Re: [meteorite-list] What is and isn't a Widmanstatten Pattern was Cooling rates
I think there are some misconceptions here, although this is not my specialty. Most of the metallic minerals in iron meteorites are described as "alloys" in that they are composed of various metals combined together. These alloys have specific structures, e.g., the metal atoms in kamacite are arranged in a body-centered cubic structure and those in taenite are face-centered cubic. The minerals Kamacite and taenite are solid solutions of mainly Fe and Ni which can have a range of compositions without altering the basic structure. Tetrataenite is another alloy, but this time with a fixed composition (FeNi) and an ordered structure. Formation of the Widmanstatten structure is pretty well understood. It does not happen as the metal cools from the liquid state and solidifies. That process leads to the formation of just taenite. Only when the alloy cools to much lower temperature, after it is completely solid, can the Widmanstatten pattern form. High pressures are not involved. Goldstein and coworkers have shown that the process is controlled by the Fe-Ni-P phase diagram. Depending on the exact composition of the alloy, a variety of phase transformations take place over a range of temperatures, ultimately leading to the formation of kamacite and taenite. Composition and cooling rate play roles in determining in the structures we now observe. You can read about it in: http://articles.adsabs.harvard.edu/full/2005M%26PS...40..239Y jeff Mr EMan wrote: We had a metallurgist on the list a few years back that insisted Widmanstatten patterns were found everywhere and posted some micrographs supporting his assertion. As I recall he got very ill with us when we pointed out why, what he had photos of, weren't Widmanstatten patterns. It was focused on a physical "casual" similarity not "causal" chemistry. Once again Widmanstatten patterns aren't stress fractures nor alloy specific patterns. I further assert that metal in meteorites is NOT an alloy in that the nickel is in a specific locus within a molecule. It is therefore not a mixture but a compound, chemically speaking. Widmanstatten patterns are a cross-sectional view of crystal latices that result from the migration of nickel atoms over eons into two distinct unusual, zoned, crystalline arrangements. Bandwidth is actually plate thickness. The migration is chemically driven while the metal is molten and only occurs in a specific range of temperatures. This is a subtle but distinct difference. This migration may even be a molecule by molecule transfer of nickel atoms which takes millions of years to clear out a 3mm band. This is to say a nickel atom may move in one side of a molecule and forces the central nickel atom to the face and lacking stability is ejected out the other side--maybe not, as the actual displacement/sorting is still an enigma. The nickel iron content may assemble from a single form as it accretes and represent a move to homogeneity interupted when the mass ran out of thermal energy. It may all start out as taenite and part of it converts to kamacite or vice versa. Who really knows? I fully believe collisions would impede if not stop the process-- not speed it up. It is easy and natural to try to infer a similar pattern might be from a similar process but the only similarity is in low contrast photographs when the scale is ignored. Elton --- On Sun, 9/6/09, E.P. Grondine wrote: From: E.P. Grondine Subject: Re: [meteorite-list] Cooling rates To: meteorite-list@meteoritecentral.com, "Steve Dunklee" Date: Sunday, September 6, 2009, 1:47 PM Hi Steve, all - I don't think they're due to repeated collisions. Suppose that we have molten iron/nickle under incredible compression, which is then almost instantaneously released. 250 parent bodies seems like a lot. Perhaps instead there was more differentiation within fewer parent bodies. Ed __ http://www.meteoritecentral.com Meteorite-list mailing list Meteorite-list@meteoritecentral.com http://six.pairlist.net/mailman/listinfo/meteorite-list -- Dr. Jeffrey N. Grossman phone: (703) 648-6184 US Geological Survey fax: (703) 648-6383 954 National Center Reston, VA 20192, USA __ http://www.meteoritecentral.com Meteorite-list mailing list Meteorite-list@meteoritecentral.com http://six.pairlist.net/mailman/listinfo/meteorite-list __ http://www.meteoritecentral.com Meteorite-list mailing list Meteorite-list@meteoritecentral.com http://six.pairlist.net/mailman/listinfo/meteorite-list
