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1. Voluminous silica precipitated from martian waters during late-stage aqueous alteration

Author

Pan, L., Carter, J., Quantin-Nataf, C., Pineau, M., Chauviré, B., Mangold, N., Deit, L. Le, Rondeau, B., and Chevrier, V.

Subjects
Astrophysics - Earth and Planetary Astrophysics
Abstract

Mars' transition from an early "warm and wet" to the "cold and dry" environment left fingerprints on the geological record of fluvial activity on Mars. The morphological and mineralogical observations of aqueous activity provided varying constraints on the condition and duration of liquid water on martian surface. In this study, we surveyed the mineralogy of martian alluvial fans and deltas and investigated the hydrated silica-bearing deposits associated with these landforms. Using CRISM data, we identified 35 locations across Mars with hydrated silica in proximity to fan/deltas, where the spectral characteristics are consistent with immature or dehydrated opal-A. In a few stepped fan/deltas, we find hydrated silica occurs within the bulk fan deposits and form sedimentary layers correlated with elevation, corroborating the formation of hydrated silica through precipitation. Meanwhile in the older fan/deltas silica mostly occur at distal locations and the relation to primary sedimentary deposits is more complex. We propose that the hydrated silica-bearing deposits in stepped fan/deltas likely formed authigenically from martian surface waters, mainly during the Late Hesperian and Early Amazonian [Hauber et al., 2013]. These silica-bearing deposits could be a tracer for the temperature of water involved in the formation of these deposits, given more precise and detailed observations of the sedimentary context, accessory minerals, the concentration of hydrated silica and sediment-to-water ratio. Therefore, we consider that silica-bearing deposits should be among the most critical samples to investigate for future Mars missions, which are accessible in the landing sites of Mars 2020 and ExoMars missions.

Published
2020
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4. FT-NIR and DSC characterisation of water in opal

Author

Thomas, P, Chauviré, B, Flower-Donaldson, K, Aldridge, L, Smallwood, A, and Liu, B

Subjects
03 Chemical Sciences, 09 Engineering, 19 Studies in Creative Arts and Writing, Materials
Abstract

© 2020 Elsevier Ltd and Techna Group S.r.l. Opal is a hydrous silica (Si02.nH2O) formed through a dissolution-precipitation process. The formation process incorporates water into the structure as bound silanol and molecular water. As the water is distributed in a range of states, multiple methods of characterisation are required to identify each state. This study reports the results of temperature dependent FT-NIR and DSC investigation on natural opal samples of the opal-A (amorphous) and opal-CT (poorly crystalline cristobalite with tridymitic stacking faults) types. Significant differences in the melting behaviour of crystallisable water as well as differences in the spectral characteristics of the non-crystallisable molecular water are observed. These differences are ascribed to the different microstructures of the opal types.

Published
2020

5. DSC of natural opal: insights into the incorporation of crystallisable water in the opal microstructure

Author

Chauviré B and Thomas PS

Subjects
0303 Macromolecular and Materials Chemistry, 0306 Physical Chemistry (incl. Structural), 0399 Other Chemical Sciences, Physical Chemistry
Abstract

© 2019, Akadémiai Kiadó, Budapest, Hungary. Low-temperature DSC on a wide range of opal-A and opal-CT samples was carried out to estimate the proportion of crystallisable water and to determine the size of water-filled cavities. A wide range of crystallisable water contents in the range 4.9 to 41.9% of the water contained in opals were observed, although the proportion of crystallisable water did not correlate with structure. Pore size and pore size distribution were estimated from the melt temperature depression and heat flow data, respectively. Opal-CT was observed to have smaller water-filled pores (radii < 2 nm) than opal-A (radii from 2.5 to 4.9 nm), suggesting that molecular water may be contained between nanograins in the microstructural units (spheres or lepispheres). A narrower pore size distribution was calculated for opal-CT, and no melting of the crystallisable water was observed where bulk water would be expected to melt, suggesting the absence of larger voids. The melting peaks for opal-A, on the other hand, transitioned into the melting of bulk water suggesting the presence of significantly larger water-filled pores, an observation consistent with the microstructure observed in SEM micrographs.

