Optical Microscopy of Meteoritic Metal

METAL SCIENCE- Development of Chemical Zoning Meteorite Thermal Histories | Expected Phase Transformations for Slow Cooling | Development of Chemical Zoning | Alternative Reaction Paths for Slow Cooling | Reaction Path for Fast Cooling The equilibrium compositions of taenite and kamacite change continuously during cooling.  The equilibrium kamacite Ni concentration, defined by the a/(g+a) boundary, increases from about 4 wt. % at 700 C to about 6 wt. % at 460 C.  The a/(g+a) boundary changes slope at temperatures below 460 C, causing kamacite Ni concentrations to decrease slightly during additional cooling. Over the same total temperature range, the equilibrium taenite composition becomes highly Ni-enriched from the starting composition of 10 wt. % at 700 C to more than 50 wt. % at temperatures below 350 C. If meteoritic taenite and kamacite had maintained total equilibrium throughout cooling, these phases would end up homogeneous with compositions defined by the low temperature portion of Fe-Ni phase diagram.  In reality, electron microprobe traverses across taenite and kamacite show that both minerals are chemically zoned.  The most marked zoning occurs in taenite, which has high Ni at the taenite rim (~50 wt.%) and lower Ni at the taenite core (this Ni distribution is referred to as a M-shaped profile).  Kamacite exhibits less zoning, with a slight Ni depletion at the kamacite rim.    INSERT PHOTO!   Nickel distribution across taenite and kamacite, showing the M-shaped Ni profile of taenite and the Ni dip in kamacite. To understand the development of taenite and kamacite zoning, we again consider the behavior of an FeNi10 alloy during cooling.  However, we now examine the important effect of sluggish diffusion kinetics on the Ni distributions.  The parent taenite will cool into the taenite + kamacite field at 700 C, where kamacite is expected to nucleate and grow.  As the taenite + kamacite assemblage cools, the taenite Ni concentration will increase along the g/(g+a) solvus and the kamacite Ni will change along the a/(g + a) solvus.  At high temperatures volume diffusion is sufficiently fast that taenite and kamacite remain homogeneous with compositions defined by the equilibrium tie-lines.  However, diffusion rates decrease exponentially as the temperature drops.  We assume that taenite and kamacite maintain equilibrium at their mutual interface throughout cooling, but volume diffusion eventually becomes so slow that the taenite and kamacite interiors fail to equilibrate at each new temperature. Taenite thus develops its M-shaped Ni profile with the high Ni equilibrium composition at the taenite/kamacite interface, and the lower Ni disequilibrium composition at the taenite core.  The Ni dip observed in kamacite at its interface with taenite reflects the reversal of the a/(g + a) slope at temperatures below 500 C. The shape of the taenite M-profile is a function of the metal composition (which determines the kamacite precipitation temperature and the diffusion rates) and the rate of meteorite cooling.  Several workers have used computer simulations of the diffusion-controlled g ® g + a reaction to model the development of taenite Ni profiles.  The cooling rates of meteorites can be deduced by matching the taenite M-profiles measured in meteorites to the taenite M-profiles generated by computer simulation.  This technique typically yields cooling rates on the order of 1-100 degrees per million years.