Ni‑rich mineral nepouite explains the exceptional green color of speleothems

Martin Vlieghe; Gaetan Rochez; Stéphane Pire-Stevenne; Jean-Yves Storme; Augustin Dekoninck; Yves VANBRABANT; Olivier Namur; Yishen Zhang; Alicia Van Ham-Meert; Jean-Pierre Donnadieu; Michel Berbigé; Jean-Luc Hasbroucq; Johan Yans

doi:10.1038/s41598-023-41977-7

Ni‑rich mineral nepouite explains the exceptional green color of speleothems

Martin Vlieghe
, Gaetan Rochez
, Stéphane Pire-Stevenne
, Jean-Yves Storme
, Augustin Dekoninck
, Yves VANBRABANT
, Olivier Namur
, Yishen Zhang
, Alicia Van Ham-Meert
, Jean-Pierre Donnadieu
, Michel Berbigé
, Jean-Luc Hasbroucq
, Johan Yans

Research output: Contribution to journal › Article › peer-review

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Abstract

Speleothems are secondary mineral structures typically found in karstic caves and usually composedof calcite or aragonite. Despite being naturally white, some might exhibit unusual colors, such asblue, black, red, yellow or green. The causes of these exceptional colorations are poorly understood,especially for green speleothems, which are barely reported. Here we describe the occurrence of thegreen Ni‑bearing serpentine nepouite in green aragonite and calcite speleothems, in the Aven duMarcou (Hérault, France). Nepouite is mainly found as flat lamellar crystals in the outer rim of greenspeleothems and crystallized alongside radially grown aragonite crystals. This supports nepouitebeginning to crystallize recently, due to a change in the chemical composition of the water. Nepouitealso exhibits extensive substitution between Ni, Mg and Zn. The various elements responsible fornepouite precipitation are thought to come from the weathering of pyrite crystals in the overlyingrocks, which is consistent with the pH conditions of the cave and the Al‑free composition of nepouite.This study explains the crystallization mechanisms and stability conditions of silicate minerals incolored caves.

Original language	English
Article number	15017
Journal	Scientific Reports
Volume	13
Issue number	1
DOIs	https://doi.org/10.1038/s41598-023-41977-7
Publication status	Published - Dec 2023

Funding

This work was funded by an Institute of Life, Earth and Environment doctoral grant and a UNamur F.S.R. Grant both awarded to M.V. We thank RBINS and KULeuven for providing access to Raman and EPMA facilities respectively, as well as Jean-Claude Gayet and Anthony Alliès (Association Mont Marcou) for allowing sampling in the Aven du Marcou. We also thank University of Montpellier, and particularly Michel Lopez, for the realization of X-Ray Fluorescence analyses. We thank David Alford who agreed proof-reading the document to improve English language expression. We also thank Scientific Reports Editor Hiroaki Ohfuji for his help and support. This work was funded by an Institute of Life, Earth and Environment doctoral grant and a UNamur F.S.R. Grant both awarded to M.V. We thank RBINS and KULeuven for providing access to Raman and EPMA facilities respectively, as well as Jean-Claude Gayet and Anthony Alliès (Association Mont Marcou) for allowing sampling in the Aven du Marcou. We also thank University of Montpellier, and particularly Michel Lopez, for the realization of X-Ray Fluorescence analyses. We thank David Alford who agreed proof-reading the document to improve English language expression. We also thank Scientific Reports Editor Hiroaki Ohfuji for his help and support.

Funders
KULeuven
UNamur F.S.R.
Laboratoire de Paléontologie, Institut des Sciences de l'Évolution de Montpellier , C.c. 064, Université Montpellier 2

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title = "Ni‑rich mineral nepouite explains the exceptional green color of speleothems",

abstract = "Speleothems are secondary mineral structures typically found in karstic caves and usually composedof calcite or aragonite. Despite being naturally white, some might exhibit unusual colors, such asblue, black, red, yellow or green. The causes of these exceptional colorations are poorly understood,especially for green speleothems, which are barely reported. Here we describe the occurrence of thegreen Ni‑bearing serpentine nepouite in green aragonite and calcite speleothems, in the Aven duMarcou (H{\'e}rault, France). Nepouite is mainly found as flat lamellar crystals in the outer rim of greenspeleothems and crystallized alongside radially grown aragonite crystals. This supports nepouitebeginning to crystallize recently, due to a change in the chemical composition of the water. Nepouitealso exhibits extensive substitution between Ni, Mg and Zn. The various elements responsible fornepouite precipitation are thought to come from the weathering of pyrite crystals in the overlyingrocks, which is consistent with the pH conditions of the cave and the Al‑free composition of nepouite.This study explains the crystallization mechanisms and stability conditions of silicate minerals incolored caves.",

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AU - Vlieghe, Martin

AU - Rochez, Gaetan

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AU - Storme, Jean-Yves

AU - Dekoninck, Augustin

AU - VANBRABANT, Yves

AU - Namur, Olivier

AU - Zhang, Yishen

AU - Van Ham-Meert, Alicia

AU - Donnadieu, Jean-Pierre

AU - Berbigé, Michel

AU - Hasbroucq, Jean-Luc

AU - Yans, Johan

PY - 2023/12

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N2 - Speleothems are secondary mineral structures typically found in karstic caves and usually composedof calcite or aragonite. Despite being naturally white, some might exhibit unusual colors, such asblue, black, red, yellow or green. The causes of these exceptional colorations are poorly understood,especially for green speleothems, which are barely reported. Here we describe the occurrence of thegreen Ni‑bearing serpentine nepouite in green aragonite and calcite speleothems, in the Aven duMarcou (Hérault, France). Nepouite is mainly found as flat lamellar crystals in the outer rim of greenspeleothems and crystallized alongside radially grown aragonite crystals. This supports nepouitebeginning to crystallize recently, due to a change in the chemical composition of the water. Nepouitealso exhibits extensive substitution between Ni, Mg and Zn. The various elements responsible fornepouite precipitation are thought to come from the weathering of pyrite crystals in the overlyingrocks, which is consistent with the pH conditions of the cave and the Al‑free composition of nepouite.This study explains the crystallization mechanisms and stability conditions of silicate minerals incolored caves.

AB - Speleothems are secondary mineral structures typically found in karstic caves and usually composedof calcite or aragonite. Despite being naturally white, some might exhibit unusual colors, such asblue, black, red, yellow or green. The causes of these exceptional colorations are poorly understood,especially for green speleothems, which are barely reported. Here we describe the occurrence of thegreen Ni‑bearing serpentine nepouite in green aragonite and calcite speleothems, in the Aven duMarcou (Hérault, France). Nepouite is mainly found as flat lamellar crystals in the outer rim of greenspeleothems and crystallized alongside radially grown aragonite crystals. This supports nepouitebeginning to crystallize recently, due to a change in the chemical composition of the water. Nepouitealso exhibits extensive substitution between Ni, Mg and Zn. The various elements responsible fornepouite precipitation are thought to come from the weathering of pyrite crystals in the overlyingrocks, which is consistent with the pH conditions of the cave and the Al‑free composition of nepouite.This study explains the crystallization mechanisms and stability conditions of silicate minerals incolored caves.

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Ni‑rich mineral nepouite explains the exceptional green color of speleothems

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