3
$\begingroup$

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?

$\endgroup$
2
$\begingroup$

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:

enter image description here

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.

$\endgroup$
-1
$\begingroup$

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. ]

$\endgroup$
  • 1
    $\begingroup$ The Widmanstattten pattern is due to exolution of taenite and kamacite. $\endgroup$ – bon Feb 13 '18 at 8:24
  • $\begingroup$ Taenite and kamacite are the mineral names for austenite and ferrite ; aka , FCC and BCC. $\endgroup$ – blacksmith37 Feb 14 '18 at 21:07

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.