Published
2020

6. Quantification of water content by laser induced breakdown spectroscopy on Mars

Author

William Rapin, Y Meslin, P., Maurice, S., Wiens, R. C., Didier Laporte, Chauviré, B., Olivier Gasnault, Schröders, S., Beck, P., Bender, S., Beyssac, O., Cousin, A., Dehouck, E., Drouet, C., Forni, O., Nachon, M., Melikechi, N., Rondeau, B., Mangold, N., Thomas, N., Lasue, J., Fabre, C., Frydenvang, J., J-L, Lacour, Institut de recherche en astrophysique et planétologie (IRAP), Institut national des sciences de l'Univers (INSU - CNRS)-Université Toulouse III - Paul Sabatier (UT3), Université Fédérale Toulouse Midi-Pyrénées-Université Fédérale Toulouse Midi-Pyrénées-Observatoire Midi-Pyrénées (OMP), Météo France-Centre National d'Études Spatiales [Toulouse] (CNES)-Université Fédérale Toulouse Midi-Pyrénées-Centre National de la Recherche Scientifique (CNRS)-Institut de Recherche pour le Développement (IRD)-Météo France-Centre National d'Études Spatiales [Toulouse] (CNES)-Centre National de la Recherche Scientifique (CNRS)-Institut de Recherche pour le Développement (IRD)-Centre National de la Recherche Scientifique (CNRS), Los Alamos National Laboratory (LANL), Laboratoire Magmas et Volcans (LMV), Centre National de la Recherche Scientifique (CNRS)-Institut de Recherche pour le Développement et la société-Institut national des sciences de l'Univers (INSU - CNRS)-Observatoire de Physique du Globe de Clermont-Ferrand (OPGC), Institut national des sciences de l'Univers (INSU - CNRS)-Université Clermont Auvergne [2017-2020] (UCA [2017-2020])-Centre National de la Recherche Scientifique (CNRS)-Institut national des sciences de l'Univers (INSU - CNRS)-Université Clermont Auvergne [2017-2020] (UCA [2017-2020])-Centre National de la Recherche Scientifique (CNRS)-Université Clermont Auvergne [2017-2020] (UCA [2017-2020])-Université Jean Monnet [Saint-Étienne] (UJM), Institut de Planétologie et d'Astrophysique de Grenoble (IPAG), Institut national des sciences de l'Univers (INSU - CNRS)-Centre National d'Études Spatiales [Toulouse] (CNES)-Centre National de la Recherche Scientifique (CNRS)-Observatoire des Sciences de l'Univers de Grenoble (OSUG ), Institut polytechnique de Grenoble - Grenoble Institute of Technology (Grenoble INP )-Institut national des sciences de l'Univers (INSU - CNRS)-Institut national de recherche en sciences et technologies pour l'environnement et l'agriculture (IRSTEA)-Université Savoie Mont Blanc (USMB [Université de Savoie] [Université de Chambéry])-Centre National de la Recherche Scientifique (CNRS)-Université Grenoble Alpes [2016-2019] (UGA [2016-2019])-Institut polytechnique de Grenoble - Grenoble Institute of Technology (Grenoble INP )-Institut national de recherche en sciences et technologies pour l'environnement et l'agriculture (IRSTEA)-Université Savoie Mont Blanc (USMB [Université de Savoie] [Université de Chambéry])-Centre National de la Recherche Scientifique (CNRS)-Université Grenoble Alpes [2016-2019] (UGA [2016-2019]), Optical Science Center for Applied Research (OSCAR), Delaware State University (DSU), University of Copenhagen = Københavns Universitet (KU), Université Toulouse III - Paul Sabatier (UT3), Université de Toulouse (UT)-Université de Toulouse (UT)-Institut national des sciences de l'Univers (INSU - CNRS)-Observatoire Midi-Pyrénées (OMP), Institut de Recherche pour le Développement (IRD)-Université Toulouse III - Paul Sabatier (UT3), Université de Toulouse (UT)-Université de Toulouse (UT)-Institut national des sciences de l'Univers (INSU - CNRS)-Centre National d'Études Spatiales [Toulouse] (CNES)-Centre National de la Recherche Scientifique (CNRS)-Météo-France -Institut de Recherche pour le Développement (IRD)-Institut national des sciences de l'Univers (INSU - CNRS)-Centre National d'Études Spatiales [Toulouse] (CNES)-Centre National de la Recherche Scientifique (CNRS)-Météo-France -Centre National de la Recherche Scientifique (CNRS), Institut national des sciences de l'Univers (INSU - CNRS)-Université Jean Monnet - Saint-Étienne (UJM)-Institut de Recherche pour le Développement et la société-Université Clermont Auvergne [2017-2020] (UCA [2017-2020])-Centre National de la Recherche Scientifique (CNRS)-Observatoire de Physique du Globe de Clermont-Ferrand (OPGC), Institut national des sciences de l'Univers (INSU - CNRS)-Université Clermont Auvergne [2017-2020] (UCA [2017-2020])-Centre National de la Recherche Scientifique (CNRS)-Institut national des sciences de l'Univers (INSU - CNRS)-Université Clermont Auvergne [2017-2020] (UCA [2017-2020])-Centre National de la Recherche Scientifique (CNRS), and University of Copenhagen = Københavns Universitet (UCPH)

