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Date: 16 August 2018
Phosphorus under high pressure  

Topic Name: Phosphorus under high pressure
Category: Geo sciences & technology
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Research persons: Brownish Fabrice and his team

Location: Henri Horn Department of seismology CNRS-IPG Paris University Paris VII, France


Phosphorus under high pressure

Phosphorus is always surrounded by 4 oxygen atoms in the mineral and organic forms (PO4). For the first time, the element phosphorus related to six oxygen atoms (PO6) was obtained in experiments, by compressing a mixture of quartz and aluminium phosphates under conditions of very high temperature and pressure. The researchers of the Laboratory of geology (CNRS/Ecole normal higher of Paris) and of the synchrotron Sun, authors of this work, think that this new configuration of phosphorus could exist in the terrestrial coat .
Phosphorus, chemical element essential to the animal and vegetable life, is present in all the cells of the alive one. In the organic world (ARN, ADN) as in the mineral world (Os, teeth), phosphorus is always surrounded by four oxygen atoms (grouping PO4). Although there are only 0,1 percent in phosphorus weight in the earth's crust, the phosphatic minerals are important by their capacity to incorporate tracer elements of the formation of the Earth (rare earths) and indicators of the age of the rocks (uranium, thorium). The geochemical behavior of phosphorus to the depths of the terrestrial coat remained unknown. This element in silicates or form are its built-in own minerals, the phosphates, as on the surface of the sphere? Is there phosphorus in the metal of the terrestrial core? Like the silicon of which the number of first neighbors oxygenates master key from 4 to 6 in the zone of transition, it was imagined theoretically, by the calculation of molecular dynamics, which a phosphorus atom can also be surrounded by six atoms of oxygen (PO6) but with much greater depth. Brownish Fabrice and his team of the laboratory of Geology of CNRS and the higher Teacher training school, in collaboration with a group of researchers in physics of materials of the synchrotron SUN and Geodynamics Research Center of the university of Ehime in Japan, thought that, although phosphorus is essential with the life on the surface of the sphere, this element could concentrate primarily in the deep envelopes of the Earth. In that, they are based on the phosphorus contents of the meteorites most primitive of the solar system whose composition is supposed to represent that of the Earth as a whole. This assumption is corroborated by the contents of almost 0,5 percent of weight of oxide of phosphorus, P2O5, analyzed in minerals resulting from the coat lower and brought back to surface in the form of inclusions in rare diamonds Thus the researchers imagined that phosphorus could be present, in form PO6, in the structure of the silicates which constitute the interior of our planet. To check their assumption, they reproduced in experiments the extreme conditions of pressure (200 000 atmospheres) and of temperature (1600°C) which reign in the depths of the Earth at the base of the higher coat (Figure 1). Instead of compressing phosphates, they used a mixture of quartz (SiO2 with SiO4 groupings) and of aluminium phosphate (AlPO4), like starting product. Under the effect of the pressure and temperature, quartz was transformed into stishovite (SiO6 groupings). The produced grains of stishovite incorporated, during their growth, of the considerable quantities of phosphorus (about 1 percent in phosphorus oxide weight, P2O5). By analyzing a grain of stishovite of a few tens of microns (Figure 2) per microphone-spectroscopy of absorption to the threshold of phosphorus by using the synchrotron radiation, the researchers highlighted, for the first time, the shape PO6 of phosphorus…This result will not only make it possible to better include/understand the mineralogy of the deep coat but also the exchanges chemical between the core and the terrestrial coat and the role of the zones of subduction in the transfer of elements towards the internal Earth.
References:Brunet, F., Flank, A. - Mr., Itié, J. - P., Irifune, T., Lagarde, P. (2007) Experimental obviousness of six-fold oxygen coordination for phosphorus. American Mineralogist, 92, 989-993. Enquiring contact: BROWNISH Fabrice Laboratory of geology (CNRS/ENS) Mél: brunet@geologie.ens.fr Tel.: 01 44 32 22 74
In The Images-
Figure 1. The eight tungsten carbide cubes which transmit the pressure on the sample (device multi-anvil) after they were left the jaws of a press of 2000 tons. The material grisâtre, very deformed, ensures the role of joint in this device so that the assembly containing the sample is not entirely extruded under the effect of the pressure. The properly spoken sample is contained in a cylindrical platinum capsule 1,5 mms in diameter. The two wire which leave the cubes are the bits of the thermocouple for in-situ measurement of temperature. © Brownish Fabrice, CNRS 2007 (this image is available near the photographic library of CNRS, 01 45 07 57 90, phototheque@cnrs-bellevue.fr)
Figure 2. Image in electronic scan microscopy of a polished grain of stishovite (18 GPa and 1873 K). The fractured grains have as a AlPO4 composition whereas the analysis of the crystals of stishovite (points of impact of the electronic beam used for the analysis visible on the stereotype) shows nearly 1 percent in phosphorus oxide weight. This phosphorus is in an octahedral oxygen environment (6 close close relations oxygenates) as shows it the data of spectroscopy of absorption (XANES). © Brownish Fabrice, CNRS 2007 (this image is available near the photographic library of CNRS, 01 45 07 57 90, phototheque@cnrs-bellevue.fr)

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