Special Issue "Agates: Types, Mineralogy, Deposits, Host Rocks, Ages and Genesis"

A special issue of Minerals (ISSN 2075-163X). This special issue belongs to the section "Mineral Deposits".

Deadline for manuscript submissions: closed (15 July 2021) | Viewed by 13275

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Special Issue Editor

Dr. Galina Palyanova
E-Mail Website
Guest Editor
1. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, 630090 Novosibirsk, Russia
2. Department of Geology and Geophysics, Novosibirsk State University, 630090 Novosibirsk, Russia
Interests: ore-forming processes; experiment; thermodynamic modeling; minerals-indicators; fluid−mineral−rock interactions; gold mineralization; gold deposits; mechanisms of ore formation; reconstruction of T,P,X-conditions
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Special Issue Information

Dear Colleagues,

You are welcome to contribute papers to the Special Issue "Agates: Types, Mineralogy, Deposits, Host Rocks, Ages, and Genesis".

Agates are beautiful and fascinating stones found all around the world. They are microcrystalline quartz nodules found in veins and cavities in host rocks. Agates have been observed in various kinds of host rocks. They are classically associated with volcanic rocks and can be found in sedimentary, metamorphic, and igneous environments. Agates are characterized by a wide variety of colors, shades, and patterns. There are more than 100 varieties of this stone. Agates are unique in their original patterns, but a layered texture is considered to be more common. Although agates are composed almost entirely of SiO2, it is the trace quantities of various other elements that give agates their color and lead to their characteristic banding. Carbonates, zeolites, and iron and manganese oxides are often found in agates. Ore-forming minerals, such as sulfides of iron, zinc, lead, and copper, and native metals are rarely found in them.

Despite many in-depth studies and improving technologies and investigative methods, the source of the silica required for the growth of agate remains largely unknown. The complex, multi-step process of agate formation is not yet completely understood. In these small eternal pebbles, as in the microworld, powerful natural processes that occurred hundreds of millions of years ago are also reflected.

The aim of this Special Issue is to bring together researchers from different disciplines to further our understanding of the genesis of agates.

Dr. Galina Palyanova
Guest Editor

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Keywords

  • varieties of agates
  • textures in agates
  • the growth of agates
  • mineralogy of agates
  • sulfides, oxides, native metals, and other associated minerals in agates
  • xenoliths in agates
  • deposits of agates
  • T,P,X-conditions of formation
  • age of agates
  • trace element abundance in agates
  • occurrence and formation of agates

Published Papers (9 papers)

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Editorial

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Editorial
Editorial for Special Issue “Agates: Types, Mineralogy, Deposits, Host Rocks, Ages and Genesis”
Minerals 2021, 11(10), 1035; https://doi.org/10.3390/min11101035 - 24 Sep 2021
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Abstract
Agates are famous, beautiful, and fascinating stones found all around the world [...] Full article
(This article belongs to the Special Issue Agates: Types, Mineralogy, Deposits, Host Rocks, Ages and Genesis)

