I have recently been learning and researching about meteorite types and classification. Pallasites are mainly composed of a 'solid-solution' of olivine crystals and an iron-nickel alloy. My question is: what is the phase of the iron-nickel alloy, in a body-centered cubic crystal structure (alpha-iron, and hexahedrites) or face-centered cubic?
First, a correction. A solid solution is between different compositions in the same phase. For example, olivine is a solid solution of forsterite (Mg2SiO4) and fayalite (Fe2SiO4).
There are two common iron–nickel alloys found in meteorites (pallasites and others): kamacite and taenite. They are commonly intergrown into lamallae called "Widmanstätten pattern". It looks like this:
Kamacite has up to about 10% nickel, whereas taenite can get up to ~60% nickel, but commonly found up to 30%.
Kamacite has the space group Im3m, making it hexoctahedral body-centered. Taenite has the space group Fm3m, making it hexoctahedral face-centered.
The metallic part ( essentially all two elements= Iron + nickel) is two phases mixed together : Body Centered Cubic - magnetic, and Face Centered Cubic - nonmagnetic. So the answer to your question is that it is both . Iron ( BCC) starts to form FCC when Ni exceeds 6 %. The proverbial 18 - 8 stainless steel is 8 % Ni and generally non magnetic austenite phase. My educated guess is the classic Widmidststten pattern is ferrite ( BCC ) grains/crystals forming from the original austenite ( FCC ) grains/crystals. But it is not that simple as when hot ( 1500 F ) it is 100 % austenite ( FCC ). Further complication is the austenite ( FCC , nickel rich) makes crystal twins . The twins can also give the Widmanstratten structure. And more complicated because temperature history can also affect the FCC / BCC morphology. [ Maybe I cheated but I searched meteors on the net and learned some but also found some mistakes in the references. ]