Special Issue "Mineralogy of Quartz and Silica Minerals"

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

Deadline for manuscript submissions: closed (28 February 2018)

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Special Issue Editor

Guest Editor
Prof. Dr. Jens Götze

Institut für Mineralogie, TU Bergakademie Freiberg, 09599 Freiberg, Germany
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Interests: applied mineralogy; non-metallic raw materials; quartz and silica minerals; agate, cathodoluminescence; crystal growth

Special Issue Information

Dear Colleagues,

The various modifications of silica, especially quartz, play a central role in the composition of geological materials. In addition, quartz is widely used as raw material in numerous industrial fields. Therefore, the knowledge about specific properties of SiO2 rocks and minerals is indispensable for the understanding and reconstruction of geological processes, as well as for specific technical applications. This Special Issue aims to bring together studies dealing with the formation, mineralogy and geochemistry of quartz and other silica minerals. These topics include the formation of quartz deposits and problems of processing, aspects of the analysis of high-purity quartz, as well as specifics of SiO2 modifications and varieties (e.g., opal, chalcedony, agate, quartz).

Prof. Dr. Jens Götze
Guest Editor

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Keywords

  • quartz
  • opal
  • agate
  • SiO2 deposits and raw materials
  • processing of quartz raw materials
  • analytics of high-purity quartz
  • quartz mineralogy
  • structure and properties of quartz
  • geochemistry/trace elements
  • spectroscopic methods

Published Papers (14 papers)

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Editorial

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Open AccessEditorial Editorial for Special Issue “Mineralogy of Quartz and Silica Minerals”
Minerals 2018, 8(10), 467; https://doi.org/10.3390/min8100467
Received: 12 October 2018 / Accepted: 17 October 2018 / Published: 19 October 2018
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Abstract
Quartz and other silica minerals make up 12. [...] Full article
(This article belongs to the Special Issue Mineralogy of Quartz and Silica Minerals) Printed Edition available