Research

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Article
Black Agates from Paleoproterozoic Pillow Lavas (Onega Basin, Karelian Craton, NW Russia): Mineralogy and Proposed Origin
Minerals 2021, 11(9), 918; https://doi.org/10.3390/min11090918 - 25 Aug 2021
Cited by 1 | Viewed by 638
Abstract
The present study provides the first detailed investigation of black agates occurring in volcanic rocks of the Zaonega Formation within the Onega Basin (Karelian Craton, Fennoscandian Shield). Three characteristic texture types of black agates were identified: monocentric concentrically zoning agates, polycentric spherulitic agates, [...] Read more.
The present study provides the first detailed investigation of black agates occurring in volcanic rocks of the Zaonega Formation within the Onega Basin (Karelian Craton, Fennoscandian Shield). Three characteristic texture types of black agates were identified: monocentric concentrically zoning agates, polycentric spherulitic agates, and moss agates. The silica matrix of black agates is only composed of length-fast and zebraic chalcedony, micro- and macro-crystalline quartz, and quartzine. In addition to silica minerals, calcite, chlorite, feldspar, sulphides, and carbonaceous matter were also recognised. The black colour of agates is related to the presence of disseminated carbonaceous matter (CM) with a bulk content of less than 1 wt.%. Raman spectroscopy revealed that CM from black agates might be attributed to poorly ordered CM. The metamorphic temperature for CM from moss and spherulitic agates was determined to be close to 330 °C, whereas CM from concentrically zoning agates is characterised by a lower temperature, 264 °C. The potential source of CM in moss and spherulitic agates is associated with the hydrothermal fluids enriched in CM incorporated from underlaying carbon-bearing shungite rocks. The concentrically zoning agates contained heterogeneous CM originated both from the inter-pillow matrix and/or hydrothermal fluids. Full article
(This article belongs to the Special Issue Agates: Types, Mineralogy, Deposits, Host Rocks, Ages and Genesis)
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Article
Occurrence and Distribution of Moganite and Opal-CT in Agates from Paleocene/Eocene Tuffs, El Picado (Cuba)
Minerals 2021, 11(5), 531; https://doi.org/10.3390/min11050531 - 18 May 2021
Cited by 2 | Viewed by 1170
Abstract
Agates in Paleocene/Eocene tuffs from El Picado/Los Indios, Cuba were investigated to characterize the mineral composition of the agates and to provide data for the reconstruction of agate forming processes. The volcanic host rocks are strongly altered and fractured and contain numerous fissures [...] Read more.
Agates in Paleocene/Eocene tuffs from El Picado/Los Indios, Cuba were investigated to characterize the mineral composition of the agates and to provide data for the reconstruction of agate forming processes. The volcanic host rocks are strongly altered and fractured and contain numerous fissures and veins mineralized by quartz and chalcedony. These features indicate secondary alteration and silicification processes during tectonic activities that may have also resulted in the formation of massive agates. Local accumulation of manganese oxides/hydroxides, as well as uranium (uranyl-silicate complexes), in the agates confirm their contemporaneous supply with SiO2 and the origin of the silica-bearing solutions from the alteration processes. The mineral composition of the agates is characterized by abnormal high bulk contents of opal-CT (>6 wt%) and moganite (>16 wt%) besides alpha-quartz. The presence of these elevated amounts of “immature” silica phases emphasize that agate formation runs through several structural states of SiO2 with amorphous silica as the first solid phase. A remarkable feature of the agates is a heterogeneous distribution of moganite within the silica matrix revealed by micro-Raman mapping. The intensity ratio of the main symmetric stretching-bending vibrations (A1 modes) of alpha-quartz at 465 cm−1 and moganite at 502 cm−1, respectively, was used to depict the abundance of moganite in the silica matrix. The zoned distribution of moganite and variations in the microtexture and porosity of the agates indicate a multi-phase deposition of SiO2 under varying physico-chemical conditions and a discontinuous silica supply. Full article
(This article belongs to the Special Issue Agates: Types, Mineralogy, Deposits, Host Rocks, Ages and Genesis)
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Article
Copper-Containing Agates of the Avacha Bay (Eastern Kamchatka, Russia)
Minerals 2020, 10(12), 1124; https://doi.org/10.3390/min10121124 - 14 Dec 2020
Cited by 2 | Viewed by 966
Abstract
