[meteorite-list] Question about nickel [nickel-poor iron meteorites]

From: Piper R.W. Hollier <piper_at_meteoritecentral.com>
Date: Thu Apr 22 10:00:06 2004
Message-ID: <5.1.0.14.2.20020709105021.02038600_at_pop.xs4all.nl>

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.
Received on Tue 09 Jul 2002 05:19:22 AM PDT