Cathodoluminescence of Rare Earth Doped Zircons. II. Relationship Between the Distribution of the Doping Elements and the Contrasts of Images

Cathodoluminescence (CL) color photographs using an optical CL microscope with a cold cathode electron gun are compared with non-spectrally resolved (polychromatic) and selected wavelength CL images obtained by means of a scanning electron microscope equipped with a CL spectrometer. It is the aim of this paper to show how the interpretation of the contrasts of CL images depends on the knowledge of the CL photon energy distributions participating to the observed contrasts as well as the matrix effects modifying the number of emitted photons compared to that of generated photons. It is shown that the impurities different from the rare earth elements (REE) activators are responsible for charge trapping mechanisms leading to the development of internal electric fields modifying the energy and spatial distribution of the electrons within the insulators and consequently modifying the relative intensities of the intrinsic (host lattice) emission and characteristic emission of a REE activator. In addition, the mechanisms of production of photons must be better understood before trying to express the CL intensity of a monochromatic line as a function of the corresponding REE activator

Recent advances in understanding the similarities and differences of colombian euclases

Colombian euclase is rare and associated with emerald in medium-temperature hydrothermal veins hosted by Lower Cretaceous black shales (BS). The original sources of euclase production were the mining districts of Gachala and Chivor in the eastern emerald belt, but in 2016 euclases were also found at the La Marina mine in the western emerald belt. The present study is centered on a chemical and mineralogical examination of zoned Colombian euclase sold on the gem market as "trapiche'. Its texture is characterized by growth bands and sectors distinguished by the presence of numerous inclusions (mainly pyrite, carbonates, and organic matter) which represent around 0.2% of the total volume of the crystals. Xray computed tomography showed that the largest inclusions are randomly located, whereas the small inclusions are concentrated in the center of the crystals, along the crystallographic b axis, between neighboring growth sectors and between growth bands in each sector. The texture cannot be defined as "trapiche', like that of Colombian emeralds, because there is no matrix material from the surrounding BS trapped between the growth sectors and accumulated as dendrites. Three-phase fluid inclusions (FI) containing halite, liquid, and vapor phases are also observed in the euclase, and their volume is identical to that of the inclusions in emerald. Chromium and vanadium are the main chromophores, and the highest concentrations (1240 and 400 ppm, respectively) were found in deep blue-colored zones. Surprisingly, the euclase crystals have high Ge contents, between 230 and 530 ppm. The Rare Earth Element (REE) patterns of euclase are inherited from the enclosed BS or albitized and carbonatized BS. Euclase has the same REE pattern as emerald from the Gachala mines with an Eu anomaly (Eu/Eu* similar to 0.40) and a depletion in Heavy Rare Earth Elements (HREE). The present study allows for the reconstruction of the formation conditions of "trapiche' euclase and discussion about its probable geographic origin, i.e., the eastern emerald belt

Kinetic and metallographic study of oxidation at high temperature of cast Ni 25Cr alloy in water vapour rich air

International audienceThe high temperature oxidation behaviour of a Ni-25 wt-% Cr alloy in air enriched with water vapour (180 mbars H2O) was studied at 1000, 1100, 1200 and 1300 degrees C. The oxidised samples were characterised by X-ray diffraction, electron microscopy and wavelength dispersion spectroscopy. The obtained data were compared to the ones earlier obtained for the same alloy oxidised in dry air. Water vapour globally induced at all temperatures a decrease of the parabolic constant Kp and an increase in the chromia volatilisation constant Kv. The oxide scales do not present morphologic difference between the two atmospheres. After oxidation in humidified air the scale thickness is thinner and the Cr depleted depth is lower than in dry air