The copper-containing agates of the Avacha Bay (Eastern Kamchatka, Russia) have been investigated in this study. Optical microscopy, scanning electron microscopy, electron microprobe analysis, X-ray powder diffraction, Raman spectroscopy, and fluid inclusions were used to investigate the samples. It was found that copper [...] Read more.
The copper-containing agates of the Avacha Bay (Eastern Kamchatka, Russia) have been investigated in this study. Optical microscopy, scanning electron microscopy, electron microprobe analysis, X-ray powder diffraction, Raman spectroscopy, and fluid inclusions were used to investigate the samples. It was found that copper mineralization in agates is represented by native copper, copper sulphides (chalcocite, djurleite, digenite, anilite, yarrowite, rarely chalcopyrite) and cuprite. In addition to copper minerals, sphalerite and native silver were also found in the agates. Native copper is localized in a siliceous matrix in the form of inclusions usually less than 100 microns in size—rarely up to 1 mm—forming dendrites and crystals of a cubic system. Copper sulphides are found in the interstices of chalcedony often cementing the marginal parts of spherule aggregates of silica. In addition, they fill the micro veins, which occupy a cross-cutting position with respect to the concentric bands of chalcedony. The idiomorphic appearance of native copper crystals and clear boundaries with the silica matrix suggest their simultaneous crystallization. Copper sulphides, cuprite, and barite micro veins indicate a later deposition. Raman spectroscopy and X-ray powder diffraction results demonstrated that the Avacha Bay agates contained cristobalite in addition to quartz and moganite. The fluid inclusions study shows that the crystalline quartz in the center of the nodule in agates was formed with the participation of solutions containing a very low salt concentration (<0.3 wt.% NaCl equivalent) at the temperature range 110–50 °C and below. The main salt components were CaCl2 and NaCl, with a probable admixture of MgCl2. The copper mineralization in the agates of the Avacha Bay established in the volcanic strata can serve as a direct sign of their metallogenic specialization. Full article
(This article belongs to the Special Issue Agates: Types, Mineralogy, Deposits, Host Rocks, Ages and Genesis)
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Article
Mineralogy and Geochemistry of Agates from Paleoproterozoic Volcanic Rocks of the Karelian Craton, Southeast Fennoscandia (Russia)
Minerals 2020, 10(12), 1106; https://doi.org/10.3390/min10121106 - 09 Dec 2020
Cited by 2 | Viewed by 1151
Abstract
Agates of Paleoproterozoic volcanics (2100−1920 Ma) within the Onega Basin (Karelian Craton, Southeast Fennoscandia) were studied using optical and scanning electron microscopy, X-ray powder diffraction, X-ray fluorescence spectrometry XRF, laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), and C-O isotope analysis. Agate mineralization [...] Read more.
Agates of Paleoproterozoic volcanics (2100−1920 Ma) within the Onega Basin (Karelian Craton, Southeast Fennoscandia) were studied using optical and scanning electron microscopy, X-ray powder diffraction, X-ray fluorescence spectrometry XRF, laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), and C-O isotope analysis. Agate mineralization is widespread in the lavas gas vesicles, inter-pillow space of basalts, picrobasalts, basaltic andesites, as well as agglomerate tuffs. Agates are characterized by fine and coarse banding concentric zoning; moss, spotted, veinlet, and poor-fancy texture types were identified. Agate mineralization is represented by silicates, oxides, and hydroxides, carbonates, phosphates, sulfides, and sulfates. Among the silica minerals in agates only chalcedony, quartz and quartzine were found. The parameters of the quartz structure according to the X-ray diffraction data (well-develops reflections (212), (203), (301), large crystallite sizes (Cs 710–1050 Å) and crystallinity index (CI 7.8–10.3) give evidence of multi-stage silica minerals recrystallization due to a metamorphic (thermal) effect. The decreasing trend of trace element concentration in the banded agates from the outer zone to the core suggests a chemical purification process during crystallization. C-O isotope characteristic of agate-associated calcite reflects primary magmatic origin with the influence of hydrothermal activity and/or low-thermal meteoric fluids. Agates were formed under low PT-parameters and related to hydrothermal activity on the first stage of Svecofenian orogeny within 1780−1730 Ma. Thus, it can be believed that the temporal gap between continental flood basalts outflow and agate formation is about 190 Ma. Full article
