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Advances in the Petrography of Natural Fine-Grained Materials: A Microscale...
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Advances in the Petrography of Natural Fine-Grained Materials: A Microscale Answer to Their Genetic Pathways and Applications

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

Deadline for manuscript submissions: 30 September 2025 | Viewed by 1550

Special Issue Editors

Prof. Dr. Manuel Pozo Rodríguez

E-Mail Website
Guest Editor
Department of Geology and Geochemistry, Universidad Autónoma of Madrid, 28049 Madrid, Spain
Interests: geology and geochemistry of clays; special clays applications; sepiolite–palygorskite; bentonite; talc–kerolite; clays and health; mineral characterization
Special Issues, Collections and Topics in MDPI journals
Prof. Dr. Emilia García Romero

E-Mail Website
Guest Editor
Department of Mineralogy and Petrology, Universidad Complutense of Madrid, 28040 Madrid, Spain
Interests: mineralogy and crystallochemistry of clay minerals, bentonite; sepiolite-palygorskite; special clays; SEM-TEM of clays

Special Issue Information

Dear Colleagues,

Fine-grained materials are difficult to study petrographically due to the small size of their constituents, especially those consisting of clay-sized particles (<2–4 µm). Many papers include petrographic descriptions of fine-grained materials, especially in the silt and clay size range (<63 µm), as a complement to other procedures and techniques—either a facies analysis or compositional characterization. Commonly, these fine-grained materials correspond to sediments and rocks formed by inherited minerals and, more rarely, those formed by authigenic phases.

This Special Issue is focused on the petrography of fine-grained (mostly clay-sized) materials composed of minerals resulting from authigenesis, either from the transformation of previous crystalline or amorphous phases (precursors) or from direct precipitation (neoformation). The correct interpretation of their textural features is of great help in the genetic interpretation of their constituent minerals and thus of their formation environment. These genetic environments include both continental and marine settings in which syngenetic, diagenetic and edaphic processes are responsible for the textural variations presented by the materials. Fine-grained materials related to hydrothermal processes are also included in this.

In addition, many fine-grained materials of authigenic origin, especially claystones, exhibit physical and physico-chemical properties that lend them to important industrial and technological applications. These properties may vary in materials of the same mineral composition, even within the same deposit, with one of the causes being texture. For this reason, in addition to the determination of mineralogical and chemical composition, petrographic characterization and its correct interpretation is fundamental in order for these materials to provide an adequate response during specific applications. The use of polarized light microscopy supplemented with both scanning and transmission electron microscopy, and associated analytical procedures and techniques, are of particular relevance.

Prof. Dr. Manuel Pozo Rodríguez
Prof. Dr. Emilia García Romero
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

  • clay and clay minerals
  • petrography
  • electron microscopy
  • texture and microfabric
  • properties
  • genesis
  • paleoenvironment
  • applications

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Published Papers (2 papers)

