Solid state chemistry of iron-nickel phosphides and the composition of the Earth core

Malcolm F. Nicol, University Nevada Las Vegas, DMR 0521179

Seismic evidence shows that the density of Earth core is about 10% lower than density of elemental iron at the core PT conditions, suggesting presence of light elements alloyed with iron in the outer core region. Based on cosmochemical abundances and occurrence in meteoritic samples, prime candidates considered for light element core constituents are S, Si, C, P, N, O and H. (Fe1-xNix)2P occurs in meteorites in two polymorphic forms: hexagonal barringerite and orthorhombic allabogdanite. Presence of Ni and Co in natural allabogdanite has been thought to be responsible for the stability of the orthorohombic phase.

Our microdiffracrtion experiments with synthetic Fe2P single-crystals reveal that occurrence of orthorhombic barringerite in meteoritic samples is not caused by presence of Ni, but rather triggered by heating at high pressure, experienced either inside the meteorite parent body, or during the delivery to Earth, placing important constraints on the thermodynamic history of the meteorite and providing clues as to the size of the parent body [1]. (Fe1-xNix)2P, if present in the Earth’s core, would have slight density decreasing effect. We also found Ni2P to melt incongruently at pressure between 5 and 40 GPa, leading to formation of pyrite-type cubic NiP2 and Ni-rich Ni-P glass [2]. The pyrite-type NiP2 could form in the cores of Mars and Mercury.

[1] Dera, P. et al. (2008) Geophys. Res. Lett., 35, L10301.[2] Dera, P. et al. (2008), J. Geophys. Res. In review

6.5 GPa after heatingNi2PNiP2

Ne

2deg

Barringerite-allabogdanite transition in Fe2P

Incongruent melting in Ni2P

Development of six new, synchrotron-based approaches for structure determination of microcrystals

Malcolm F. Nicol, University Nevada Las Vegas, DMR 0521179

The crystal structure determination methods for synchrotron microcrystallography, with special emphasis on high-pressure experiments, developed within this project have been thoroughly tested and successfully applied to several important materials science, solid state chemistry and mineral physics problems (iron-nickel phosphides, boron carbide, graphite, corundum-type transition metal oxides, iron-magnesium carbonates).

The new methods allow full determination of crystal structure, as well as strain/stress analysis for micrograins (10 micrometers and less) of crystalline solids and extend the pressure range of single-crystal methods beyond 1 megabar (1 million atmospheres).

The newly built synchrotron setups at sectors 13 and 16, Advanced Photon Source Laboratory (APS), as well as custom data collection and analysis software have been designed and commissioned, with focus on automation, simplicity and user friendliness to make microdiffraction experiments accessible to large community of users.

The details and principles of operation of the new synchrotron setups, as well as the capabilities and operation of the data collection and analysis software have been discussed at a workshop in Colorado Springs, during the 2008 annual COMPRES meeting. To further disseminate the results of our development to the international community two similar workshops are planned for the 2009, at the International School of Crystallography in Erice, Italy and AIRAPT meeting in Tokyo, Japan.

Two microcrystals of Cr2O3 at 60 GPa and a corresponding single-crystal diffraction pattern

Energy dispersive SXD setup at Sector 16, APS