(This article belongs to the Special Issue Agates: Types, Mineralogy, Deposits, Host Rocks, Ages and Genesis)
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Article
Gemological Characteristics and Origin of the Zhanguohong Agate from Beipiao, Liaoning Province, China: A Combined Microscopic, X-ray Diffraction, and Raman Spectroscopic Study
Minerals 2020, 10(5), 401; https://doi.org/10.3390/min10050401 - 30 Apr 2020
Cited by 6 | Viewed by 1268
Abstract
The Zhanguohong agate from Beipiao (Liaoning province, China), which occurs in the intermediate–felsic volcanic breccias of the Early Cretaceous Yixian Formation, generally shows massive and banded structures, with red, yellow, and/or white layers or zones. Little research has been done on its mineralogical [...] Read more.
The Zhanguohong agate from Beipiao (Liaoning province, China), which occurs in the intermediate–felsic volcanic breccias of the Early Cretaceous Yixian Formation, generally shows massive and banded structures, with red, yellow, and/or white layers or zones. Little research has been done on its mineralogical and gemological characteristics or its genesis. In this study, we present petrographic and spectroscopic constraints on the mineral composition and micro-texture of the silica matrix, as well as the ferruginous inclusions within the agates, in order to deduce the origin of the Zhanguohong agate. According to the microscopic observations, sandwich-like interlayered micro-granular quartz, fibrous chalcedony, and jigsaw quartz bands are common in the banded agates. X-ray diffraction (XRD) and Raman spectroscopic analyses revealed that all of the samples were mainly composed of α-quartz and moganite, with minor hematite and goethite. The moganite content (17–54 wt%) of the silica matrix decreases by varying degrees from the outermost to the innermost part of the banded agates. The crystal defects and ferric iron in the microcrystalline silica grains probably contributed to the moganite crystallization. The red, yellow, and orange zones are rich in hematite, goethite, and their mixtures, respectively. The ore-forming fluids fluctuated between acidic and alkaline within a temperature range of 100–200 °C and at a sustained positive Eh. Combined with the field observations, these results suggest that the multiperiod precipitation of the agates probably resulted from the episodic volcanic activity during the Early Cretaceous lithospheric extension in eastern China. Full article
(This article belongs to the Special Issue Agates: Types, Mineralogy, Deposits, Host Rocks, Ages and Genesis)
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Article
Agates from Western Atlas (Morocco)—Constraints from Mineralogical and Microtextural Characteristics
Minerals 2020, 10(2), 198; https://doi.org/10.3390/min10020198 - 22 Feb 2020
Cited by 4 | Viewed by 1481
Abstract
Agate samples collected from the vicinity of Asni and Agouim (Western Atlas, Morocco) were investigated using microscopic observations supported by Raman micro-spectroscopy. The agates are marked by the presence of various microtextures typical of epithermal vein deposits, including jigsaw-puzzle, feathery, and lattice-bladed. The [...] Read more.
Agate samples collected from the vicinity of Asni and Agouim (Western Atlas, Morocco) were investigated using microscopic observations supported by Raman micro-spectroscopy. The agates are marked by the presence of various microtextures typical of epithermal vein deposits, including jigsaw-puzzle, feathery, and lattice-bladed. The first two indicate that the formation of agates was likely marked by recrystallization of metastable silica phases (i.e., opaline silica or massive chalcedony). The presence of lattice-bladed (after barite and calcite) quartz may be, in turn, ascribed to the boiling-related conditions that could have triggered the formation of abundant copper and iron sulfides found within silica matrix. Additionally, the local occurrence of growth lines (so-called Bambauer quartz) and intergrowth of length-slow and length-fast chalcedony are linked to the variations of physico-chemical conditions during rock formation (alkaline-acidic). According to Raman spectroscopy, silica matrix of the agates is made of α-quartz with a local admixture of moganite (from 0.0 up to 78 wt.%), but also contains numerous solid inclusions of hematite, celadonite, as well as poorly-organized carbonaceous material and rutile. These phases were likely emplaced during low-temperature hydrothermal activity of SiO2-bearing fluids that originated from post-magmatic hydrothermal activity developed within host rocks and/or meteoric waters. Full article
(This article belongs to the Special Issue Agates: Types, Mineralogy, Deposits, Host Rocks, Ages and Genesis)
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Review