Subjects
[SDU.STU.PE]Sciences of the Universe [physics]/Earth Sciences/Petrography, ComputingMilieux_MISCELLANEOUS
Abstract

International audience

Published
2019

7. Geological significance of hydrated silica on Mars as seen by CRISM data compared to terrestrial analogs

Author

Pineau, M., Le Deit, L., Chauviré, B., Carter, J., Rondeau, B., Mangold, N., Laboratoire de Planétologie et Géodynamique UMR6112 (LPG), Centre National de la Recherche Scientifique (CNRS)-Institut national des sciences de l'Univers (INSU - CNRS)-Université de Nantes - Faculté des Sciences et des Techniques, Université de Nantes (UN)-Université de Nantes (UN)-Université d'Angers (UA), Institut des Sciences de la Terre (ISTerre), Université Grenoble Alpes (UGA)-Centre National de la Recherche Scientifique (CNRS)-Université Savoie Mont Blanc (USMB [Université de Savoie] [Université de Chambéry])-PRES Université de Grenoble-Institut de recherche pour le développement [IRD] : UR219-Institut national des sciences de l'Univers (INSU - CNRS)-Institut Français des Sciences et Technologies des Transports, de l'Aménagement et des Réseaux (IFSTTAR)-Université Joseph Fourier - Grenoble 1 (UJF), Institut d'astrophysique spatiale (IAS), Université Paris-Sud - Paris 11 (UP11)-Institut national des sciences de l'Univers (INSU - CNRS)-Centre National de la Recherche Scientifique (CNRS), Laboratoire de Planétologie et Géodynamique [UMR 6112] (LPG), Université d'Angers (UA)-Université de Nantes - UFR des Sciences et des Techniques (UN UFR ST), Université de Nantes (UN)-Université de Nantes (UN)-Institut national des sciences de l'Univers (INSU - CNRS)-Centre National de la Recherche Scientifique (CNRS), Institut Français des Sciences et Technologies des Transports, de l'Aménagement et des Réseaux (IFSTTAR)-Institut national des sciences de l'Univers (INSU - CNRS)-Institut de recherche pour le développement [IRD] : UR219-Université Savoie Mont Blanc (USMB [Université de Savoie] [Université de Chambéry])-Centre National de la Recherche Scientifique (CNRS)-Université Grenoble Alpes [2016-2019] (UGA [2016-2019]), Pineau, Maxime, and Université Paris-Sud - Paris 11 (UP11)-Institut national des sciences de l'Univers (INSU - CNRS)-Centre National de la Recherche Scientifique (CNRS)-Centre National d’Études Spatiales [Paris] (CNES)

