Reviewing Statistical Data, Classificational Profiles, and Other Problematics — A. The Iron Meteorites.

Reviewing Statistical Data, Classificational Profiles, and Other Problematics — A. The Iron Meteorites.

"If you think this is complicated, it is."

A discussion which centers on the N= 1109 iron meteorites listed at  The Meteoritical Bulletin Database as of 4 March 2019 and the development of a "Clean List" of N= 1090 iron meteorites whose classification is on firmer and more consistent grounds. Some preliminary — but quite significant — implications of our inquiry are proffered as well.

First, a brief historical resume.
Second, untangling levels of classificatory completeness… [The main dance]
Conclusions
Bibliography (abbreviated)
Appendix A: Counting and Classification Statistics for 164 Antarctic Iron meteorites
Appendix B: Counting and Classification Statistics for 99 NWA Iron meteorites
Finally, a personal epistemological note.


A Historical Brief: From the Stone Age to the 3rd Millennium in 6 paragraphs.

For countless millennia, our foreparents have stumbled across odd, dense, and peculiar hunks of Fe-rich metal as they have farmed and travelled over mountains and valleys and thru forests and deserts. Today, we call these peculiar objects "iron meteorites" (aka, irons) as in the large they are (1) chemically dominated by iron-invariably-accompanied by nickel and (2) mineralogically dominated by "alloys" of unoxidized or "free" iron alloyed with nickel and a number of accessory Fe-rich minerals and mineralites. Meteoriticists chemically label the "free" iron as "Fe-Ni metal" and, mineralogically, this Fe-Ni metal usually includes prominent kamacite, taenite, and other phases (mineralites). The accessory phases in unweathered irons normally include some Fe-rich (or, Fe-Ni rich) sulfides, phosphides, carbides. These phases include a number of phases which were first discovered in meteorites and are rarely — if ever — present in natural terrestrial lithologies. Some of these preserved masses are quite massive — 13 irons are characterized by masses of over 10 tons — and most readers have seen, at least, slices or portions of iron meteorites on display in science museums. The mass range of these displayed meteorites or meteorite fragments run the gamut from small 1 kg or smaller to over several tons.

There is, however, a very interesting complication — or, rather, a very interesting set of complications — associated with our irons. To wit, when a meteorite fall is observed and the meteorite itself is recovered, the great majority of meteorites are not irons — they are silicate-rich "stony meteorites" (aka, stones). Stony meteorites are quiet interesting objects and are often preserved (the oldest witnessed fall which has been preserved is the Nogata stone which fell during the nite of 19 May 861 CE. Still, meteoritic stones are not as large and not as durable as meteoritic irons and, in addition, weathered stones are much less prominently peculiar than weathered irons — which are often ignored by a human with an untrained eye. And that brings us into a thicket of developments of the half past century.

During the middle of the 20th Century, the great meteorite collector, Harvey Nininger (1887-1986), begin to realize that a significant number of additional fragments from a meteorite fall could often be recovered from a meteorite which had been observed to fall or had been recovered years or decades before. In addition, he stumbled upon the fact that occasionally the "strewn field" [area where the dispersed fragments of a meteorite can be recovered] of a known meteorite may overlap with the strewn field of another meteorite. Over the next few decades the implications of this discovery gradually were realized as humans begin to undertake planned searches of arid regions in the United States, Australia, Chile, and elsewhere so that the number of meteorite recoveries rapidly accelerated. In 1973 an explosion in additional meteorite recoveries from Antarctica began in earnest when 12 meteorites were retrieved by Japanese scientists near the Yamato mountains  [Yamato 7301— Yamato 7301].

In 1985, The Catalogue of Meteorites (4/e), Graham et al. listed 2,784 meteorites including 725 irons. In this iteration irons represented 26% of the total number of recorded meteorites. In 2000,  The Catalogue of Meteorites (5/e), Grady et al. listed 22,507 meteorites including 865 irons.  In this iteration irons represented 3.8% of the total number of recorded meteorites. In early 2019 [4 March 2019 to be exact],  The Meteoritical Bulletin Database  has listed 60,656 meteorites with approved "valid" (unique) names including 1211 irons.  In this latest iteration irons represent only 2.0% of the total number of recorded meteorites. Of the 60,656 recognized meteorites, 38,677 (64%) had been recovered in Antarctica.

 With such large numbers, every student of meteoritics must pay some attention to "statistical" considerations. We mention what appear to the author to be the most problematic issues underlying our numbers. One, meteorites listed at The Meteoritical Bulletin Database — for both practical and historical reasons — are classified with varying degrees of completeness. In the author's opinion some of the classification labels (and associated data) is useless for most statistical studies while other classification labels and associated data are models are of precision. A significant fraction of the classification labels (group membership and petrologic type) are of varying intermediate value. [Mass and temporal data are sometimes compromised, but to a much smaller extent.] Two, while — for starters — this inquiry is largely restricted to the problematics of iron meteorite classification, a much deeper inquiry must include a more detailed look at the problematics of classification of all meteorites. Three and most importantly, the iron meteorites recovered in Antarctica have dramatically different post-earth impact histories from those of almost all other recovered meteorites. Specifically, most recovered iron meteorites in the rest of the world are found on the ground or (partially) buried very near to the location of their original impact site. On the other hand, most irons recovered from Antarctica have been buried beneath the ice for most of their post-impact history and have only recently been brought to the surface because under-ice obstacles have blocked the slow glacial movements that have removed most of the Antarctic irons from their original impact site. Four, consideration of additional physical dynamics in other geographical meteorite cohorts (e.g., Northwest Africa meteorites) should be illuminating.