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Research

20 pages, 9535 KiB  
Article
Hydrothermal Retrogradation from Chlorite to Tosudite: Effect on the Optical Properties
by Zahra Ahmadi, Fernando Nieto, Farhad Khormali, Nicolás Velilla, Morteza Einali, Abbas Maghsoudi and Arash Amini
Minerals 2025, 15(3), 326; https://doi.org/10.3390/min15030326 - 20 Mar 2025
Viewed by 341
Abstract
In the argillic alteration zone of the SinAbad area of the Urumieh–Dokhtar magmatic belt (Iran), Mg-rich, Fe-poor chlorites, which crystallised at temperatures between 160 °C and 260 °C, were affected by extensive alteration to smectite mixed-layering at the micro- and nano-scales during the [...] Read more.
In the argillic alteration zone of the SinAbad area of the Urumieh–Dokhtar magmatic belt (Iran), Mg-rich, Fe-poor chlorites, which crystallised at temperatures between 160 °C and 260 °C, were affected by extensive alteration to smectite mixed-layering at the micro- and nano-scales during the retrograde evolution of the hydrothermal system. Chlorites retain their usual optical aspect and properties, except for the index of refraction perpendicular to the (001) layers, which becomes lower than those parallel to the layers, producing an increase in birefringence and change in the optic and elongation signs, in comparison to the ordinary ones for Mg chlorites. Scanning electron microscopy (SEM) maps and compositions, and electron microprobe (EMP) analyses indicate minor but ubiquitous Ca (and K) content. X-ray diffraction (XRD) of chloritic concentrates allowed the identification of chlorite and tosudite. High-resolution transmission electron microscopy (HRTEM) images show major 14 Å (chlorite), with the frequent presence of 24 Å (contracted tosudite) individual layers and small packets up to five layers thick. Lateral change from 14 Å to 24 Å individual layers has been visualised. High-resolution chemical maps obtained in high-angle annular dark-field (HAADF) mode confirm the existence of areas preferentially dominated by chlorite or tosudite. The overall chemical compositions obtained by SEM, EMP, and transmission electron microscopy (TEM) align from the chlorite to the tosudite end-members, whose pure compositions could be determined from extreme analytical electron microscopy (AEM) analyses. The described intergrowths and interlayers, under the optical resolution, could provide a clue to explain changes in the normal optic properties of chlorite, which are mentioned, but not explained, in the literature. Full article
(This article belongs to the Special Issue Advances in the Petrography of Natural Fine-Grained Materials: A Microscale Answer to Their Genetic Pathways and Applications)
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23 pages, 14258 KiB  
Article
Geochemical Variations of Kerolite, Stevensite, and Saponite from the Pre-Salt Sag Interval of the Santos Basin: An Approach Using Electron Probe Microanalysis
by Maurício Dias da Silva, Márcia Elisa Boscato Gomes, André Sampaio Mexias, Manuel Pozo, Susan Martins Drago, Everton Marques Bongiolo, Paulo Netto, Victor Soares Cardoso, Lucas Bonan Gomes and Camila Wense Ramnani
Minerals 2025, 15(3), 285; https://doi.org/10.3390/min15030285 - 11 Mar 2025
Viewed by 693
Abstract
This study investigates the mineralogy and chemical characteristics of pre-salt clay minerals, classifies them, and defines assemblages in reactive microsites. Using Electron Probe Micro-Analysis (EPMA), the chemical formulas of Mg-rich clays were determined. Stevensite exhibited low interlayer charge and aluminum content, while kerolite [...] Read more.
This study investigates the mineralogy and chemical characteristics of pre-salt clay minerals, classifies them, and defines assemblages in reactive microsites. Using Electron Probe Micro-Analysis (EPMA), the chemical formulas of Mg-rich clays were determined. Stevensite exhibited low interlayer charge and aluminum content, while kerolite was characterized by a minimal charge. K/S (kerolite/stevensite) mixed layer showed intermediate compositions and charges between these endmembers. Saponite was distinguished by higher levels of Al, K, and Fe, along with a higher interlayer charge. The proposed assemblages are as follows: saponite in mudstone facies (without spherulites/shrubs), with a hybrid matrix; pure kerolite in spherulstone and shrubstone facies, marked by the absence of significant reactions and high preservation of matrix and textures; stevensite in facies with extensive matrix replacement by dolomitization/silicification; and K/S and kerolite in similar facies with intermediate matrix replacement levels and the coexistence of two intimately related clay mineral compositions. This study enables reliable differentiation of these species based on point mineral chemistry and mapping, combined with a microsite approach and conventional techniques. Additionally, it discusses the formation of pre-salt clays, influenced by significant kinetic and chemical interactions during their genesis and burial to depths of approximately 5 km. Full article
(This article belongs to the Special Issue Advances in the Petrography of Natural Fine-Grained Materials: A Microscale Answer to Their Genetic Pathways and Applications)
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