Subjects
[SDU.STU.PL]Sciences of the Universe [physics]/Earth Sciences/Planetology, [SDU.STU.PL] Sciences of the Universe [physics]/Earth Sciences/Planetology, ComputingMilieux_MISCELLANEOUS, [SDU.STU.MI] Sciences of the Universe [physics]/Earth Sciences/Mineralogy, [SDU.STU.MI]Sciences of the Universe [physics]/Earth Sciences/Mineralogy
Abstract

International audience

Published
2019

8. Investigation of opals on MARS using CRISM data and terrestrial analogs

Author

Pineau, M., Le Deit, L., Rondeau, B., Chauviré, B., Carter, J., Mangold, N., Laboratoire de Planétologie et Géodynamique [UMR 6112] (LPG), Université d'Angers (UA)-Université de Nantes - UFR des Sciences et des Techniques (UN UFR ST), Université de Nantes (UN)-Université de Nantes (UN)-Institut national des sciences de l'Univers (INSU - CNRS)-Centre National de la Recherche Scientifique (CNRS), Institut des Sciences de la Terre (ISTerre), Institut Français des Sciences et Technologies des Transports, de l'Aménagement et des Réseaux (IFSTTAR)-Institut national des sciences de l'Univers (INSU - CNRS)-Institut de recherche pour le développement [IRD] : UR219-Université Savoie Mont Blanc (USMB [Université de Savoie] [Université de Chambéry])-Centre National de la Recherche Scientifique (CNRS)-Université Grenoble Alpes [2016-2019] (UGA [2016-2019]), Institut d'astrophysique spatiale (IAS), Université Paris-Sud - Paris 11 (UP11)-Institut national des sciences de l'Univers (INSU - CNRS)-Centre National de la Recherche Scientifique (CNRS)-Centre National d’Études Spatiales [Paris] (CNES), Université Paris-Sud - Paris 11 (UP11)-Institut national des sciences de l'Univers (INSU - CNRS)-Centre National de la Recherche Scientifique (CNRS), Pineau, Maxime, Laboratoire de Planétologie et Géodynamique UMR6112 (LPG), Centre National de la Recherche Scientifique (CNRS)-Institut national des sciences de l'Univers (INSU - CNRS)-Université de Nantes - Faculté des Sciences et des Techniques, Université de Nantes (UN)-Université de Nantes (UN)-Université d'Angers (UA), and Université Grenoble Alpes (UGA)-Centre National de la Recherche Scientifique (CNRS)-Université Savoie Mont Blanc (USMB [Université de Savoie] [Université de Chambéry])-PRES Université de Grenoble-Institut de recherche pour le développement [IRD] : UR219-Institut national des sciences de l'Univers (INSU - CNRS)-Institut Français des Sciences et Technologies des Transports, de l'Aménagement et des Réseaux (IFSTTAR)-Université Joseph Fourier - Grenoble 1 (UJF)

Subjects
[SDU.STU.PL]Sciences of the Universe [physics]/Earth Sciences/Planetology, [SDU.STU.PL] Sciences of the Universe [physics]/Earth Sciences/Planetology, ComputingMilieux_MISCELLANEOUS, [SDU.STU.MI] Sciences of the Universe [physics]/Earth Sciences/Mineralogy, [SDU.STU.MI]Sciences of the Universe [physics]/Earth Sciences/Mineralogy
Abstract

