Genèse de silice supergène sur Terre et implications sur Mars

Silica (SiO2), significant constituent of telluric planet, can be found under various forms. Genesis of amorphous and cryptocrystalline polymorphs (opal-A, opal-CT, chalcedony) is not well understood on Earth. In 2008, several areas on Mars surface exhibit spectroscopic feature of hydrated amorphous silica (“opaline silica”). To understand the geological significance of this silica, two approaches have been used. The first one is the study of a terrestrial case: the opal deposit of Wegel Tena, in Ethiopia. The geology shows that these opals are pedogenetic: illuvial clays, pedogenic texture, geochemical analysis, plant fossils. Stratiform organization associated with clays, is common with silica observed on Mars. The second approach is based on the accurate understanding of the infrared signature of silica from well-constrained origin. Criteria based on the infrared signature of hydration in silica (bands about 4500, 5200 et 7000 cm-1) were created. They enable to discriminate structure (opal-A, opal-CT, chalcedony) and genesis processes (hydrothermal or weathering process) of silica. This work establishes a new classification based on genesis processes to identify differences implied by them. Extracting all information from the infrared signature of martian silica is also discussed. In conclusion, amorphous hydrated silica records the geological conditions of their formation. Instruments on board of the future martian rovers could bring some clues to understand the geology of these silica.La silice (SiO2), composant important des planètes telluriques, se trouve sous diverses formes. La genèse de ses polymorphes amorphes et mal cristallisés (opale-A, opale-CT, calcédoine) est encore mal comprise sur Terre. En 2008, des régions de Mars montrent des signatures spectroscopiques de silice amorphe hydratée (« opaline silica »). Pour comprendre la signification géologique de cette silice, deux approches ont été utilisées. La première est l’étude d’un cas terrestre : le gisement d’opale de Wegel Tena (Éthiopie). La géologie montre que ces opales sont issues de la pédogenèse : argiles illuviales, textures pédogéniques, analyses géochimiques, fossiles végétaux. L’organisation stratiforme et la présence d’argile sont communes aux détections martiennes. La seconde approche consiste à décrypter la signature infrarouge de silices d'origine géologique terrestre connue. Des critères basés sur la signature infrarouge de l’hydratation des silices (bandes vers 4500, 5200 et 7000 cm-1) permettant de déduire la structure (opale-CT, opale-A ou calcédoine) et le mode de genèse de la silice (hydrothermalisme ou altération continentale) ont été créés. Il est démontré que la silice amorphe hydratée enregistre les conditions géologiques de sa formation. Ce travail propose une nouvelle classification basée sur les processus de genèse, afin d’identifier les différences induites par ceux-ci. Il est également discuté d’extraire de la signature infrarouge de la silice martienne toutes les informations possibles (structure, mode de formation). Les instruments qui seront embarqués sur les futurs rovers martiens pourraient amener à une meilleure compréhension de la géologie de cette silice

Cracking of Gem Opals

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

Near-infrared signature of hydrothermal opal: a case study of Icelandic silica sinters

Abstract. Silica minerals constitute a main target to assess the origin of life or the possibility of its emergence. On Earth, ancient hydrothermal silica deposits have preserved the oldest forms of life. Beyond Earth, such silica-rich hydrothermal systems have been observed on Mars by orbital near-infrared (NIR) remote sensing and in situ rover exploration. This work investigates the variations of texture and NIR properties of opal with temperature, within a single geological context of hot springs. Silica sinters have been sampled in Icelandic hot-spring fields, in the Reykholt region, and at the Hveravellir site, with water temperature ranging from 14 to 101 ∘C. Variations in the NIR spectral features (concavity ratio criteria, CRC) vary with fluid temperature, lithofacies, and microtexture. Only high-temperature samples display high CRC values (CRC5200>0.85), but low CRC values (CRC5200 < 0.75) are measured for any temperature. Hence, temperature is not the only parameter controlling spectral properties of opal. Several other parameters such as the hydrodynamic context, the microbial activity, silica micro-textures, and porosity may also affect silica precipitation, the incorporation and speciation of water in it, and thus its NIR signature. The observations suggest a limitation in the use of NIR spectral features for the interpretation of the geological context of fossil opal on Earth or Mars: only opal with high CRC values can be inferred as being formed by hydrothermal activity. Low CRC values can be attributed to either low-temperature hydrothermal activity (< 50–60 ∘C) or to continental weathering

Near infrared signature of opaline silica at Mars-relevant pressure and temperature

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

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Blue Spinel from the Luc Yen District of Vietnam

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Cracking of Gem Opals

International audienceThe 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

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

International audienceAnimal 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

Pedogenic origin of precious opals from Wegel Tena (Ethiopia): Evidence from trace elements and oxygen isotopes

International audienceThe trace element and oxygen isotope composition of Wegel Tena (Ethiopia) gem opals was measured to provide evidence of the conditions of their genesis. Elemental measurements display several behaviors, especially for K, Ca, Sr and Ba suggesting that-in agreement with previous assumptions-the silica-rich fluids that precipitate into opal are fed by the weathering of ignimbrite at several degrees. The distribution of elements in the opals indicates that the sources of silica in the ignimbrite are both glass and feldspar. Rare Earth Element (REE) signatures are also consistent with a weathering process, but underline that a wide range of physical and chemical conditions prevail at the regional, local and even intra-sample scales. The Ce anomaly emphasizes the variations in redox conditions during opal precipitation, whereas Eu anomaly indicates that feldspar dissolution feeds some of the silica-rich fluids. This suggests that the fluid responsible for opal precipitation is not homogenous across the area with underground water circulation, but rather that each sample reflects formation conditions specific to its very local environment. The oxygen isotope signatures of the opals (from 26.52 to 30.98‰ vs SMOW) allow us to formulate several hypotheses concerning their temperature of formation and the isotopic composition of the fluid. The hypothesis consistent with our other measurements is the pedogenic formation of the opals at ambient temperature (18-21 °C) involving a slightly evaporated soil water fed by meteoric water with an isotopic composition lower than at present, during an Oligocene period likely warmer and wetter than today