Research

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Open AccessArticle Deciphering Silicification Pathways of Fossil Forests: Case Studies from the Late Paleozoic of Central Europe
Minerals 2018, 8(10), 432; https://doi.org/10.3390/min8100432
Received: 5 July 2018 / Revised: 24 September 2018 / Accepted: 26 September 2018 / Published: 1 October 2018
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Abstract
The occurrence and formation of silicified wood from five late Paleozoic basins in Central Europe was investigated. Fossil wood from diverse geological settings was studied using field observations, taphonomic determinations as well as mineralogical analyses (polarizing microscopy, cathodoluminescence (CL) microscopy and spectroscopy). The [...] Read more.
The occurrence and formation of silicified wood from five late Paleozoic basins in Central Europe was investigated. Fossil wood from diverse geological settings was studied using field observations, taphonomic determinations as well as mineralogical analyses (polarizing microscopy, cathodoluminescence (CL) microscopy and spectroscopy). The results indicate that silicification is either a monophase or multiphase process under varying physico-chemical conditions. In particular, CL studies revealed complex processes of silica accumulation and crystallization. The CL characteristics of quartz phases in silicified wood can mostly be related to blue (390 and 440 nm), yellow (580 nm), and red (650 nm) emission bands, which may appear in different combinations and varying intensity ratios. Yellow CL is typical for initial silicification, reflecting quick precipitation under oxygen-deficient conditions caused by initial decay of the organic material. Blue CL is predominantly of secondary origin, resulting from replacement of precursor phases by a secondary hydrothermal quartz generation or subsequent silicification of wood. The red CL can be related to a lattice defect (non-bridging oxygen hole center—NBOHC). Full article
(This article belongs to the Special Issue Mineralogy of Quartz and Silica Minerals) Printed Edition available
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Open AccessArticle Multi-Stage Evolution of Gold-Bearing Hydrothermal Quartz Veins at the Mokrsko Gold Deposit (Czech Republic) Based on Cathodoluminescence, Spectroscopic, and Trace Elements Analyses
Minerals 2018, 8(8), 335; https://doi.org/10.3390/min8080335
Received: 13 June 2018 / Revised: 1 August 2018 / Accepted: 2 August 2018 / Published: 4 August 2018
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Abstract
We performed a detailed analysis of hydrothermal quartz at the Mokrsko gold deposit (Čelina, Mokrsko-East, and Mokrsko-West deposits). Twenty-one samples were studied by scanning electron microscopy cathodoluminescence (CL) imagining, CL emission spectra and trace elements were measured on six selected samples. Four quartz [...] Read more.
We performed a detailed analysis of hydrothermal quartz at the Mokrsko gold deposit (Čelina, Mokrsko-East, and Mokrsko-West deposits). Twenty-one samples were studied by scanning electron microscopy cathodoluminescence (CL) imagining, CL emission spectra and trace elements were measured on six selected samples. Four quartz growth generations Q1 to Q4 were described. Homogeneous early blue CL Q1 with initial emission spectra at 380 and 500 nm was observed at the Čelina deposit with typical titanium concentrations in the range of 20–50 ppm. Hydrothermal quartz at Mokrsko-West, which also includes early Q1, late subhedral faces of yellow CL Q2, and microfissures of greenish CL Q3 (both 570 nm), is characterized by titanium depletion. The titanium concentration is comparable to previous studies of crystallization temperatures proving titanium concentration in quartz as a good geothermal indicator. Q4, developed in microfissures only at Čelina, has no visual CL effect. Mokrsko-West is specific in comparison to Mokrsko-East and Čelina by germanium enrichments in hydrothermal quartz (up to 17 ppm) and the presence of fluorite. Tectonic (sheeted veinlets system, regional tectonic setting) and geochemical (germanium in quartz, the presence of fluorite) characteristics of the quartz veins link the late mineralization stages at the Mokrsko-West deposit to the temporally related Blatná intrusive suite. Full article
(This article belongs to the Special Issue Mineralogy of Quartz and Silica Minerals) Printed Edition available
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Open AccessArticle Textural Characteristics of Noncrystalline Silica in Sinters and Quartz Veins: Implications for the Formation of Bonanza Veins in Low-Sulfidation Epithermal Deposits
Minerals 2018, 8(8), 331; https://doi.org/10.3390/min8080331
Received: 25 June 2018 / Revised: 27 July 2018 / Accepted: 31 July 2018 / Published: 2 August 2018
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Abstract
Silica sinters forming at the Wairakei geothermal power plant in New Zealand are composed of noncrystalline opal-A that deposited rapidly from cooling geothermal liquids flashed to atmosphere. The sinter is laminated with alternating layers of variably compacted silicified filamentous microbes encased by chains [...] Read more.
