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Minerals
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Mineralogy, Geochemistry, and Origin of Agate: An Ongoing Challenge
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Mineralogy, Geochemistry, and Origin of Agate: An Ongoing Challenge

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

Deadline for manuscript submissions: closed (31 August 2023) | Viewed by 3888

Special Issue Editors

Dr. Magdalena Dumańska-Słowik

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Guest Editor
Department of Mineralogy, Petrography and Geochemistry, AGH University of Science and Technology, Av. Mickiewicza 30, 30-059 Krakow, Poland
Interests: gemstones; igneous petrology; alkaline rocks; ore deposits
Special Issues, Collections and Topics in MDPI journals
Prof. Dr. Jens Götze

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Guest Editor
Institute of Mineralogy, TU Bergakademie Freiberg, 09599 Freiberg, Germany
Interests: applied mineralogy; quartz and silica; cathodoluminescence
Special Issues, Collections and Topics in MDPI journals
Tomasz Powolny

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Co-Guest Editor
Department of Mineralogy, Petrography and Geochemistry, AGH University of Science and Technology, 30-059 Krakow, Poland
Interests: volcanic rocks; geochemistry; carthodoluminescence; mineralogy

Special Issue Information

Dear Colleagues,

We are pleased to announce  this Minerals Special Issue, which focuses on one of the most spectacular varieties of silica—agate. Agate has frequently been recognized among basic to acidic volcanic rocks, where it occurs as nodules or veins in association with low-temperature hydrothermal phases such as calcite or zeolites species. Agate mostly consists of alternating, multi-coloured, chalcedony-rich layers that reveal variable water and moganite contents, with an admixture of quartz and opal. The process of the recrystallization of silica phases is accompanied by changes in crystallite size and moganite content and strongly depends on the age of agate host rocks. Meanwhile, micro-textural variations within single agate bodies, along with the presence of various solid inclusions (i.e., iron oxides and carbonaceous material), may reflect changes in physicochemical parameters (i.e., pH) during the influx of hydrothermal fluids, followed by a self-purification process.

Current studies on agates aim to determine: (1) the mechanisms of the silica deposition; (2) the source and temperature of parental fluids (i.e., magmatic versus meteoric waters); (3) the spatial and genetic relationship between particular silica phases (i.e., length-slow and length-fast chalcedony); and/or (4) the origin of colouration in particular chalcedonic bands.

Experimental data involving the formation of agates as well as miscellaneous mineralogical-geochemical and related studies on these deposits around the globe would be appreciated in the following issue.

Dr. Magdalena Dumańska-Słowik
Prof. Dr. Jens Götze
Tomasz Powolny
Guest Editors

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All submissions that pass pre-check are peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

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 2400 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

  • hydrothermal solutions
  • recrystallization
  • silica
  • lithophysae
  • agates in sedimentary environment
  • volcanic alteration
  • agate microstructure
  • geochemistry of agate
  • microbial processes in agate formation

Published Papers (3 papers)

