Hello Rodrigo and list, Vagn Buchwald's "Handbook of Iron Meteorites" (1976) has a rather detailed discussion of nickel content on pages 76-77 with several histograms showing the frequency distribution of percentage nickel content. He states, "In iron meteorites, nickel varies from a minimum of 5.1-5.3% (Tombigbee River, Auburn, Holland's Store) to a maximum of 35% (Santa Catharina). If Oktibbeha County, page 947, is confirmed as an independent meteorite, the nickel maximum increases to about 60%."
In Buchwald's histograms, the most nickel-poor iron group, the IIA hexahedrites, show a bell-shaped distribution which drops off very sharply below 5.3%. Of course, Buchwald's data set is now 26 years old and meteoritic science has not stood still in the years since his masterpiece was published. Still, the only irons which I am aware of with nickel content below the 5.1% lower bound which Buchwald claims are the so-called Bellsbank trio, which includes Bellsbank, La Primitiva, and Tombigbee River. These are classified as chemically and structurally anomalous (UNGR) but appear to be related to the IIAB irons. A nickel content of 4.3% was determined for Tombigbee River in 1973, a fact which Buchwald was apparently unaware of at the time his Handbook was published. A paper published in 1984 by Malvin, Zong, and Wasson presents a hypothesis for a process which may have allowed the Bellsbank trio to form under very exceptional circumstances in isolated pockets of phosphorus-rich melt in the IIAB core, with their anomalous composition as a result. This paper is available online at http://adsabs.harvard.edu/abstract_service.html (search for "Bellsbank"). All things considered, it seems much more likely that an iron specimen with nickel content below around 4% is manmade rather than a representative of an as yet undiscovered group of iron meteorites. Buchwald points out that nickel is extremely siderophile ("iron loving"), even more so than iron itself. Processes of fractionation in the metal phase allow for a rather wide range of iron/nickel ratios, but the laws of physical chemistry seem to draw the line at around 4% as a lower bound. Best wishes to all, Piper Standard disclaimer: I am not now and have never been a professional meteoriticist or geochemist, and any personal opinions not supported by references to authorities more qualified than myself should be taken with the usual grain of salt. ______________________________________________ Meteorite-list mailing list [EMAIL PROTECTED] http://www.pairlist.net/mailman/listinfo/meteorite-list
