# Editorial for the Special Issue of Minerals, “Advances in Low-Temperature Mineralogy and Geochemistry”

Iuliu Bobos, Franck Bourdelle

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Iuliu Bobos, Franck Bourdelle. Editorial for the Special Issue of Minerals, “Advances in Low-Temperature Mineralogy and Geochemistry”. Minerals, 2025, 15 (1), pp.84. 10.3390/min15010084. hal-05300234

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Editorial

# Editorial for the Special Issue of *Minerals*, "Advances in Low-Temperature Mineralogy and Geochemistry"

Iuliu Bobos <fnref n="1" />,* and Franck Bourdelle <fnref n="2" />,*

Low-temperature geochemical processes are ubiquitous in geological areas related to weathering, diagenesis, very low-grade metamorphism, and post-magmatic processes, and provide an explanation of the P–T–X history of the Earth’s crust. These processes may provide challenging topics of the broad qualitative questions related to near-surface Earth processes involving chemical transfer, mineralogical reactions, recycling of chemical elements, mineralization, etc.

In this way, light (δ⁷Li; δ¹¹B), stable (δ¹⁸O, δD), and radiogenic isotope (¹⁴⁴Nd/¹⁴³Nd, ²⁰⁷Pb/²⁰⁶Pb, ¹⁷⁶Hf/¹⁷⁷Hf, etc.) ratios are potentially powerful tracers of fluid–mineral interactions, while thermo-barometer indicators (e.g., chlorite thermometer, phengite barometer, phengite–chlorite pairs, muscovite-paragonite solvus, etc.) based on fine mineralogical compositions are appropriate probes to reconstruct pro- and retrograde mineralogical reactions. Furthermore, remarkable advances have recently been made in understanding chemical element recycling within the diagenesis and low-grade metamorphism environment, hydrothermal systems, and fluid circulation. Over the past decade, analytical techniques (e.g., X-ray diffraction, secondary-ion mass spectroscopy, laser-ablation mass spectroscopy, high-resolution transmission electron microscopy, etc.) have also advanced and offered a new vision for scientists in mineralogy and geochemistry, making data available with an unparalleled analytical and spatial resolution.

Contributions on low-temperature mineralogy and geochemistry from a broad range of fields were selected and published. The Special Issue begins with a review of chlorite geothermometry and recent related analytical advances [1]. Chlorite, a ubiquitous clay mineral in metamorphic, diagenetic, and hydrothermal systems, exhibits a chemical composition partly dependent on temperature (T) and pressure (P) conditions. Recent analytical developments that allow for circumventing or responding to certain criticisms regarding the low-temperature application of thermometers are shown. Furthermore, new perspectives in terms of analysis (e.g., Mn redox in Mn-chlorite) and geothermometers (molecular solid-solution model, oxychlorite end-member) are addressed.

The ore-forming processes and subsequent processes of alteration–mineralization explain the role of fluids during mineralogical reactions and element recycling. A very interesting mineralogical reaction from hübnerite to scheelite and Fe,Mn-chlorite was studied in the magmatic–hydrothermal system of Borralha (Portugal) [2]. Replacing wolframite with scheelite is common among tungsten deposits where these two tungstates may coexist. Additionally, a genetic linkage between W (Cu, Mo) and chlorite minerals was recognized in this ore-system deposit [3,4]. The replacement of wolframite with scheelite in CO₂-rich fluids was identified in the Borralha magmatic–hydrothermal system, where fluid pressure decreases (or simple cooling) and the ratios of total Ca<fnref n="2" />⁺ (from feldspar albitization) to

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Received: 18 December 2024

Revised: 9 January 2025

Accepted: 14 January 2025

Published: 17 January 2025

**Citation:** Bobos, I.; Bourdelle, F. Editorial for the Special Issue of *Minerals*, "Advances in Low-Temperature Mineralogy and Geochemistry". *Minerals* 2025, 15, 84. https://doi.org/10.3390/min15010084

**Copyright:** © 2025 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).

<footnote n="1">¹ Institute of Earth Sciences, University of Porto, Rua do Campo Alegre 687, 4168-007 Porto, Portugal</footnote>

<footnote n="2">² CY Cergy Paris Université, Sorbonne Université, CNRS-INSU, Institut des Sciences de la Terre de Paris, ISTEP, F-95000 Cergy, France</footnote>

<footnote>* Correspondence: ibobos@fc.up.pt (I.B.); franck.bourdelle@cyu.fr (F.B.)</footnote>
mal/geothermal systems) are described around the world [13–17]. Sulphate effluent minerals precipitated on the argillic facies as the result of water-rock interaction and fumarole emission covering the surface of the Donnoe and Dachnoe fields of the Mutnovsky volcano are described by Zhitova et al. [18]. Ca, Ba, $(NH_4)^+$, Na-Fe$^{3+}$, $(NH_4)^+$-Al, $(NH_4)^+$-Fe$^{3+}$, Na-Al, K-Al, and K-Fe$^{3+}$ sulphates were reported, where ammonia species were concentrated around fumaroles.

Finally, basalt-derived carnelian was identified in the Newark basin as medium- to coarse-size pebbles in fluvial gravel and alluvium and colluvium formed from erosion of Lower Jurassic Preakness Basalt [19]. Carnelian contains 97–98 wt.% SiO$_2$, ~1.0 wt.% Fe$_2$O$_3$, and 1.0–1.4 wt.% LOI, and trace element abundances are low except for Y, Nb, Ta, W, Th, and U. Rare earth element (REE) patterns display heavy REE enrichment and large negative Eu anomalies. Si was likely derived from the alteration of basaltic glass. $\delta^{18}O_{VSMOW}$ values measured on carnelian are high and range from +18.3 to +31.2‰, comparable to global occurrences of volcanic rock-derived chalcedony. According to authors, carnelian was precipitated from the mixing of hydrothermal fluid with meteoric water under conditions of low temperature (20–80 °C) and neutral to slightly alkaline pH.

**Acknowledgments:** We are grateful to all our dear collaborators, contributors, and authors who contributed to this Special Issue. Many thanks to Urs Klötzli (University of Wien, Austria) for reading the manuscript and making suggestions.

**Conflicts of Interest:** The authors declare no conflicts of interest.

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