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Research

28 pages, 6836 KiB  
Article
Agate Mineralization in Paleoproterozoic Organic Carbon-Rich Sedimentary Rocks of the Onega Basin (NW Russia): Insights into Genesis
by Evgeniya N. Svetova, Sergei A. Svetov and Oleg B. Lavrov
Minerals 2024, 14(5), 447; https://doi.org/10.3390/min14050447 - 24 Apr 2024
Abstract
In this contribution, we present the results of mineralogical investigation of the agates in Paleoproterozoic organic carbon-rich sedimentary rocks within the Onega Basin (Fennoscandian shield, Russia) aimed at reconstructing the agate-forming processes. Optical and scanning electron microscopy, EDS microanalysis, thermal analysis, X-ray powder [...] Read more.
In this contribution, we present the results of mineralogical investigation of the agates in Paleoproterozoic organic carbon-rich sedimentary rocks within the Onega Basin (Fennoscandian shield, Russia) aimed at reconstructing the agate-forming processes. Optical and scanning electron microscopy, EDS microanalysis, thermal analysis, X-ray powder diffraction, Raman spectroscopy, and carbon isotope analysis were used for the study. Three main varieties of agates differing in morphology and texture were identified, including concentrically zoned nodules, fine-banded, and carbon-rich moss agates. Mineralogical evidence indicates the participation of hydrothermal fluids in agate formation. Concentrically zoned nodules could be formed due to the dissolution of carbonate concretions in the organic carbon-rich siltstones and their silicification as a result of late hydrothermal processes. Fine-banded vein agates occur in stockworks crosscutting organic carbon-rich rocks and are widely accompanied by sulfides, selenides, carbonates, sulfates, and iron oxides. Carbonaceous matter in moss agates is present as poorly ordered carbon and is characterized by a low δ13Corg value (−25.64‰), suggesting a biogenic origin. Raman spectroscopy data showed an elevated amount of moganite besides alpha quartz in the concentrically zoned nodules compared to other agate varieties, indicating different ages of the mineralization processes. We suggest that the revealed varieties of agates were formed at different stages of long-term hydrothermal processes occurring in the Onega Basin. Full article
(This article belongs to the Special Issue Mineralogy, Geochemistry, and Origin of Agate: An Ongoing Challenge)
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19 pages, 14121 KiB  
Article
Chemically Oscillating Reactions as a New Model for the Formation of Mineral Patterns in Agate Geodes and Concretions
by Dominic Papineau
Minerals 2024, 14(2), 203; https://doi.org/10.3390/min14020203 - 16 Feb 2024
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Abstract
Agate geodes contain spheroidal patterns characterized by spectacularly coloured and circularly concentric laminations with radially aligned quartz crystals, yet the origin of these geometric patterns has remained enigmatic. Here, detailed comparisons are documented between these kinds of patterns in a selection of geodes [...] Read more.
Agate geodes contain spheroidal patterns characterized by spectacularly coloured and circularly concentric laminations with radially aligned quartz crystals, yet the origin of these geometric patterns has remained enigmatic. Here, detailed comparisons are documented between these kinds of patterns in a selection of geodes and concretions and those produced by abiotic chemically oscillating reactions. We find strikingly comparable self-similar, fractal patterns in both natural volcanogenic geodes and sedimentary concretions as well as in these benchtop experiments. In addition, the mineralogical composition of patterns and associated organic matter point to the oxidation of organic compounds in both geodes and concretions. This process occurred during diagenetic or supergene alteration, and it is consistent with spontaneous and abiotic chemically oscillating reactions. It is concluded that the oxidation of organic acids was involved in the formation of these patterns and that these rocks indicate oxidation–reduction reactions involving organic carbon, which itself may be abiotic or biological in origin. Hence, agate geodes and concretions represent the abiotic biosignatures of possible biological origin in volcanic and sedimentary rocks. Full article
(This article belongs to the Special Issue Mineralogy, Geochemistry, and Origin of Agate: An Ongoing Challenge)
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31 pages, 33833 KiB  
Article
Mineralogy, Geochemistry, and Genesis of Agates from Chihuahua, Northern Mexico
by Maximilian Mrozik, Jens Götze, Yuanming Pan and Robert Möckel
Minerals 2023, 13(5), 687; https://doi.org/10.3390/min13050687 - 18 May 2023
Viewed by 2250
Abstract
The present study aimed to investigate the genesis and characteristics of some of the world-famous agate deposits in the state of Chihuahua, Mexico (Rancho Coyamito, Ojo Laguna, Moctezuma, Huevos del Diablo, Agua Nueva). Geochemical and textural studies of host rocks showed that all [...] Read more.
The present study aimed to investigate the genesis and characteristics of some of the world-famous agate deposits in the state of Chihuahua, Mexico (Rancho Coyamito, Ojo Laguna, Moctezuma, Huevos del Diablo, Agua Nueva). Geochemical and textural studies of host rocks showed that all the studied deposits are related to the same rock type within the geological unit of Rancho el Agate andesite, a quartz-free latite that shows clear indications of magma mixing. As a result of their large-scale distribution and various differentiation processes, as well as transport separation, different textures and local chemical differences between rocks of different localities can be observed. These differences have also influenced the properties of SiO2 mineralization in the rocks. The mixing of near-surface fluids from rock alterations with magmatic hydrothermal solutions led to the accumulation of various elements in the SiO2 matrix of the agates, which were, on the one hand, mobilized during secondary rock alteration (Fe, U, Ca, K, Al, Si) and, on the other hand, transported with magmatic fluids (Zn, Sb, Si, Zr, Cr). Different generations of chalcedony indicate a multi-stage formation as well as multiple cycles of filling the cavities with fluids. The hydrothermal fluids are presumably related to the residual solutions of a rhyolitic volcanism, which followed the latitic extrusions in the area and probably caused the formation of polymetallic ore deposits in the Chihuahua area. The enrichment of highly immobile elements indicates the involvement of volatile fluids in the agate formation. The vivid colors of the agates are almost exclusively due to various mineral inclusions, which consist mainly of iron compounds. Full article
(This article belongs to the Special Issue Mineralogy, Geochemistry, and Origin of Agate: An Ongoing Challenge)
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