Silica sinters forming at the Wairakei geothermal power plant in New Zealand are composed of noncrystalline opal-A that deposited rapidly from cooling geothermal liquids flashed to atmosphere. The sinter is laminated with alternating layers of variably compacted silicified filamentous microbes encased by chains of fused silica microspheres. Microscopic inspection of bonanza quartz vein samples from the Buckskin National low-sulfidation epithermal precious metal deposit in Nevada showed that colloform bands in these veins exhibit relic microsphere textures similar to those observed in the silica sinters from the Wairakei power plant. The textural similarity suggests that the colloform bands were originally composed of noncrystalline opal-A that subsequently recrystallized to quartz. The colloform bands contain dendrites of electrum and naumannite that must have grown in a yielding matrix of silica microspheres deposited at the same time as the ore minerals, implying that the noncrystalline silica exhibited a gel-like behavior. Quartz bands having other textural characteristics in the crustiform veins lack ore minerals. This suggests that ore deposition and the formation of the colloform bands originally composed of compacted microspheres of noncrystalline silica are genetically linked and that ore deposition within the bonanza veins was only episodic. Supersaturation of silica and precious metals leading to the formation of the colloform bands may have occurred in response to transient flashing of the hydrothermal liquids. Flashing of geothermal liquids may thus represent a key mechanism in the formation of bonanza precious metal grades in low-sulfidation epithermal deposits. Full article
(This article belongs to the Special Issue Mineralogy of Quartz and Silica Minerals) Printed Edition available
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Open AccessArticle Amethyst Occurrences in Tertiary Volcanic Rocks of Greece: Mineralogical, Fluid Inclusion and Oxygen Isotope Constraints on Their Genesis
Minerals 2018, 8(8), 324; https://doi.org/10.3390/min8080324
Received: 19 June 2018 / Revised: 20 July 2018 / Accepted: 25 July 2018 / Published: 28 July 2018
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Abstract
Epithermally altered volcanic rocks in Greece host amethyst-bearing veins in association with various silicates, carbonates, oxides and sulfides. Host rocks are Oligocene to Pleistocene calc-alkaline to shoshonitic lavas and pyroclastics of intermediate to acidic composition. The veins are integral parts of high to [...] Read more.
Epithermally altered volcanic rocks in Greece host amethyst-bearing veins in association with various silicates, carbonates, oxides and sulfides. Host rocks are Oligocene to Pleistocene calc-alkaline to shoshonitic lavas and pyroclastics of intermediate to acidic composition. The veins are integral parts of high to intermediate sulfidation epithermal mineralized centers in northern Greece (e.g., Kassiteres–Sapes, Kirki, Kornofolia/Soufli, Lesvos Island) and on Milos Island. Colloform–crustiform banding with alternations of amethyst, chalcedony and/or carbonates is a common characteristic of the studied amethyst-bearing veins. Hydrothermal alteration around the quartz veins includes sericitic, K-feldspar (adularia), propylitic and zeolitic types. Precipitation of amethyst took place from near-neutral to alkaline fluids, as indicated by the presence of various amounts of gangue adularia, calcite, zeolites, chlorite and smectite. Fluid inclusion data suggest that the studied amethyst was formed by hydrothermal fluids with relatively low temperatures (~200–250 °C) and low to moderate salinity (1–8 wt % NaCl equiv). A fluid cooling gradually from the external to the inner parts of the veins, possibly with subsequent boiling in an open system, is considered for the amethysts of Silver Hill in Sapes and Kassiteres. Amethysts from Kornofolia, Megala Therma, Kalogries and Chondro Vouno were formed by mixing of moderately saline hydrothermal fluids with low-salinity fluids at relatively lower temperatures indicating the presence of dilution processes and probably boiling in an open system. Stable isotope data point to mixing between magmatic and marine (and/or meteoric) waters and are consistent with the oxidizing conditions required for amethyst formation. Full article
(This article belongs to the Special Issue Mineralogy of Quartz and Silica Minerals) Printed Edition available
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Open AccessArticle Cathodoluminescence (CL) Characteristics of Quartz from Different Metamorphic Rocks within the Kaoko Belt (Namibia)
Minerals 2018, 8(5), 190; https://doi.org/10.3390/min8050190
Received: 11 November 2017 / Revised: 24 March 2018 / Accepted: 28 March 2018 / Published: 1 May 2018
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Abstract
Quartz of metamorphic rocks from the Kaoko belt (Namibia) representing metamorphic zones from greenshist to granulite facies were investigated by cathodoluminescence (CL) microscopy and spectroscopy to characterize their CL properties. The samples cover P-T conditions from the garnet zone (500 ± 30 °C, [...] Read more.
