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Nanoscale characterization of chrysocolla, black chrysocolla, and pseudomalachite from supergene copper deposits of Atacama Desert in northern Chile

  • Zia Steven Kahou ORCID logo EMAIL logo , Anne-Magali Seydoux-Guillaume , Pierre-Marie Zanetta , Stéphanie Duchêne , Stéphanie Brichau and Eduardo Campos
Published/Copyright: August 14, 2025
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American Mineralogist
From the journal American Mineralogist Volume 110 Issue 8

Abstract

We present the first textural and chemical characterization at nanometer scale of chrysocolla [(Cu2–xAlx)H2–xSi2O5(OH)4·nH2O], black chrysocolla (a Mn-rich variety of chrysocolla), and pseudo-malachite [Cu5(PO4)2(OH)4] from two distinct supergene copper deposits from Atacama Desert in northern Chile. These minerals are the most common copper minerals found in the supergene deposits associated with copper porphyries from Atacama Desert. However, the lack of nanoscale morphological information prevents a deeper understanding of their formation process. Nanoscale characterization using transmission electron microscope (TEM) imaging allows further characterization of the structural states of chrysocolla, black chrysocolla, and pseudomalachite, offering valuable insights into their genesis. Chrysocolla and black chrysocolla are not single crystals but assemblages of Cu nanoparticles embedded in an Si-rich amorphous matrix. Scanning TEM (STEM) images reveal that chrysocolla consists of rounded Cu-rich nanoparticles embedded in an amorphous matrix, while black chrysocolla consists of rounded Cu-rich nanoparticles with few needle-shaped Mn-rich particles, all embedded in an amorphous matrix. The richness in nanoparticles defines a layering that mimics the colloform texture observed in optical microscopy. In contrast, pseudomalachite is a massive polycrystalline mineral consisting of a juxtaposition of large nanocrystal grains of ∼500 nm. The STEM-electron energy loss spectrometry (EELS) spectra show that copper in chrysocolla and black chrysocolla is in a reduced state. This suggests that chrysocolla and black chrysocolla form under reducing conditions, probably just below the water table. Alternatively, it could be that water table oscillation allows for the cyclical precipitation of Cu0-rich nanoparticles and oxidized copper-rich silicates. Conversely, pseudomalachite crystallization requires oxidative conditions. The oxidation state variations, from chrysocolla (Cu0) to pseudomalachite (Cu2+), certainly occur during the episodic switch of the water table linked to tectonic events or climatic changes. The findings also have implications for the U-Pb dating of supergene copper deposits, since black chrysocolla and pseudomalachite can incorporate significant U contents. The different structural states of the three minerals may explain their different behaviors regarding U and Pb mobility and, therefore, the preservation of the U-Pb chronometric signal.

Keywords: Chrysocolla; black chrysocolla; pseudomalachite; transmission electron microscope; nanoscale characterization

Acknowledgments and Funding

A.M.S.G. thanks S. Reynaud (Lab Hubert Curien, Saint-Etienne, France) for FIB-preparation, and the Consortium Lyon St-Etienne de Microscopie (CLyM) for access to the TEM NeoARM in Saint-Etienne (France). A.M.S.G. and P.M.Z. acknowledge funding for a direct electron detector from the French National Research Agency (ANR), project ANR-22-CE29-0019 (PI M. Bugnet, MATEIS, France), and from the LabEx iMUST (ANR-10-LABX-0064), LabEx MANU-TECH-SISE (ANR-10-LABX-0075), and EUR MANUTECH SLEIGHT (ANR-17-EURE-0026) of the University of Lyon under the “Investissements d’Avenir” and “France 2030” investment plans set up by the French government. For open access, the authors have applied a CC-BY public copyright license to any Author Accepted Manuscript version arising from this submission.

This work was supported by the program funding of the Institut Carnot ISIFoR (project Dali ref. no. 450808 and Focus ref. no. 450906), the TelluS Program of CNRS/INSU and ANID (Chilean National Research of Development Agency) by means of the FONDECYT Regular project No. 1220062. We acknowledge the LMI Copedim, an IRD research program, and the Université Paul Sabatier (Toulouse, France), as well as Universidad Católica del Norte (Antofagasta, Chile). Finally, we gratefully acknowledge the Society of Economic Geologists Foundation for the Hugh McKinstry fund (SRG 20-15) received by Zia Steven Kahou.

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Received: 2023-12-12
Accepted: 2024-12-14
Published Online: 2025-08-14
Published in Print: 2025-08-26

© 2025 Mineralogical Society of America

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https://doi.org/10.2138/am-2024-9300
Keywords for this article
Chrysocolla; black chrysocolla; pseudomalachite; transmission electron microscope; nanoscale characterization

Articles in the same Issue

  1. Hematite (U-Th)/He thermochronometry unveils unique exhumation history: An example from the Dexing porphyry copper deposit, Southern China
  2. Viscosity measurements of selected lunar regolith simulants
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  4. Identification of the nature of recycled carbonates in the mantle: Insights from the Mo-Mg isotopic pair
  5. Discriminating ionic mobility between diffusivity and electrical conductivity experiments on Earth’s silicate materials
  6. Morphological approach to understanding mineral alteration and nanoparticle formation under alkaline conditions using granitic rock thin sections
  7. Identification of hydroandradite in CM carbonaceous chondrites: Aproduct of calc-silicate alteration on C-complex asteroids
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  11. High-temperature Raman spectroscopy of K2Ca(CO3)2 bütschliite and Na2Ca2(CO3)3 shortite
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