[meteorite-list] ATQ: Taenite and Kamacite and Widmanstatten Formation

From: Mr EMan <mstreman53_at_meteoritecentral.com>
Date: Sat, 3 Jan 2009 22:16:13 -0800 (PST)
Message-ID: <899795.63657.qm_at_web55205.mail.re4.yahoo.com>

--- On Sat, 1/3/09, Pete Shugar wrote:
When looking at?the irons, the Widmenstatten pattern and the thickness tell whether it is a Coarsest?Octahedrite? or just an Octahedrite.
Q: And just for giggles which are the iron and which are the nickle bands?
Pete

A: Actually BOTH are iron and nickel bands! The two main minerals accounting for "Widmanstatten" patterns are the minerals taenite(P: TAA-nite) and kamacite(aka gamma iron in foundry parlance). They are both orthorhombic iron-nickel crystals, however, taenite is the face-centered form and kamacite is the body-centered version. That difference allows one mineral to contain more atoms than the other.

Between the Widmanstatten bands are schreibersite bands(Fe,Ni)P and very fine grained non crystalized bands of taenite and kamacite molecules forming the mineral plessite. This is believed to be the "left-over stock" of migrating molecules when the molten metal froze in the process of moving toward their respective plates-- more later regarding coarseness of the bands.

Proportions in the two minerals by weight and atom ratios:
taenite: 79.19 % Fe 20.81 % Ni 8:6 max
kamacite: 89.54 % Fe 10.46 % Ni 8:1 max
Note: these ratios and counts are variable owing to several reasons not discussed here but close enough for discussion. Just remember taenite has more room for nickel atoms and kamacite has little room without forcing things too far. Diagrams of these molecular/ crystal models can be seen at:
<http://en.wikipedia.org/wiki/Crystal_structure>

A face-centered "unit cell" has 14 atoms while a body-centered unit cell has 9. Ergo, a face-centered crystal has room for 5 more atoms and according to the stacking rules those are usually nickel. Overall nickel ratios vary and usually, this translates to roughly 25-30% nickel content over all in taenite. The body-centered kamacite stack favors just one: approximately 12%.

Nickel is slightly more resistant to acid than is iron so the mineral taenite doesn't etch as fast as kamacite thus permitting the inducement of the Widmanstatten pattern. Taenite is the taller band.

Coarseness is an indication of the length of time the crystal growing process was allowed to run within the body of the asteroid. Growth of both mineral plates occurs so long as the temperature remains above 400*C and below 900*C Generally this process is measured in declines of tens of degrees C per million years.

This explains why terrestrial iron can only contain kamacite or plessite and can't show Widmanstatten patterns. There is simply not enough time or incentive for the separate minerals to migrate to their respective corners in human timeframes. This is also why only meteorites can contain Widmanstatten patterns and can't be faked with any known process.

Note 1: This discussion pertains to Octahederites which are medium nickel to iron ratio containing meteorite types. Many points may not apply to to high nickle ratio, taenite-containing, Ataxites-- Nor low nickel ratio, kamacite-bearing, Hexahederites. The crystal forms and ratios don't always hold up at each end of the process.

Note 2:: The occurrence of the two types of iron crystals occurs under a band of high temperature and pressure conditions. Taenite will revert to kamacite if the process were allowed to run to the end at lower temperatures. Technically solidified taenite is unstable and will revert to tetrataenite over "geological" timeframes. The transport processes which form kamacite over taenite are not the same and are driven by several factors and trace elements and phosphorous. Phosphorous bearing Schreibersite seems to form bands at the taenite plate surface and may act as a catalyst that forces its formation. It could also act as a reverse throttle making the atom sorting process slow beyond imagination.

The generalized discussion above is close enough for understanding the basic process; it is the normal state of things but, as with most things in life, there are many situation-specific exceptions and nuances. I encourage anyone interested to take the plunge and explore them all.
Received on Sun 04 Jan 2009 01:16:13 AM PST