International audience

Published
2018

9. In Situ Analysis of Opal in Gale Crater, Mars

Author

Rapin, W., Chauviré, B., Gabriel, T. S. J., Mcadam, A. C., Ehlmann, B. L., Hardgrove, C., Meslin, P.-y., Rondeau, B., Dehouck, E., Franz, H. B., Mangold, N., Chipera, S. J., Wiens, R. C., Frydenvang, J., Schröder, S., Rapin, W., Chauviré, B., Gabriel, T. S. J., Mcadam, A. C., Ehlmann, B. L., Hardgrove, C., Meslin, P.-y., Rondeau, B., Dehouck, E., Franz, H. B., Mangold, N., Chipera, S. J., Wiens, R. C., Frydenvang, J., and Schröder, S.

Published
2018

10. The precious opal deposit at Wegel Tena, Ethiopia: Formation via successive pedogenesis events

Author

Chauviré, B, Rondeau, B, Mazzero, F, and Ayalew, D

Subjects
Geochemistry & Geophysics
Abstract

Since its discovery in 2008, the opal deposit in the Wegel Tena area of Ethiopia, located within an Oligocene ignimbritic trap, has become a major provider of precious opal. This study provides new field evidence and laboratory results that reinforce the pedogenetic model for gem opal formation. In the eight mines investigated, opal occurs in lenticular horizons within an unwelded ignimbrite bed. Moreover, several deposits are superimposed in the sequence, indicating that gem opal formation occurred several times in the same place. We observed that, in contrast to the other beds, the opal-bearing horizons are strongly weathered: volcanic glass is altered to smectite, rocks with a granular microstructure are apparent, and cavities are covered by illuvial clays. These features are typical of pedogenetic processes. Plant fossils in the opals reveal a well preserved cellular structure composed of chalcedony. These observations are consistent over all the mines, supporting the pedogenetic model for precious opal formation in the Wegel Tena area.

Published
2017

12. In Situ Analysis of Opal in Gale Crater, Mars

Author

Rapin, W., primary, Chauviré, B., additional, Gabriel, T. S. J., additional, McAdam, A. C., additional, Ehlmann, B. L., additional, Hardgrove, C., additional, Meslin, P.‐Y., additional, Rondeau, B., additional, Dehouck, E., additional, Franz, H. B., additional, Mangold, N., additional, Chipera, S. J., additional, Wiens, R. C., additional, Frydenvang, J., additional, and Schröder, S., additional

Published
2018
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13. Quantification of water content by laser induced breakdown spectroscopy on Mars

Author

Rapin, W, Meslin, PY, Maurice, S, Wiens, RC, Laporte, D, Chauviré, B, Gasnault, O, Schröder, S, Beck, P, Bender, S, Beyssac, O, Cousin, A, Dehouck, E, Drouet, C, Forni, O, Nachon, M, Melikechi, N, Rondeau, B, Mangold, N, Thomas, NH, Rapin, W, Meslin, PY, Maurice, S, Wiens, RC, Laporte, D, Chauviré, B, Gasnault, O, Schröder, S, Beck, P, Bender, S, Beyssac, O, Cousin, A, Dehouck, E, Drouet, C, Forni, O, Nachon, M, Melikechi, N, Rondeau, B, Mangold, N, and Thomas, NH

Abstract

© 2017 Elsevier B.V. Laser induced breakdown spectroscopy (LIBS), as performed by the ChemCam instrument, provides a new technique to measure hydrogen at the surface of Mars. Using a laboratory replica of the LIBS instrument onboard the Curiosity rover, different types of hydrated samples (basalts, calcium and magnesium sulfates, opals and apatites) covering a range of targets observed on Mars have been characterized and analyzed. A number of factors related to laser parameters, atmospheric conditions and differences in targets properties can affect the standoff LIBS signal, and in particular the hydrogen emission peak. Dedicated laboratory tests were run to identify a normalization of the hydrogen signal which could best compensate for these effects and enable the application of the laboratory calibration to Mars data. We check that the hydrogen signal increases linearly with water content; and normalization of the hydrogen emission peak using to oxygen and carbon emission peaks (related to the breakdown of atmospheric carbon dioxide) constitutes a robust approach. Moreover, the calibration curve obtained is relatively independent of the samples types.

