Relations entre nanostructure, propriétés physiques et mode de formation des opales A et CT.

L'opale, matériau amorphe ou mal cristallisé, possède une grande variété de structures, dont le bloc élémentaire est un nanograin de silice de 25 nm de diamètre environ. Dans l'opale A, les nanograins s'agencent en sphères, le plus souvent de façon concentrique, mais parfois radialement. Dans l'opale CT, ils peuvent s'organiser ou non, avec jusqu'à quatre degrés d'ordre, incluant certaines structures inédites. La détermination d'inclusions est possible par spectrométrie Raman. Les inclusions montrent souvent un environnement de formation spécifique qui permet de remonter jusqu'aux conditions de formation, et parfois jusqu'à la provenance. Des critères géochimiques ont été élaborés permettant l'identification de l'origine géographique de tous les gisements étudiés. L'origine géologique est déterminée par la teneur en baryum. Certains éléments modifient des caractères physiques tels que la couleur ou la luminescence. La luminescence est activée par la présence d'uranium ou celle de défauts structuraux liés à l'oxygène.Opal, which is an amorphous (opal-A) or poorly crystallized (opal-CT) material, shows a wide variety of structures, and is built from elementary silica nanograins of about 25 nm in diameter. In opal-A, nanograins arrange themselves in spheres, most often in a concentric way, sometimes radialy. In opal-CT, they can organize or not, with up to four degrees of order, including several structures described here for the first time. We have established that inclusions determination is possible by Raman spectrometry. Inclusions are often characteristic of a specific environment, helping to identify formation conditions, even sometimes the exact locality. Geochemical criteria have been elaborated, making the identification of the geographical origin of all deposits studied possible. Geological origin is determining through barium content. Several elements influence some physical properties, such as color or luminescence. Luminescence is activated by uranium or by oxygen-related defects, and inhibited by iron.NANTES-BU Sciences (441092104) / SudocSudocFranceF

An Historical Alexandrite from the Mineralogy Museum of Paris School of Mines

International audienceA spectacular 42.54 ct faceted alexandrite belonging to the collection of the Mineralogy Museum of Paris School of Mines was non-destructively characterised for this report. This historical gem was exhibited in at least one exposition in Paris during the late 1870s and/or in 1880 before it was donated to the museum by lapidary Henri Garreaud in 1882. However, its geographic origin was not stated, and its gemmological properties have not been examined until now. Our study reveals that the gem's microscopic, chemical and spectroscopic characteristics are similar to those of alexandrite from Sri Lanka

Study of an ‘Egyptian’ Emerald from the Mineralogy Museum at the Paris School of Mines

International audienceA 3.19 ct rectangular faceted emerald, reportedly from Egypt, has been in the collectionof the Mineralogy Museum at the Paris School of Mines (Mines Paris – PSL) most likely since the midtolate nineteenth century. Its microscopic features (e.g. multiphase inclusions with jagged outlinesand gota de aceite structures), and its chemical and spectroscopic characteristics (e.g. mainlyrelated to Cr and V rather than Fe, as well as a low alkali content), are consistent with emeralds fromColombia and completely different from those of known Egyptian stones. Apparently, the speciesand variety of the specimen were correctly established when the gem arrived in the collection,but its origin was not. The locality may have been deduced from the relatively low gem quality ofthis faceted emerald, resembling those that were discovered around the same time (early to midnineteenthcentury) in Egypt

From the lithosphere to the lower mantle: An aqueous-rich metal-bearing growth environment to form type IIb blue diamonds

International audienceA study of five diamonds containing mineral and fluid inclusions, selected among forty-nine specimens from the Cullinan Mine, South Africa, was carried out to better document the origin and formation of N-absent B-poor (type IIb) diamonds. The combination of several in-situ non-destructive techniques was used to identify the mineralogy and the chemical composition of primary and secondary inclusions. These include breyite, larnite, graphite, Fe-Ni-Cu native metallic alloys, sulfides of the pyrrhotite group, Ni-rich oxide and potential hydrous ferric sulfates. A common and abundant hydrous fluid containing H2O + CH4 was also identified. From the various observations, we suggest that these type IIb diamonds grew in an aqueous oxidized fluid reacting with a reduced mantle characterized by low oxygen fugacity. Remnant pressures recorded in primary breyite by Raman shifts and XRD measurements enabled the calculation of minimal entrapment pressures of inclusions using elastic geothermobarometry. Applying pressure corrections caused by elastic relaxation, minimum trapping pressures from 4.9 GPa to 5.6 GPa were calculated, suggesting lithospheric depths consistent with the occurrence of numerous graphite inclusions. The association of breyite and larnite, which is often considered as an indicator of sublithospheric origin, also occurs at pressures of 6 GPa or lower in a H2O-rich and carbonate/Ca-rich environment. The B-poor and N-absent features of type IIb diamonds do not require the classic subduction-related model of their formation. Whereas high-pressure minerals would host boron in cold subducting slabs, slabs are also important carriers of nitrogen into the deep mantle, with this latter element mostly absent in these diamonds. In our alternative model, the mantle is proposed as an alternative source of boron, whereby metallic alloys or N speciation between fluid and melt would still prevent the incorporation of nitrogen, leading to the expression of the blue, boron-related and N-absent features of type IIb diamonds. The observed mineralogical assemblage neither proves sublithospheric origin nor does it exclude lithospheric depths of formation for these diamonds. Hence, we propose that type IIb diamonds form in a mantle continuum, from sublithospheric to lithospheric depths