas glaciers can only transport minerals and rocks but do not sort and separate placer-type minerals. Nevertheless till (unconsolidated mineral matter transported by the ice without re-deposition of fluvio-glacial processes) exploration is a technique successfully used to delineate ore bodies of, for example, diamonds. The general parameters that matter during accumulation of gemstones in placers are their intrinsic value controlled by the size and hardness and the extrinsic factors controlling the evolution of the landscape through time such as weathering, erosion, and vertical movements and fertility of the hinterland as to the minerals targeted upon. Morphoclimatic processes take particular effect in the humid tropical and mid humid mid-latitude zones (chemical weathering) and in the periglacial/glacial and the high-altitude/mountain zones, where mechanical weathering and the paleogradients are high. Some tectono-geographic elements such as unconformities, hiatuses, and sequence boundaries (often with incised valley fills and karstic landforms) are also known as planar architectural elements in sequence stratigraphy and applied to marine and correlative continental environments where they play a significant role in forward modeling of gemstone accumulation. The present study on gems and gemstone placers is a reference example of fine-tuning the “Chessboard classification scheme of mineral deposits” (Dill 2010) and a sedimentary supplement to the digital maps that form the core of the overview “Gemstones and geosciences in space and time” (Dill and Weber 2013). Full article
(This article belongs to the Special Issue Mineralogy and Geochemistry of Gems)
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Open AccessArticle Femtosecond Laser Ablation-ICP-Mass Spectrometry and CHNS Elemental Analyzer Reveal Trace Element Characteristics of Danburite from Mexico, Tanzania, and Vietnam
Minerals 2018, 8(6), 234; https://doi.org/10.3390/min8060234
Received: 7 May 2018 / Revised: 22 May 2018 / Accepted: 28 May 2018 / Published: 29 May 2018
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Abstract
Danburite is a calcium borosilicate that forms within the transition zones of metacarbonates and pegmatites as a late magmatic accessory mineral. We present here trace element contents obtained by femtosecond laser ablation-inductively coupled plasma (ICP)-mass spectrometry for danburite from Mexico, Tanzania, and Vietnam. [...] Read more.
Danburite is a calcium borosilicate that forms within the transition zones of metacarbonates and pegmatites as a late magmatic accessory mineral. We present here trace element contents obtained by femtosecond laser ablation-inductively coupled plasma (ICP)-mass spectrometry for danburite from Mexico, Tanzania, and Vietnam. The Tanzanian and Vietnamese samples show high concentrations of rare earth elements (∑REEs 1900 µg∙g−1 and 1100 µg∙g−1, respectively), whereas Mexican samples are depleted in REEs (<1.1 µg∙g−1). Other traces include Al, Sr, and Be, with Al and Sr dominating in Mexican samples (325 and 1611 µg∙g−1, respectively). Volatile elements, analyzed using a CHNS elemental analyzer, reach <3000 µg∙g−1. Sr and Al are incorporated following Ca2+ = Sr2+ and 2 B3+ + 3 O2− = Al3+ + 3 OH + □ (vacancy). REEs replace Ca2+ with a coupled substitution of B3+ by Be2+. Cerium is assumed to be present as Ce4+ in Tanzanian samples based on the observed Be/REE molar ratio of 1.5:1 following 2 Ca2+ + 3 B3+ = Ce4+ + REE3+ + 3 Be2+. In Vietnamese samples, Ce is present as Ce3+ seen in a Be/REE molar ratio of 1:1, indicating a substitution of Ca2+ + B3+ = REE3+ + Be2+. Our results imply that the trace elements of danburite reflect different involvement of metacarbonates and pegmatites among the different locations. Full article
(This article belongs to the Special Issue Mineralogy and Geochemistry of Gems)
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Review

Jump to: Research

Open AccessReview Emerald Deposits: A Review and Enhanced Classification
Minerals 2019, 9(2), 105; https://doi.org/10.3390/min9020105
Received: 12 January 2019 / Revised: 7 February 2019 / Accepted: 9 February 2019 / Published: 13 February 2019
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Abstract
Although emerald deposits are relatively rare, they can be formed in several different, but
specific geologic settings and the classification systems and models currently used to describe
emerald precipitation and predict its occurrence are too restrictive, leading to confusion as to the
exact [...] Read more.