Re: [meteorite-list] What is and isn't a Widmanstatten Pattern was Cooling rates
I'd like to correct what I said two weeks back about alloys. I've talked with a specialist (Joe Goldstein) and he clarified the terminology for me as material scientists would use it. An "alloy" refers to a batch of metal containing more than one element (only one of which needs to be a metallic element). The term carries no implications about how the mixture formed or what its structure is. The term "alloy" should not be used to refer to a specific phase that crystallizes from an alloy or is present in the alloy. Taenite and kamacite are not alloys. They are phases (minerals). Therefore, hundreds of websites (including some professional ones) use the term incorrectly. An iron meteorite IS an alloy, even if it only contains one phase like taenite. Alloys can and often are composed of crystalline phases. Jeff Mr EMan wrote: --- On Mon, 9/7/09, Jeff Grossman wrote: <"alloys" in that they are composed of various metals combined together.>> I understood the distinction was that the Fe Ni formed a "chemical compound" not merely a mixture like copper and tin to make brass but even brass can form crystalline plates so that may be a bad example. It was my understanding that were it not for the mineral structure The Fe-Ni would be called an alloy. Agreed that it is frequently discussed in terms of alloy. <> I stand corrected, 30 years is a lot of facts to keep in just one's head. I did recall correctly that there is a temperature range and below which all translocation stops. Seems off the top of my head it is 800°C. <>. I have long suspected that phosphorus was a key component in the process--likely as a catalyst. The Schreibersite seems to exist largely at the boundaries in thin laminae even surrounding trolite nodules. I look forward to reading Goldstein's paper. Thanks again, Elton -- Dr. Jeffrey N. Grossman phone: (703) 648-6184 US Geological Survey fax: (703) 648-6383 954 National Center Reston, VA 20192, USA __ http://www.meteoritecentral.com Meteorite-list mailing list Meteorite-list@meteoritecentral.com http://six.pairlist.net/mailman/listinfo/meteorite-list
Re: [meteorite-list] What is and isn't a Widmanstatten Pattern was Cooling rates
I guess once again as with CCDs my education is outdated. I see that any metal compound or mixture can be called an alloy. OR it has come into such common use the distinction between mixture and compound is obsolete when talking about metals. Elton --- On Tue, 9/8/09, Mr EMan wrote: > From: Mr EMan > Subject: Re: [meteorite-list] What is and isn't a Widmanstatten Pattern was > Cooling rates > To: "Meteorite-list" , "Jeff Grossman" > > Date: Tuesday, September 8, 2009, 6:38 PM > --- On Mon, 9/7/09, Jeff Grossman > > wrote: > < are described as > "alloys" in that they are composed of various metals > combined together.>> > > I understood the distinction was that the Fe Ni formed a > "chemical compound" not merely a mixture like copper and tin > to make brass but even brass can form crystalline plates so > that may be a bad example. It was my understanding that were > it not for the mineral structure The Fe-Ni would be called > an alloy. Agreed that it is frequently discussed in > terms of alloy. > > < liquid state and solidifies.>> > > I stand corrected, 30 years is a lot of facts to keep in > just one's head. I did recall correctly that there is a > temperature range and below which all translocation stops. > Seems off the top of my head it is 800°C. > > < is controlled by the Fe-Ni-P phase diagram>>. > > I have long suspected that phosphorus was a key component > in the process--likely as a catalyst. The > Schreibersite seems to exist largely at the boundaries in > thin laminae even surrounding trolite nodules. I look > forward to reading Goldstein's paper. > > Thanks again, > Elton > __ > http://www.meteoritecentral.com > Meteorite-list mailing list > Meteorite-list@meteoritecentral.com > http://six.pairlist.net/mailman/listinfo/meteorite-list > __ http://www.meteoritecentral.com Meteorite-list mailing list Meteorite-list@meteoritecentral.com http://six.pairlist.net/mailman/listinfo/meteorite-list