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Review
Mineralogy, Geochemistry and Genesis of Agate—A Review
Minerals 2020, 10(11), 1037; https://doi.org/10.3390/min10111037 - 20 Nov 2020
Cited by 10 | Viewed by 2759
Abstract
Agate—a spectacular form of SiO2 and a famous gemstone—is commonly characterized as banded chalcedony. In detail, chalcedony layers in agates can be intergrown or intercalated with macrocrystalline quartz, quartzine, opal-A, opal-CT, cristobalite and/or moganite. In addition, agates often contain considerable amounts of [...] Read more.
Agate—a spectacular form of SiO2 and a famous gemstone—is commonly characterized as banded chalcedony. In detail, chalcedony layers in agates can be intergrown or intercalated with macrocrystalline quartz, quartzine, opal-A, opal-CT, cristobalite and/or moganite. In addition, agates often contain considerable amounts of mineral inclusions and water as both interstitial molecular H2O and silanol groups. Most agate occurrences worldwide are related to SiO2-rich (rhyolites, rhyodacites) and SiO2-poor (andesites, basalts) volcanic rocks, but can also be formed as hydrothermal vein varieties or as silica accumulation during diagenesis in sedimentary rocks. It is assumed that the supply of silica for agate formation is often associated with late- or post-volcanic alteration of the volcanic host rocks. Evidence can be found in association with typical secondary minerals such as clay minerals, zeolites or iron oxides/hydroxides, frequent pseudomorphs (e.g., after carbonates or sulfates) as well as the chemical composition of the agates. For instance, elements of the volcanic rock matrix (Al, Ca, Fe, Na, K) are enriched, but extraordinary high contents of Ge (>90 ppm), B (>40 ppm) and U (>20 ppm) have also been detected. Calculations based on fluid inclusion and oxygen isotope studies point to a range between 20 and 230 °C for agate formation temperatures. The accumulation and condensation of silicic acid result in the formation of silica sols and proposed amorphous silica as precursors for the development of the typical agate micro-structure. The process of crystallisation often starts with spherulitic growth of chalcedony continuing into chalcedony fibers. High concentrations of lattice defects (oxygen and silicon vacancies, silanol groups) detected by cathodoluminescence (CL) and electron paramagnetic resonance (EPR) spectroscopy indicate a rapid crystallisation via an amorphous silica precursor under non-equilibrium conditions. It is assumed that the formation of the typical agate microstructure is governed by processes of self-organization. The resulting differences in crystallite size, porosity, kind of silica phase and incorporated color pigments finally cause the characteristic agate banding and colors. Full article
(This article belongs to the Special Issue Agates: Types, Mineralogy, Deposits, Host Rocks, Ages and Genesis)
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Review
Agate Genesis: A Continuing Enigma
Minerals 2020, 10(11), 953; https://doi.org/10.3390/min10110953 - 26 Oct 2020
Cited by 8 | Viewed by 1654
Abstract
This review covers the last 250 years of major scientific contributions on the genesis of agates found in basic igneous host rocks. From 1770 to 1955, the genesis question was frequently limited to discussions based on observations on host rock and agate thick [...] Read more.
This review covers the last 250 years of major scientific contributions on the genesis of agates found in basic igneous host rocks. From 1770 to 1955, the genesis question was frequently limited to discussions based on observations on host rock and agate thick sections. Over the next 25 years, experimental investigations examined phase transformations when silica glass and various forms of amorphous silica were heated to high temperatures. This work demonstrated that the change from the amorphous state into chalcedony was likely to be a multi-stage process. The last 40 years has seen modern scientific instrumentation play a key role in identifying the physical and chemical properties of agate. The outcome of this work has allowed limited evidence-based comment on the conditions of agate formation. There is a general consensus that agates in these basic igneous hosts form at <100 °C. However, the silica source and the nature of the initial deposit remain to be proven. Full article
(This article belongs to the Special Issue Agates: Types, Mineralogy, Deposits, Host Rocks, Ages and Genesis)
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