Although emerald deposits are relatively rare, they can be formed in several different, but
specific geologic settings and the classification systems and models currently used to describe
emerald precipitation and predict its occurrence are too restrictive, leading to confusion as to the
exact mode of formation for some emerald deposits. Generally speaking, emerald is beryl with
sufficient concentrations of the chromophores, chromium and vanadium, to result in green and
sometimes bluish green or yellowish green crystals. The limiting factor in the formation of emerald
is geological conditions resulting in an environment rich in both beryllium and chromium or
vanadium. Historically, emerald deposits have been classified into three broad types. The first and
most abundant deposit type, in terms of production, is the desilicated pegmatite related type that
formed via the interaction of metasomatic fluids with beryllium-rich pegmatites, or similar granitic
bodies, that intruded into chromium- or vanadium-rich rocks, such as ultramafic and volcanic rocks,
or shales derived from those rocks. A second deposit type, accounting for most of the emerald of
gem quality, is the sedimentary type, which generally involves the interaction, along faults and
fractures, of upper level crustal brines rich in Be from evaporite interaction with shales and other
Cr- and/or V-bearing sedimentary rocks. The third, and comparatively most rare, deposit type is the
metamorphic-metasomatic deposit. In this deposit model, deeper crustal fluids circulate along faults
or shear zones and interact with metamorphosed shales, carbonates, and ultramafic rocks, and Be
and Cr (±V) may either be transported to the deposition site via the fluids or already be present in
the host metamorphic rocks intersected by the faults or shear zones. All three emerald deposit
models require some level of tectonic activity and often continued tectonic activity can result in the
metamorphism of an existing sedimentary or magmatic type deposit. In the extreme, at deeper
crustal levels, high-grade metamorphism can result in the partial melting of metamorphic rocks,
blurring the distinction between metamorphic and magmatic deposit types. In the present paper,
we propose an enhanced classification for emerald deposits based on the geological environment,
i.e., magmatic or metamorphic; host-rocks type, i.e., mafic-ultramafic rocks, sedimentary rocks, and
granitoids; degree of metamorphism; styles of minerlization, i.e., veins, pods, metasomatites, shear
zone; type of fluids and their temperature, pressure, composition. The new classification accounts
for multi-stage formation of the deposits and ages of formation, as well as probable remobilization
of previous beryllium mineralization, such as pegmatite intrusions in mafic-ultramafic rocks. Such
new considerations use the concept of genetic models based on studies employing chemical,
geochemical, radiogenic, and stable isotope, and fluid and solid inclusion fingerprints. The emerald occurrences and deposits are classified into two main types: (Type I) Tectonic magmatic-related
with sub-types hosted in: (IA) Mafic-ultramafic rocks (Brazil, Zambia, Russia, and others); (IB)
Sedimentary rocks (China, Canada, Norway, Kazakhstan, Australia); (IC) Granitic rocks (Nigeria).
(Type II) Tectonic metamorphic-related with sub-types hosted in: (IIA) Mafic-ultramafic rocks
(Brazil, Austria); (IIB) Sedimentary rocks-black shale (Colombia, Canada, USA); (IIC) Metamorphic
rocks (China, Afghanistan, USA); (IID) Metamorphosed and remobilized either type I deposits or
hidden granitic intrusion-related (Austria, Egypt, Australia, Pakistan), and some unclassified
deposits. Full article
(This article belongs to the Special Issue Mineralogy and Geochemistry of Gems)
Minerals EISSN 2075-163X Published by MDPI AG, Basel, Switzerland RSS E-Mail Table of Contents Alert
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