Quartz of metamorphic rocks from the Kaoko belt (Namibia) representing metamorphic zones from greenshist to granulite facies were investigated by cathodoluminescence (CL) microscopy and spectroscopy to characterize their CL properties. The samples cover P-T conditions from the garnet zone (500 ± 30 °C, 9 ± 1 kbar) up to the garnet-cordierite-sillimanite-K-feldspar zone (750 ± 30 °C, 4.0–5.5 kbar). Quartz from 10 different localities and metamorphic environments exclusively exhibits blue CL. The observed CL colors and spectra seem to be more or less independent of the metamorphic grade of the host rocks, but are determined by the regional geological conditions. Quartz from different localities of the garnet-cordierite-sillimanite-K-feldspar zone shows a dominant 450 nm emission band similar to quartz from igneous rocks, which might be related to recrystallization processes. In contrast, quartz from different metamorphic zones in the western part of the central Kaoko zone (garnet, staurolite, kyanite, and kyanite-sillimanite-muscovite zone) is characterized by a heterogeneous blue-green CL and a dominant 500 nm emission band that strongly decreases in intensity under electron irradiation. Such CL characteristics are typical for quartz of pegmatitic and/or hydrothermal origin and indicate the participation of fluids during neoformation of quartz during metamorphism. Full article
(This article belongs to the Special Issue Mineralogy of Quartz and Silica Minerals) Printed Edition available
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Open AccessArticle The Hydrothermal Breccia of Berglia-Glassberget, Trøndelag, Norway: Snapshot of a Triassic Earthquake
Minerals 2018, 8(5), 175; https://doi.org/10.3390/min8050175
Received: 26 February 2018 / Revised: 16 April 2018 / Accepted: 17 April 2018 / Published: 23 April 2018
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Abstract
The quartz-K-feldspar-cemented breccia of Berglia-Glassberget in the Lierne municipality in central Norway forms an ellipsoid structure 250 m × 500 m in size. The hydrothermal breccia is barren in terms of economic commodities but famous among mineral collectors for being a large and [...] Read more.
The quartz-K-feldspar-cemented breccia of Berglia-Glassberget in the Lierne municipality in central Norway forms an ellipsoid structure 250 m × 500 m in size. The hydrothermal breccia is barren in terms of economic commodities but famous among mineral collectors for being a large and rich site of crystal quartz of various colours and habits. Despite being a famous collector site, the mineralization is rather unique in respect to its geological setting. It occurs within Late Palaeoproterozoic metarhyolites of the Lower Allochthon of the Norwegian Caledonides regionally isolated from any other contemporaneous hydrothermal or magmatic event. In order to understand better the formation of the Berglia-Glassberget breccia, the chemistry, fluid inclusion petrography and age of the breccia cement were determined. Structural features indicate that the Berglia-Glassberget is a fault-related, fluid-assisted, hydraulic breccia which formed by single pulse stress released by a seismic event. 40Ar-39Ar dating of K-feldspar cement revealed a middle Triassic age (240.3 ± 0.4 Ma) for this event. The influx into the fault zone of an aqueous CO2-bearing fluid triggered the sudden fault movement. The high percentage of open space in the breccia fractures with cavities up 3 m × 3 m × 4 m in size, fluid inclusion microthermometry, and trace element chemistry of quartz suggests that the breccia was formed at depths between 4 and 0.5 km (1.1 to 0.1 kbar). The origin of the breccia-cementing, CO2-bearing Na-HCO3-SO4 fluid may have been predominantly of metamorphic origin due to decarbonation reactions (T > 200 °C) of limestones of the underlying Olden Nappe. The decarbonation reactions were initiated by deeply derived, hot fluids channelled to sub-surface levels by a major fault zone, implying that the breccia is situated on a deep-seated structure. Regionally, the Berglia-Glassberget occurs at a supposed triple junction of long-lived fault zones belonging to the Møre-Trøndelag, Lærdal-Gjende and the Kollstraumen fault complexes. These fault systems and the associated Berglia-Glassberget earthquake are the expression of rifting and faulting in northern Europe during the middle/late Triassic. Full article
(This article belongs to the Special Issue Mineralogy of Quartz and Silica Minerals) Printed Edition available
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Open AccessArticle The Use of Infrared Spectroscopy to Determine the Quality of Carbonate-Rich Diatomite Ores
Minerals 2018, 8(3), 120; https://doi.org/10.3390/min8030120
Received: 28 February 2018 / Revised: 15 March 2018 / Accepted: 17 March 2018 / Published: 20 March 2018
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Abstract
Diatomite, a rock formed by the accumulation of opaline diatom frustules, is a preferred raw material for the manufacturing of filters. Its uniqueness relies on the high porosity and inertness of the frustules. The presence of carbonates in some diatomite ores hinders these [...] Read more.
Diatomite, a rock formed by the accumulation of opaline diatom frustules, is a preferred raw material for the manufacturing of filters. Its uniqueness relies on the high porosity and inertness of the frustules. The presence of carbonates in some diatomite ores hinders these properties. The purpose of this study was to identify the type of carbonates and their association with the ore in a diatomite deposit, and to assess the suitability of determining the quality of the ore using techniques with potential for in-pit implementation. For this, run-of-mine samples were analysed using environmental scanning electron microscopy (ESEM) and infrared spectroscopy. The ESEM images showed that carbonate is present as cement and laminae. The infrared data revealed that the carbonate minerals correspond to aragonite and calcite, and that their occurrence is linked to the total amount of carbonate in the sample. By using a portable spectral instrument that uses diffuse reflectance, it was possible to classify the spectra of the ore samples based on the carbonate content. These results indicate that infrared technology could be used on-site for determining the quality of the ore, thus providing relevant information to assist the optimisation of mining and beneficiation activities. Full article