Re: [meteorite-list] What is and isn't a Widmanstatten Pattern was Cooling rates
--- On Mon, 9/7/09, Jeff Grossman wrote: <> I understood the distinction was that the Fe Ni formed a "chemical compound" not merely a mixture like copper and tin to make brass but even brass can form crystalline plates so that may be a bad example. It was my understanding that were it not for the mineral structure The Fe-Ni would be called an alloy. Agreed that it is frequently discussed in terms of alloy. <> I stand corrected, 30 years is a lot of facts to keep in just one's head. I did recall correctly that there is a temperature range and below which all translocation stops. Seems off the top of my head it is 800°C. <>. I have long suspected that phosphorus was a key component in the process--likely as a catalyst. The Schreibersite seems to exist largely at the boundaries in thin laminae even surrounding trolite nodules. I look forward to reading Goldstein's paper. Thanks again, Elton __ http://www.meteoritecentral.com Meteorite-list mailing list Meteorite-list@meteoritecentral.com http://six.pairlist.net/mailman/listinfo/meteorite-list
Re: [meteorite-list] What is and isn't a Widmanstatten Pattern was Cooling rates
Excellent article and info! Thank you Jeff! Tom --- "I don't like country music, but I don't mean to denigrate those who do. And for the people who like country music, denigrate means 'put down.'" -- Bob Newhart http://home.roadrunner.com/~kb2sms/ __ http://www.meteoritecentral.com Meteorite-list mailing list Meteorite-list@meteoritecentral.com http://six.pairlist.net/mailman/listinfo/meteorite-list
Re: [meteorite-list] What is and isn't a Widmanstatten Pattern was Cooling rates
I think there are some misconceptions here, although this is not my specialty. Most of the metallic minerals in iron meteorites are described as "alloys" in that they are composed of various metals combined together. These alloys have specific structures, e.g., the metal atoms in kamacite are arranged in a body-centered cubic structure and those in taenite are face-centered cubic. The minerals Kamacite and taenite are solid solutions of mainly Fe and Ni which can have a range of compositions without altering the basic structure. Tetrataenite is another alloy, but this time with a fixed composition (FeNi) and an ordered structure. Formation of the Widmanstatten structure is pretty well understood. It does not happen as the metal cools from the liquid state and solidifies. That process leads to the formation of just taenite. Only when the alloy cools to much lower temperature, after it is completely solid, can the Widmanstatten pattern form. High pressures are not involved. Goldstein and coworkers have shown that the process is controlled by the Fe-Ni-P phase diagram. Depending on the exact composition of the alloy, a variety of phase transformations take place over a range of temperatures, ultimately leading to the formation of kamacite and taenite. Composition and cooling rate play roles in determining in the structures we now observe. You can read about it in: http://articles.adsabs.harvard.edu/full/2005M%26PS...40..239Y jeff Mr EMan wrote: We had a metallurgist on the list a few years back that insisted Widmanstatten patterns were found everywhere and posted some micrographs supporting his assertion. As I recall he got very ill with us when we pointed out why, what he had photos of, weren't Widmanstatten patterns. It was focused on a physical "casual" similarity not "causal" chemistry. Once again Widmanstatten patterns aren't stress fractures nor alloy specific patterns. I further assert that metal in meteorites is NOT an alloy in that the nickel is in a specific locus within a molecule. It is therefore not a mixture but a compound, chemically speaking. Widmanstatten patterns are a cross-sectional view of crystal latices that result from the migration of nickel atoms over eons into two distinct unusual, zoned, crystalline arrangements. Bandwidth is actually plate thickness. The migration is chemically driven while the metal is molten and only occurs in a specific range of temperatures. This is a subtle but distinct difference. This migration may even be a molecule by molecule transfer of nickel atoms which takes millions of years to clear out a 3mm band. This