Published
2017

14. Near infrared signature of opal and chalcedony as a proxy for their structure and formation conditions

Author

Chauviré, B, Rondeau, B, Mangold, N, Chauviré, B, Rondeau, B, and Mangold, N

Abstract

© 2017 E. Schweizerbart'sche Verlagsbuchhandlung, D-70176 Stuttgart. Opal-A, opal-CT and chalcedony develop in various geological environments mainly through continental weathering and hydrothermal processes. Although some spectroscopic criteria already differentiate the structural varieties of silica, no criterion distinguishes the formation process of opaline silica. The originality of this study is based on a unique collection of 38 hydrous silica samples of different structures formed in various geological contexts and lithologies. This large and diverse sampling of silica emphasizes that several new spectroscopic criteria distinguish opal-A, opal-CT and chalcedony: the apparent maximum of the absorption at 5200 cm-1, the area of the absorption of silanol groups as well as the area ratio of the 5200 and 4500 cm-1 bands. Moreover, we observed that the shape of the molecular-water band, quantified by a new criterion developed here (concavity-ratio criterion, CRC), differentiates opals formed by weathering (CRC<0.74) from opals precipitated by hydrothermal processes (CRC>0.82), regardless of their structure (-A or-CT). This new method quickly assesses the geological conditions of opal formation (on Earth or other terrestrial planets) for which data are unclear or missing.

Published
2017

15. WATER CONTENT OF OPALINE SILICA AT GALE CRATER.

Author

Rapin, W., Chauviré, B., Meslin, P.-Y., Maurice, S., Rondeau, B., Mangold, N., Dehouck, E., Gasnault, O., Cousin, A., Forni, O., Frydenvang, J., Wiens, R. C., Schröder, S., and Ehlmann, B. L.

Subjects
GALE Crater (Mars), MARTIAN craters, WATER on Mars
Published
2017

16. Cracking of Gem Opals

Author

Chauviré, B, Mollé, V, Guichard, F, Rondeau, B, Thomas, PS, Fritsch, E, Chauviré, B, Mollé, V, Guichard, F, Rondeau, B, Thomas, PS, and Fritsch, E

Abstract

The value of gem opals is compromised by their potential susceptibility to “crazing”, a phenomenon observed either in the form of whitening or cracking. To understand the latter, 26 opal samples were investigated and separated into 2 groups based on handling: “water-stored” opal samples, which are stored in water after extraction, and “air-stored” opal samples, which are stored in air for more than a year. To induce cracking, samples were thermally treated by staged heating and characterized using optical microscopy and Raman spectroscopy before and after cracking. For water-stored opals, cracking was initiated with moderate heating up to 150 °C, while for air-stored opals, higher temperatures, circa 300 °C, were required. In water-stored opals that cracked, polarized light microscopy revealed stress fields remaining around the cracks, and a red shift in the Raman bands suggested tensile stresses. These stresses were not observed in air-stored samples that cracked. Based on these observations, for air-stored samples, cracking was ascribed to super-heated water-induced decrepitation. By contrast, for water-stored samples, cracking was linked to drying shrinkage, which correlates with the anecdotal reports from the gem trade. We thus identify the physical origin of cracking, and by comparing it to current knowledge, we determine the factors leading to cracking.