(This article belongs to the Special Issue Mineralogy of Quartz and Silica Minerals) Printed Edition available
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Open AccessArticle Investigation of Fluids in Macrocrystalline and Microcrystalline Quartz in Agate Using Thermogravimetry-Mass-Spectrometry
Minerals 2018, 8(2), 72; https://doi.org/10.3390/min8020072
Received: 3 November 2017 / Revised: 12 February 2018 / Accepted: 14 February 2018 / Published: 17 February 2018
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Abstract
Gaseous and liquid fluids in agates (banded chalcedony—SiO2) of different localities were investigated systematically by thermogravimetry-mass-spectrometry within a temperature range from 25 to 1450 °C, for the first time. Chalcedony and macrocrystalline quartz from twelve agate samples were investigated, from Germany [...] Read more.
Gaseous and liquid fluids in agates (banded chalcedony—SiO2) of different localities were investigated systematically by thermogravimetry-mass-spectrometry within a temperature range from 25 to 1450 °C, for the first time. Chalcedony and macrocrystalline quartz from twelve agate samples were investigated, from Germany (Schlottwitz, St. Egidien, Chemnitz and Zwickau), Brazil (Rio Grande do Sul), Scotland (Ayrshire) and the USA (Montana). They originate from mafic and felsic volcanic rocks as well as hydrothermal and sedimentary environments. The results were evaluated regarding compounds of hydrogen with fluorine, chlorine, nitrogen, carbon and sulphur. Additionally, oxygen compounds were recognized with hydrogen, fluorine, nitrogen, sulphur and carbon. The nature of the compounds was identified based on their mass-charge-ratio and the intensity ratios of the associated fragments. Due to interferences of different compounds with the same mass-charge-ratio, only H2O, HF, NO, S, SO, CO3—as well as several hydrocarbon compounds (for example CO32− or CO)—could be properly identified. The main degassing temperatures were detected at around 500 and 1000 °C. Generally, a difference between quartz and chalcedony regarding the composition of their fluids could not be found. The results indicate a silica source for the agate formation from aqueous solutions but also a possible role of fluorine compounds. Additionally, CO2 and other fluids were involved in the alteration of volcanic rocks and the mobilization and transport of SiO2. Full article
(This article belongs to the Special Issue Mineralogy of Quartz and Silica Minerals) Printed Edition available
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Open AccessArticle Dissolution Behaviors of Trace Muscovite during Pressure Leaching of Hydrothermal Vein Quartz Using H2SO4 and NH4Cl as Leaching Agents
Minerals 2018, 8(2), 60; https://doi.org/10.3390/min8020060
Received: 26 December 2017 / Revised: 2 February 2018 / Accepted: 7 February 2018 / Published: 11 February 2018
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Abstract
Dissolution behaviors of trace muscovite during pressure leaching of hydrothermal vein quartz using H2SO4 and NH4Cl as leaching agents have been studied by means of optical and electronic microscopes. Phase transformations of pure muscovite during calcination and the [...] Read more.
Dissolution behaviors of trace muscovite during pressure leaching of hydrothermal vein quartz using H2SO4 and NH4Cl as leaching agents have been studied by means of optical and electronic microscopes. Phase transformations of pure muscovite during calcination and the pressure leaching were analyzed by powder X-ray diffraction (XRD) and thermal analysis (TG-DSC), which are used for indirectly discussing dissolution mechanisms of the trace muscovite. Structure damages of trace muscovite are caused by calcination, and further developed during pressure leaching of the quartz sand using H2SO4 and NH4Cl as leaching agents. The trace muscovite is dissolved, and then efficiently separated from quartz sand by coupling effects of calcination and fluorine-free pressure leaching. Full article
(This article belongs to the Special Issue Mineralogy of Quartz and Silica Minerals) Printed Edition available
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Open AccessArticle Silica Colloid Ordering in a Dynamic Sedimentary Environment
Minerals 2018, 8(1), 12; https://doi.org/10.3390/min8010012
Received: 1 December 2017 / Revised: 4 January 2018 / Accepted: 4 January 2018 / Published: 7 January 2018
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Abstract
The formation of ordered particle arrays plays an essential role in nanotechnology, biological systems, and inorganic photonic structures in the geosphere. Here, we show how ordered arrays of amorphous silica spheres form in deeply weathered lithologies of the Great Artesian Basin (central Australia). [...] Read more.