is to say a nickel atom may move in one side of a molecule and forces the central nickel atom to the face and lacking stability is ejected out the other side--maybe not, as the actual displacement/sorting is still an enigma. The nickel iron content may assemble from a single form as it accretes and represent a move to homogeneity interupted when the mass ran out of thermal energy. It may all start out as taenite and part of it converts to kamacite or vice versa. Who really knows? I fully believe collisions would impede if not stop the process-- not speed it up. It is easy and natural to try to infer a similar pattern might be from a similar process but the only similarity is in low contrast photographs when the scale is ignored. Elton --- On Sun, 9/6/09, E.P. Grondine wrote: From: E.P. Grondine Subject: Re: [meteorite-list] Cooling rates To: meteorite-list@meteoritecentral.com, "Steve Dunklee" Date: Sunday, September 6, 2009, 1:47 PM Hi Steve, all - I don't think they're due to repeated collisions. Suppose that we have molten iron/nickle under incredible compression, which is then almost instantaneously released. 250 parent bodies seems like a lot. Perhaps instead there was more differentiation within fewer parent bodies. Ed __ http://www.meteoritecentral.com Meteorite-list mailing list Meteorite-list@meteoritecentral.com http://six.pairlist.net/mailman/listinfo/meteorite-list -- Dr. Jeffrey N. Grossman phone: (703) 648-6184 US Geological Survey fax: (703) 648-6383 954 National Center Reston, VA 20192, USA __ http://www.meteoritecentral.com Meteorite-list mailing list Meteorite-list@meteoritecentral.com http://six.pairlist.net/mailman/listinfo/meteorite-list
Re: [meteorite-list] What is and isn't a Widmanstatten Pattern was Cooling rates
We had a metallurgist on the list a few years back that insisted Widmanstatten patterns were found everywhere and posted some micrographs supporting his assertion. As I recall he got very ill with us when we pointed out why, what he had photos of, weren't Widmanstatten patterns. It was focused on a physical "casual" similarity not "causal" chemistry. Once again Widmanstatten patterns aren't stress fractures nor alloy specific patterns. I further assert that metal in meteorites is NOT an alloy in that the nickel is in a specific locus within a molecule. It is therefore not a mixture but a compound, chemically speaking. Widmanstatten patterns are a cross-sectional view of crystal latices that result from the migration of nickel atoms over eons into two distinct unusual, zoned, crystalline arrangements. Bandwidth is actually plate thickness. The migration is chemically driven while the metal is molten and only occurs in a specific range of temperatures. This is a subtle but distinct difference. This migration may even be a molecule by molecule transfer of nickel atoms which takes millions of years to clear out a 3mm band. This is to say a nickel atom may move in one side of a molecule and forces the central nickel atom to the face and lacking stability is ejected out the other side--maybe not, as the actual displacement/sorting is still an enigma. The nickel iron content may assemble from a single form as it accretes and represent a move to homogeneity interupted when the mass ran out of thermal energy. It may all start out as taenite and part of it converts to kamacite or vice versa. Who really knows? I fully believe collisions would impede if not stop the process-- not speed it up. It is easy and natural to try to infer a similar pattern might be from a similar process but the only similarity is in low contrast photographs when the scale is ignored. Elton --- On Sun, 9/6/09, E.P. Grondine wrote: > From: E.P. Grondine > Subject: Re: [meteorite-list] Cooling rates > To: meteorite-list@meteoritecentral.com, "Steve Dunklee" > > Date: Sunday, September 6, 2009, 1:47 PM > Hi Steve, all - > > I don't think they're due to repeated collisions. > > Suppose that we have molten iron/nickle under incredible > compression, which is then almost instantaneously released. > 250 parent bodies seems like a lot. Perhaps instead there > was more differentiation within fewer parent bodies. > > Ed __ http://www.meteoritecentral.com Meteorite-list mailing list Meteorite-list@meteoritecentral.com http://six.pairlist.net/mailman/listinfo/meteorite-list
