Classification of 49 Iron Meteorite Falls:
[22 Feb 2019, LCHj]
ABSTRACT
We provide here a cursory introduction to 49 witnessed meteorite falls that have been labelled as "Iron meteorites" [or, simply, "Irons"] by the Meteoritical Society. We provide only a very general overall view of these meteorites. However, our discussion includes some rather technical commentary on the problematics of describing these unusual meteorites in generic terms. A brief overall classificatory resume is also provided for these 49 meteorites.
The author hopes to utilize the information here in a future essay detailing a combined mineralogical-planetochemical perspective in a case study of the 25 recognized Kentucky meteorites — and their constituent minerals and mineralites. In the planned work the classificatory resume used here may be incorporated into a more complete Table as approximate.
The 49 Iron Falls [early 2019]
As of early 2019 [to be exact, 19 Feb 2019] the Meteoritical Society had listed exactly 1178 approved names for 1178 witnessed meteorite falls ["falls"]. 49 of these falls were classified as iron meteorites ["irons"]. The irons are dominated (volume-wise, mass-wise, mineralogically) by Fe-Ni metal — unoxidized iron invariably accompanied by (usually) lesser amounts of nickel, accessory Co, Cr, and phosphorus and trace amounts of Platinum Group [Pt-group] elements. Fe-Ni metal — an extra-terrestrially derived naturally unoxidized iron-dominated medley — is also referred to as (chemically) "free iron" or (mineralogically) "metallic iron." In the older mineralogical and meteoritical literature as Fe-Ni metal is referred to as "nickeliferous" iron. The isotopic ratios of the dominant "Iron Group" elements [Cr, Fe, Co, Ni] and the absolute and relative abundances of Pt-group elements in Fe-Ni metal are distinct from the rare natural terrestrial deposits of native "free" iron such as those found in Disko Island, Greenland and elsewhere on the planet Earth. [We hope to include these interesting terrestrial analogues in some later efforts.]
While iron meteorites are conventionally described as usually belonging to 13 iron meteorite "Groups" we note that only 8 groups are represented in the relatively small sample of 49 iron falls currently recognized. Furthermore, a significant number of these falls are incompletely classified.
Classification of Iron Falls
After an apparent meteorite has been discovered, its recognition is verified — usually accompanied by (nearly?) contemporaneous classification — and the meteorite's unique name and Classification are published on the Meteoritical Society's Meteoritical Bulletin Database. Currently, the great majority of iron meteorites are classified into 13 Iron meteorite "Chemical Groups" [here, more precisely, "Planetochemical" Groups]. It is conventionally thought — at least as a default starting hypothesis — that all meteorites in such groups may very well be derived from a single original homeworld ["Original Parent Body" or "OPB"]. Even more precisely, however, we can state that most meteoriticists believe that on the basis of what we know now, it appears physically implausible that the members of any of the presently recognized Iron groups could have been derived from any of the other groups. However, we must also add an important caveat for the large "Iron, IAB" group. This group — or, "Complex" — is divided into several distinct subgroups and it is thus easier to imagine that the IAB meteorites may very well have been derived from several objects that shared a (moderately large) common nebular formational region physically and/or planetochemically distinct from those of other iron meteorite homeworlds and their formational environments.
While the above outline serves to characterize current classificatory thinking about iron meteorites, we also need to note some practical problems in general descriptions of iron meteorites. One, for well-classified iron meteorites, it is quite clear that roughly 10% of the well-classified irons do not belong to any of the 13 recognized groups. These meteorites are classified as "Iron, ungrouped" meteorites. In a few cases, two or more irons may belong to an unofficial 'iron grouplet' — as all official "Groups" have a required minimum of five members. In the literature, it has been suggested and argued that the irons in our collection may be a very uneven statistical sample derived from, say, 100-200 planetoidal cores of bodies extant during an early epoch of our Solar System [pre- ~4 Ga BP]. A second and distinguishable problem with any statistical descriptions of iron meteorites is that many meteorites are incompletely classified. Thus, we note that 7 of the 49 iron falls are classified simply as "Iron" meteorites — we sometimes see such meteorites listed as "Iron-unclassified" meteorites. This problem has been amplified during the past three decades with many of the numerous small and very small irons recovered, especially, from the African and Antarctic deserts.
Finally, while the above problematics need attention, we note that a good deal of progress important to our story is steadily proceeding in the background. Thus, we stipulate here that there is a consensus among meteoriticists that most or all iron meteorites are derived from the collisionally disrupted cores of small, medium, and large small asteroidal, planetoidal, [even] comet-like, and/or planetary bodies. We also stipulate here that — as a largely separate line of inquiry— there appear to be linkages between a few stony and iron meteorite groups.
Classification Types for the 49 Iron falls [A Resume]
N= 7 "Iron"
7 Irons are classified simply as "Iron" meteorites.
N= 10 "Iron, IAB"
10 members of the IAB complex are listed here as "Iron, IAB" meteorites.
N= 6 "Iron, IIAB"
6 irons are classified as "Iron, IIAB" meteorites.
N= 2 "Iron, IID"
2 irons are classified as "Iron, IID" meteorites.
N= 2 "Iron, IIE"
2 irons are classified as "Iron, IIE" meteorites.
N= 1 "Iron, IIF"
1 iron fall is classified as an "Iron, IIF" meteorite.
N= 12 "Iron, IIIAB"
12 irons are classified as "Iron, IIIAB" meteorites.
N= 4 "Iron, IVA"
4 irons are classified as "Iron, IVA" meteorites.
N= 4 "Iron, ungrouped"
4 well-described irons are classified as "Iron, ungrouped" meteorites.
N= 1 Iron?
1 possible iron is classified as an "Iron?" meteorites.
Notables: We note below classificatory details that may become important — altho we will ignore them for the present.
Note A1- members of the IAB complex are divided into several subgroups which are not listed here.
Note A2- chemically or mineralogical "oddball" members of a Group are labelled as "anomalous."
Treysa (Germany) [1916], 63 kg — Labelled as Iron, IIIAB-an
Kavarpura (India) [2006], 6.8 kg — Labelled as Iron, IIE-an
Note A3- partially classified members of a Group or complex whose 'subgroup' is unstated are also labelled as "ungrouped" [within the group!]
Udei Station (Nigeria) [1927], 103 kg — Labelled as Iron, IAB-ung [one of 3]
Note B -- most-, median-, least- massive iron falls
Sikote-Alin (Russia) [1947], 23 tons
Repeev Khutor (Russia) [1933], 7 kg
Patti (Italy) [1922], 12 g
Note C -- Earliest & most recent iron falls
Jalandhar (India) [1627], 1.967 kg
Sokoto (Nigeria) [2008], 30 kg
