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[en] Experiments have been performed both on a peraluminous leucogranitic (DK89) and a F-, Li-, P-rich pegmatitic (B2) melt to constrain the stability of micas in evolved crustal silicic magmas and refine mica-melt partition coefficients for F, Li, and Be. The experiments were conducted in parallel in two fO2 ranges, “oxidizing” (NNO +1 to +3) and “reducing” (NNO –1.6 to –1.4). One two-stage reducing-oxidizing experiment was conducted in a vessel fitted with a H2-permeable Shaw-type membrane. The approach toward equilibrium was tested by imposing long experimental durations and combining mica crystallization experiments with mica dissolution experiments using mica seeds. Experimental micas and melts were analyzed for major elements by electron microprobe and for light elements by nuclear microprobe. At 3 kbar, 620 °C, and under oxidizing conditions, B2 crystallized only muscovite, the biotite seeds reacting to form a new mica intermediate between phengite and Li-rich phengite. Under reducing conditions, biotite (siderophyllite composition) appeared as the stable mica. The two-stage experiment yielded a composite mica assemblage with siderophyllite cores mantled by muscovite rims. At 3.5–3.8 kbar, 720 °C, and under oxidizing conditions, DK89 crystallized only aluminous biotite and muscovite seeds reacted to form biotite-bearing assemblages; muscovite appeared together with biotite at 700 °C. Under reducing conditions, Al-rich biotite is also the stable mica at 720 °C. Partition coefficients show that F and Li are preferentially incorporated in biotite rather than in muscovite, the opposite as for Be. Biotite fractionation buffers the F and increases the Li and Be contents of the residual melt. Muscovite increases the Li content of the melt and has little influence on F and Be concentrations. Our experiments reproduce mica assemblages and compositions typical of Variscan pegmatites and leucogranites, yet very Li-rich micas (e.g., lepidolites) were not obtained. The results stress the differential influence of fO2 on mica stability in moderately and highly fractionated crustal melts. Mica crystallization in leucogranites does not appear to be strongly dependent on fO2. In contrast, a very strong influence of fO2 on stable mica assemblages is demonstrated for the pegmatitic melt. The reducing experiments emphasize the existence of a stability field for biotite in melts poor in Fe, Mg, and Ti. If fO2 is reducing, biotite must crystallize in moderately to highly evolved peraluminous crustal melts. In contrast, the crystallization of muscovite as the sole mica in evolved crustal melts constitutes an indicator of oxidizing fO2. Such an oxidizing evolution that deviates from classical buffered T-logfO2 trajectories is the consequence of a mechanism of magma “self-oxidation” that is proposed to result from dissociation of H2O in the melt. (author)
[en] Since June 1976, evidence for the existence of superheavy elements is discussed about. After having recalled artificial elements with atomic numbers Z of wich go from Z=95 (Americium) to Z=107 (built in 1976), superheavy elements having Z greater than 110 are considered. They have been discovered by american searchers in giant halos seen in Madagascar micas. The samples have been recoked in the Fort-Dauphin region (Haut Mandrare). The corresponding numbers Z are 114, 115, 116, 124, 125, 125, 126, 127. It seems that the existence of Z=126 element should be accepted with a greater degree of confidence. But different experiences done by other groups seem to show the evidence of superheavy elements in micas looks weaker. Nevertheless, it is interesting to investigate the evidence or the non-evidence of those elements. We estimate the mass numbers A of those superheavy elements in the liquid drop model if we assume that they are stable versus β. The results thus obtained agree with the shell-model ones within 4 % approximation. We propose if the evidence of superheavies is confirmed to give the name of madagascarium to one of them (Z=126) by similitude with francium, lutetium, polonium, berkelium, americium, europium, ...Their evidence was first conjectured in Madagascar micas in 1976. Superheavy nuclei are to be distinguished from hypernuclei.
[fr]Depuis 1976, on parle dans les milieux specialises et dans les journaux de l'existence probable d'elements superlourds. Apres avoir rappele les elements construits de fa?on artificielle dont les numeros atomiques Z vont de Z=95 (l'americium) a Z=107(fabrique en 1976), nous considerons les elements superlourds dont les numeros Z depassent 110 et dont des chercheurs americains en etudiant les halos geants dans le micas de Madagascar recoltes dans la region de Fort-Dauphin (Region du Haut Mandrare) semblent avoir mis en evidence l'existence. Ce seraient donc des elements naturels, de numeros atomiques Z=114, 115, 116, 124, 125, 126, puisse etre acceptee avec une certaine confiance. Disons qu'elle est toutefois contestee par d'autres groupes de chercheurs. Les recherches continuent de fa?on active. Nous estimons les nombres de masse A de ces superlourds dans le cadre du modele de la goute liquide en supposant la stabilite par rapport a la desintegration β. Les resultats concordent avec ceux du modele en couches dans l'approximation de 4 pour cent. Pour eviter toute confusion possible, nous considerons de fa?on breve les hypernoyaux. Ces derniers sont batis en rempla?ant un ou plusieurs nucleons du nouyau par les hyperons. Nous proposons dans le cas ou ces superlourds existaient de donner au moins l'un d'eux (Z=126) le nom de madagascarium par similitude au francium, lutecium polonium, berkelium, americium, europium, ...d'autant plus que leur existence naturelle aurait ete soup?onnee pour la premiere fois dans les micas de Madagascar.
[en] Tracks lost during the etching process of an external mica detector can be quantified if the minimum retained dip angle, θc, is known. Using confocal microscopy a series of planar images were obtained from different depths to a resolution of 0.2 μm. A method of identifying θc from the modal dip angle is described. An image analysis system allows reconstruction of the tracks in three dimensions and track lengths measured for annealing studies. A robust algorithm is shown to identify tracks present within noisy images and identifies the modal dip angle in agreement with earlier results. This is the first time a fully automated system has been achieved. The results are encouraging but suggest that it might be necessary to decorate the tracks before imaging to obtain a better result. (Copyright (c) 1999 Elsevier Science B.V., Amsterdam. All rights reserved.)
[en] It is well known that cationic and anionic substitutions in triooctahedral micas influence considerably their ability to vermiculitization. Up to now the only information concerning this property was obtained on a statistical basis. Using wide line nuclear magnetic resonance, it has been possible to show a partial local ordering of these substitutions which must have a considerable influence on the resistance of micas to alteration
[fr]L'examen par resonance magnetique nucleaire des spectres du fluor et du proton sur un ensemble de micas trioctaedriques a mis en evidence le caractere partiellement ordonne des substitutions anioniques et cationiques dans la couche octaedrique