Article Dans Une Revue The American Mineralogist Année : 2024

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

Résumé

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 pseudomalachite [Cu5(PO4)2(OH)4] from two distinct supergene copper deposits from Atacama Desert of 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 to address new founding on 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 juxtaposition of large nanocrystal grains of ∼500 nm. The STEM-EELS (Electron Energy Loss Spectrometry) spectra show that copper in chrysocolla and black chrysocolla is in a reduced state. This suggest that chrysocolla and black chrysocolla forms under reducing conditions, probably just below the water table or, an alternative hypothesis, that water table oscillation allows cyclical precipitation of Cu0-rich nanoparticles and oxidized copper-rich silicate. Conversely, pseudomalachite crystallization requires oxidative conditions. The oxidation state variations from chrysocolla (Cu0) to pseudomalachite (Cu2+ 40 ), certainly happens during episodic switch of the water table linked to tectonic event or climatic changes. The findings have also implications for the U-Pb dating of supergene copper deposits since black chrysocolla and pseudomalachite can incorporate important U-content. The different structural states of the three minerals may explain their different behavior regarding U and Pb mobility and therefore the preservation of the U-Pb chronometric signal
Fichier sous embargo
Fichier sous embargo
0 4 25
Année Mois Jours
Avant la publication
lundi 30 juin 2025
Fichier sous embargo
lundi 30 juin 2025
Connectez-vous pour demander l'accès au fichier

Dates et versions

hal-04871340 , version 1 (07-01-2025)

Licence

Identifiants

Citer

Zia Steven Kahou, Anne-Magali Seydoux-Guillaume, Pierre-Marie Zanetta, Stéphanie Duchêne, Stéphanie Brichau, et al.. Nanoscale characterization of chrysocolla, black chrysocolla and pseudomalachite from supergene copper deposits of Atacama Desert of northern Chile. The American Mineralogist, 2024, ⟨10.2138/am-2024-9300⟩. ⟨hal-04871340⟩
34 Consultations
1 Téléchargements

Altmetric

Partager

More