17. Cracking of Gem Opals

Author

Chauviré, B, Mollé, V, Guichard, F, Rondeau, B, Thomas, PS, Fritsch, E, Chauviré, B, Mollé, V, Guichard, F, Rondeau, B, Thomas, PS, and Fritsch, E

Abstract

The value of gem opals is compromised by their potential susceptibility to “crazing”, a phenomenon observed either in the form of whitening or cracking. To understand the latter, 26 opal samples were investigated and separated into 2 groups based on handling: “water-stored” opal samples, which are stored in water after extraction, and “air-stored” opal samples, which are stored in air for more than a year. To induce cracking, samples were thermally treated by staged heating and characterized using optical microscopy and Raman spectroscopy before and after cracking. For water-stored opals, cracking was initiated with moderate heating up to 150 °C, while for air-stored opals, higher temperatures, circa 300 °C, were required. In water-stored opals that cracked, polarized light microscopy revealed stress fields remaining around the cracks, and a red shift in the Raman bands suggested tensile stresses. These stresses were not observed in air-stored samples that cracked. Based on these observations, for air-stored samples, cracking was ascribed to super-heated water-induced decrepitation. By contrast, for water-stored samples, cracking was linked to drying shrinkage, which correlates with the anecdotal reports from the gem trade. We thus identify the physical origin of cracking, and by comparing it to current knowledge, we determine the factors leading to cracking.

18. Arthropod entombment in weathering-formed opal: new horizons for recording life in rocks.

Author

Chauviré B, Houadria M, Donini A, Berger BT, Rondeau B, Kritsky G, and Lhuissier P

Subjects
Animals, Arthropods, Earth, Planet, Geologic Sediments, Silicon Dioxide, Weather, Earth Sciences methods, Fossils anatomy & histology, Hemiptera anatomy & histology
Abstract

Animal fossils preserved in various geological materials, such as limestone, claystone, or amber, provide detailed information on extinct species that is indispensable for retracing the evolution of terrestrial life. Here, we present the first record of an animal fossil preserved in opal formed by weathering with such high-resolution details that even individual cuticle hairs are observed. The fossil consists of the exoskeleton of a nymphal insect belonging to the order Hemiptera and either the family Tettigarctidae or the Cicadidae. This identification is based on anatomical details such as the tibial and femoral morphology of the forelegs. The exoskeleton of the insect was primarily zeolitized during the alteration of the host rocks and later sealed in opal deposited by silica-rich fluids derived from the continental weathering of the volcanic host rocks. Organic matter is preserved in the form of amorphous carbon. This finding makes opal formed by rocks weathering a new, complementary source of animal fossils, offering new prospects for the search for ancient life in the early history of Earth and possibly other terrestrial planets such as Mars, where weathering-formed opal occurs.

Published
2020
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19. Reverse dichromatism in gem andalusite related to total pleochroism of the Fe 2+ -Ti 4+ IVCT.

Author

Rondeau B, Chamard-Bois S, Fritsch E, Notari F, and Chauviré B

Abstract

We document a dichromatism effect in gem andalusite that shifts from light brownish pink at low thickness to medium green at high thickness. This is roughly reverse to that usually observed in other dichromatic materials. We show that this is due to a very strong pleochroism in andalusite, when dichromatism in other materials is due to an alexandrite effect. Here, colors varies from a strongly absorbing orangey-brown in the c direction to much less absorbing medium green in a and b directions. The brown absorption is so strong that it becomes opaque (total absorption) for thickness above about 1.5 mm (this value may vary according to concentration of the chromophore and intensity of the light source), and hence does not contribute to coloration anymore. For such high thicknesses, only the a and b directions remain transparent and hence only the green component contributes to the color: the sample appears green. For lower thicknesses, the green color combines with the orangey-brown color that becomes transparent along the c direction, resulting in a light brownish pink. From trace elements analysis in zoned samples, we also confirm that the dark orangey-brown color is due to Fe 2+ -Ti 4+ , and bring evidences that the light green color is mostly due to Fe, either in the form of isolated Fe 3+ or Fe 2+ -Fe 3+ IVCT, with possibly some contributions from isolated Mn 3+ ., (Copyright © 2018 Elsevier B.V. All rights reserved.)

Published
2018
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