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Title:
High-pressure behavior of iron-nickel-cobalt phosphides and its implications for meteorites and planetary cores
Authors:
Dera, P.; Lavina, B.; Borkowski, L. A.; Downs, R. T.; Prewitt, C. T.; Prakapenka, V.; Rivers, M. L.; Sutton, S.; Boctor, N.
Affiliation:
AA(The University of Chicago, Consortium for Advanced Radiation Sources, Argonne National Lab, Bldg. 434A, Argonne, IL 60439, United States ; ), AB(The University of Chicago, Consortium for Advanced Radiation Sources, Argonne National Lab, Bldg. 434A, Argonne, IL 60439, United States ; ), AC(University of Nevada, HiPSEC, Argonne National Lab, Bldg. 434A, Argonne, IL 60439, United States ; ), AD(University of Arizona, Gepartment of Geosciences, 209 Gould-Simpson Building, Tucson, AZ 85721, United States ; ), AE(University of Arizona, Gepartment of Geosciences, 209 Gould-Simpson Building, Tucson, AZ 85721, United States ; ), AF(The University of Chicago, Consortium for Advanced Radiation Sources, Argonne National Lab, Bldg. 434A, Argonne, IL 60439, United States ; ), AG(The University of Chicago, Consortium for Advanced Radiation Sources, Argonne National Lab, Bldg. 434A, Argonne, IL 60439, United States ; ), AH(The University of Chicago, Consortium for Advanced Radiation Sources, Argonne National Lab, Bldg. 434A, Argonne, IL 60439, United States ; ), AI(Carnegie Institution of Carnegie Institution Carnegie Inst. of Washington, Geophysical Lab, 5251 Broad Branch Rd., Washington, DC 20015, United States ; )
Publication:
American Geophysical Union, Fall Meeting 2008, abstract #MR13B-1706
Publication Date:
12/2008
Origin:
AGU
AGU Keywords:
1042 Mineral and crystal chemistry (3620), 3662 Meteorite mineralogy and petrology (1028, 6240), 3672 Planetary mineralogy and petrology (5410), 3914 Electrical properties, 3924 High-pressure behavior
Abstract Copyright:
(c) 2008: American Geophysical Union
Bibliographic Code:
2008AGUFMMR13B1706D

Abstract

Minerals with composition (Fe,Ni)xP, are rare, but important accessory phases present in iron and chondrite meteorites. The occurrence of these minerals in meteoritic samples is believed to originate either from the equilibrium condensation of protoplanetary materials taking place in solar nebulae or from crystallization processes in the cores of parent bodies. Fe-Ni phosphides are considered an important candidate for a minor phase present in Earth's core, and at least partially responsible for the observed core density deficit with respect to pure Fe. We report results of high-pressure high-temperature single-crystal X- ray diffraction experiments with end-members belonging to the (Fe,Ni,Co)2P family, including Fe2P, Ni2P and Co2P. A new phase transition to the Co2Si-type structure (allabogdanite) has been found in Fe2P barringerite at 8.0 GPa, upon heating. The high-pressure phase can be quenched metastably to ambient conditions and then, if heated again, it transforms back to barringerite. Ni2P barringerite does not undergo transformation to allabogdanite structure up to 50 GPa, but instead exhibits incongruent melting with formation of pyrite-type NiP2 and Ni-P glass. Our results indicate that the presence of allabogdanite in meteoritic samples places two important constraints on the thermodynamic history of the meteorite. First, it imposes a minimum pressure and temperature for the formation of the Fe2P, and additionally rules out any higher temperature low pressure alterations. If present in the Earth's core, Fe2P will have the allabogdanite rather than the barringerite structure. Crystal chemical trends in the compressibility of (Fe,Ni,Co)2P minerals, as well as polymorphic transition paths are analyzed in the context of Earth and planetary core composition and properties.
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