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Widmanstatten Patterns


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Hi Guys-

Just wanted to know what the relationship is between the fineness - coarseness of the widmanstatten patterns and the % nickel. Does more nickel produce a coarser pattern like Campo or Canyon Diablo? Or is it the other way around? I know this topic was discussed a few years ago but can't seem to find it anymore.

Steve

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This should answer your question:

http://www.arizonaskiesmeteorites.com/Widmanstatten/

Keep in mind though that iron meteorites can show more than one etch pattern. For example, I have a Toluca that shows two separate etch patterns including plessitic needles. So this makes things a little more complicated since the meteorite should be rather homogenous with its nickel content. Therefore, this brings up the question of how plessitic needles can form within other iron-nickel crystals.

Here is some information on the plessitic structures in Toluca.

http://www.minersoc.org/pages/Archive-M ... 44-413.pdf

This is the Toluca after the etch. Unfortunately it was poorly cut so I could not get a real flat surface, but the etch still came out good.


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post-26752-0-16281700-1368478507_thumb.j

post-26752-0-61592300-1368478510_thumb.j

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Steve,

The thickness of the bands in the W. pattern, fine or coarse, is a result of how long it took the parent body to cool, not necessarily a relationship with regards to nickel % (though nickel % will determine what types of crystal structures form). The coarser a crystal structure is, the larger the crystals, and thus the longer it took to cool.

Edited by Mikestang
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and remember...you won't get a wid-pattern with ataxites...too much nickel

fred

Ataxites can be nickel rich or nickel poor.

High nickel content precludes kamacite/taenite from exsolving in large quantities, and the iron mass forms as almost entirely kamacite, thus no W. pattern develops. In nickel rich ataxites, taenite still manages to form in very small quantities, and you are left with a plessite-ish intergrowth of the two (which is why etching ataxites will still, on occasion, reveal crystalline structure).

Nickel poor ataxites tend to be granular in structure.

Besides the Toluca example above, I invite you to read about the Old Woman meteorite (H. Plotkin et al. 2012) and the New Baltimore meteorite (Buchwald 1975), as they each display both hexahedrite and octahedrite structures and classify as a type IIAB and ungrouped iron, respectively.

Edited by Mikestang
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Therefore, this brings up the question of how plessitic needles can form within other iron-nickel crystals.

As pointed out by H. Plotkin (2012), "...we now recognize that transitional textural types do occur in close proximity, and can unravel the unique combinations of chemisty, nucleation, and cooling history that produce these textures."

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So this makes things a little more complicated since the meteorite should be rather homogenous with its nickel content.

One more quick comment - while the bulk nickel content of an iron will generally not vary all that much from specimen to specimen, locally within a piece there can be rather large variations in compostion.

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I remember reading about it once, but can't find where. I think it's something like 38%. I think I know who to ask, though, so lemme reach out to him and get back to you.

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Well I was way off in my recollection of the % Nickel, here's the info provided to me by my friend:

METCAT Bernd 8390 entries

IndexName Class %Ni

Cheder IID 72,4

Oktibbeha County IAB 60,1

Lafayette (iron) ATAX? 59,4

Dermbach ATAX 42,1

Santa Catharina ATAX 33,8

Tishomingo ATAX 32,5

Twin City ATAX 30

Lime Creek ATAX 29,5

Willow Grove ATAX 27,9

Barbianello IAB-IIICD 27,1

San Cristobal IAB 25,7

Wedderburn IIICD 23,2

Freda IIICD 23

Onello ATAX 21,7

Britstown IAB 19,5

Morradal ATAX 19,2

Tinnie IVB 18,4

Kofa IAB 18,27

Czestochowa Rakow I 18,25

Föllinge IAB-sLH 18,13

Warburton Range IVB 18

Santa Clara IVB 17,9

South Byron ATAX 17,8

Babb's Mill (Troost's Iron) ATAX 17,7

Dayton IAB-sLH 17,6

Weaver Mountains IVB 17,4

Ternera IVB 17,2

Skookun IVB 17,13

Tawallah Valley IVB 17

Botetourt County ATAX 17

Edited by Mikestang
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Speaking of kamacite, taenite, Ni here is one coming up for classification and submission...which once again is different! Note the Ni at about 30.10 in the taenite and about 7.18 in the kamacite!

The main phase present is kamacite Fe=91.98 ±0.16, Ni=7.18 ±0.28, Co=0.45 ±0.02 (wt%) n=12, followed by taenite Fe=69.12 ±1.72, Ni=30.10 ±1.71, Co=0.13 ±0.03 (wt%) n=7, with minor scheibersite.

At this time, no one has figured this one out....yet. it will happen but it is not what we thought it would be!

Jim

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