The formation of ordered particle arrays plays an essential role in nanotechnology, biological systems, and inorganic photonic structures in the geosphere. Here, we show how ordered arrays of amorphous silica spheres form in deeply weathered lithologies of the Great Artesian Basin (central Australia). Our multi-method approach, using optical and scanning electron microscopy, X-ray microdiffraction, Raman spectroscopy, and electron probe microanalysis, reveals that particle morphologies trace the flow of opal-forming colloidal suspensions and document syn- and post-depositional deformation. The micromorphology of amorphous silica pseudomorphs suggests that the volume-preserving replacement of non-silicate minerals proceeds via an interface-coupled dissolution precipitation process. We conclude that colloid flow and post-depositional shearing create but also destroy natural photonic crystals. Contrary to previous studies, our results indicate that purely gravitational settling/ordering is the exception rather than the rule during the formation of three-dimensional periodic sphere arrays in the highly dynamic colloidal suspensions of chemically weathered clastic sediments. Full article
(This article belongs to the Special Issue Mineralogy of Quartz and Silica Minerals) Printed Edition available
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Open AccessArticle Trace Element Compositions and Defect Structures of High-Purity Quartz from the Southern Ural Region, Russia
Minerals 2017, 7(10), 189; https://doi.org/10.3390/min7100189
Received: 14 September 2017 / Revised: 5 October 2017 / Accepted: 5 October 2017 / Published: 11 October 2017
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Abstract
Quartz samples of different origin from 10 localities in the Southern Ural region, Russia have been investigated to characterize their trace element compositions and defect structures. The analytical combination of cathodoluminescence (CL) microscopy and spectroscopy, electron paramagnetic resonance (EPR) spectroscopy, and trace-element analysis [...] Read more.
Quartz samples of different origin from 10 localities in the Southern Ural region, Russia have been investigated to characterize their trace element compositions and defect structures. The analytical combination of cathodoluminescence (CL) microscopy and spectroscopy, electron paramagnetic resonance (EPR) spectroscopy, and trace-element analysis by inductively coupled plasma mass spectrometry (ICP-MS) revealed that almost all investigated quartz samples showed very low concentrations of trace elements (cumulative concentrations of <50 ppm with <30 ppm Al and <10 ppm Ti) and low abundances of paramagnetic defects, defining them economically as “high-purity” quartz (HPQ) suitable for high-tech applications. EPR and CL data confirmed the low abundances of substitutional Ti and Fe, and showed Al to be the only significant trace element structurally bound in the investigated quartz samples. CL microscopy revealed a heterogeneous distribution of luminescence centres (i.e., luminescence active trace elements such as Al) as well as features of deformation and recrystallization. It is suggested that healing of defects due to deformation-related recrystallization and reorganization processes of the quartz lattice during retrograde metamorphism resulted in low concentrations of CL activator and other trace elements or vacancies, and thus are the main driving processes for the formation of HPQ deposits in the investigated area. Full article
(This article belongs to the Special Issue Mineralogy of Quartz and Silica Minerals) Printed Edition available
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Open AccessArticle Mineralogy and Processing of Hydrothermal Vein Quartz from Hengche, Hubei Province (China)
Minerals 2017, 7(9), 161; https://doi.org/10.3390/min7090161
Received: 19 July 2017 / Revised: 31 August 2017 / Accepted: 31 August 2017 / Published: 2 September 2017
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Abstract
Quartz occurs in many geological materials, and is used in numerous industrial fields as a raw material. Mineralogy and the processing of hydrothermal quartz were studied by optical microscope, electron probe microanalysis, scanning electron microscope, inductively coupled plasma-optical emission spectrometry, and inductively coupled [...] Read more.
Quartz occurs in many geological materials, and is used in numerous industrial fields as a raw material. Mineralogy and the processing of hydrothermal quartz were studied by optical microscope, electron probe microanalysis, scanning electron microscope, inductively coupled plasma-optical emission spectrometry, and inductively coupled plasma mass spectrometer. A combination of the geological occurrence of the quartz deposit, mineralogical studies, and the processing technologies of the hydrothermal quartz was accomplished. The results show that impurities within the quartz mainly include muscovite, hematite, apatite, and secondary fluid inclusions. The main chemical impurities are Al (353 μg·g−1), K (118 μg·g−1), Fe (61.2 μg·g−1), P (15.5 μg·g−1), Na (13.4 μg·g−1), Mg (11.8 μg·g−1), Ti (8.31 μg·g−1), and B (10.8 μg·g−1). Based on these results, a combined process consisting of calcination and fluoride-free pressure acid leaching was established to effectively decompose and dissolve the quartz, and remove gangue minerals and fluid inclusions. The calcination process not only removed volatile components; it also destroyed the crystal structure of gangue minerals and enhanced their release probabilities. The calcination process has a positive influence on the removal of impurity elements by the fluoride-free pressure acid leaching process. A total of 85.2 wt % and 84.0 wt % of impurity elements was removed using the leaching systems of HCl-NH4Cl and H2SO4-NH4Cl, respectively. Full article
(This article belongs to the Special Issue Mineralogy of Quartz and Silica Minerals) Printed Edition available
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Review

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Open AccessReview Lunar and Martian Silica
Minerals 2018, 8(7), 267; https://doi.org/10.3390/min8070267
Received: 1 May 2018 / Revised: 13 June 2018 / Accepted: 14 June 2018 / Published: 25 June 2018
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Abstract
Silica polymorphs, such as quartz, tridymite, cristobalite, coesite, stishovite, seifertite, baddeleyite-type SiO2, high-pressure silica glass, moganite, and opal, have been found in lunar and/or martian rocks by macro-microanalyses of the samples and remote-sensing observations on the celestial bodies. Because each silica [...] Read more.
Silica polymorphs, such as quartz, tridymite, cristobalite, coesite, stishovite, seifertite, baddeleyite-type SiO2, high-pressure silica glass, moganite, and opal, have been found in lunar and/or martian rocks by macro-microanalyses of the samples and remote-sensing observations on the celestial bodies. Because each silica polymorph is stable or metastable at different pressure and temperature conditions, its appearance is variable depending on the occurrence of the lunar and martian rocks. In other words, types of silica polymorphs provide valuable information on the igneous process (e.g., crystallization temperature and cooling rate), shock metamorphism (e.g., shock pressure and temperature), and hydrothermal fluid activity (e.g., pH and water content), implying their importance in planetary science. Therefore, this article focused on reviewing and summarizing the representative and important investigations of lunar and martian silica from the viewpoints of its discovery from lunar and martian materials, the formation processes, the implications for planetary science, and the future prospects in the field of “micro-mineralogy”. Full article
(This article belongs to the Special Issue Mineralogy of Quartz and Silica Minerals) Printed Edition available
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