We shall begin, then, our inquiries into the problematics of iron meteorite classification. For our initial inquiry, we will subdivide the tallies of irons groups into three initial classificational categories — Raw, Minimal, and Clean. Raw tallies are the totals for Groups and Subgroups as provided by The Meteoritical Bulletin Database. Minimal tallies are totals for Groups and Subgroups which can be used to compare general populations (e.g., clans, cohorts…) such as stones, differentiated stony irons, and irons. Clean tallies are totals for subgroups and petrologic types which can be used to compare relative abundances of sub-populations vis-à-vis other sub-populations of the same type, e.g., iron groups w. other iron groups, type 3 stony meteorites with other stones of types 4, 5, and/or 6.

Untangling the Uneven Classification of Meteoritic Multitudes.

Classificational Iron Profiles for 1209 iron meteorites

RAW TALLY (Na)
Na= 1209 Records of Iron Meteorites

This essay does not consider 2 "relict irons" which may appear on some lists. More importantly, 27 of these records do not have a listed mass. An additional iron meteorite [Bulls Run] is listed as questionable ["Iron(?)"]. We exclude these 28 records as unsuitable for comparative purposes. The remaining 1181 irons are moderately useful ["minimally" sufficient] for some general statistical purposes. [We have also excluded "relict irons."

MINIMAL TALLY (Nb)
Nb= 1181 Records.

The author deems that these minimal records are adequate to utilize in comparisons between major groupings of meteorites (stones, irons, and differentiated stony irons).
                                                         *     *     *
CLEAN TALLY (Nc)
Nc= 1090 Clean Records.

82 iron meteorites with listed masses are classified simply as "Iron" meteorites. Such incompletely classified irons are useless for comparing the relative abundances of iron meteorite groups viz-à-viz other iron groups. In addition, A total of 9 records are listed as either Iron, IAB?, Iron, IIE?, or Iron, IIIAB?. These records are also useful as evidence of "Iron" meteorites ("Irons"), but they are inadequate for comparing the relative abundances of the iron groups. We remove all 91 irons from the minimally adequate list to create our "Clean" list.

Using the 1090 Clean Records.

These records can be used to compare the relative abundances of the various Iron meteorite Groups and Complexes. Most irons are classified into 12 groups or into the IAB Complex. We also note, en passant, that the "Ungrouped" iron meteorites have been analyzed and they do not belong to either to a defined iron groups or complex.
The IAB complex includes 315 irons which are placed with various specificity into 6 major subdivisions — a main "group" and 5 high/medium/low Au-Ni abundances — plus 3 other more general subdivisions. For now we forgo further discussion of the intricacies of this unusual cohort.

The Clean Iron Groups (ordered by relative abundances)
Iron, IAB Complex;   N=315  [28.90%
Iron, IIIAB;   N=311  [28.53%]
Iron, IIAB;   N=134  [12.29%]
Iron, IVA;   N=84  [7.71%]
Iron, IID;   N=27  [2.48%]
Iron, IIE;   N=21  [1.93%]
Iron, IIIE;   N=16  [1.47%]
Iron, IVB;   N=16  [1.47%]
Iron, IC;   N=13  [1.19%]
Iron, IIIF;   N=9  [0.83%]
Iron, IIC;   N=8  [0.73%]
Iron, IIF;   N=6  [0.55%]
Iron, IIG;   N=6  [0.55%]
ALSO NOTE
Iron, ungrouped;   N=124  [11.38%]

NOTABLES
Occasional "anomalous" members are included in a few groups. The combined members of the large IAB and IIIAB groups [N= 626] represent 57.4%of the Clean Tally. The combined membership of the 4 largest iron groups (IAB, IIAB, IIIAB,IVA) [N= 844] represent 77.4%of the Clean Tally.

The Major Iron Groups: Maximum, Median, Minimum Masses

A quick look at the range of masses within the the IAB complex and the 3 largest iron groups involves displaying the maximum, median, and minimum masses. The median mass (as opposed to the average mass) is useful because it tamps down some of the statistical noise due the very large masses of a few irons.

Mass Ranges for 4 Major Iron Groups and the "ungrouped" Irons
1. Iron, IAB Complex   
MassNodes (Iron, IAB Complex): M1- 3 t; M158- 3.78 kg;  M315- 10.7 g
2. Iron, IIIAB   
MassNodes (Iron, IIIAB): M1- 58.2 t; M158.5- 13.4 kg; M308- 3 g
3. Iron, IIAB   
MassNodes (Iron, IIAB): M1- 23 t; M67.5- 10 kg; M133- 15.4 g
4. Iron, IVAB
MassNodes — (Iron, IVA): M1- 26 t;  M39.5- 11.18 kg;  M78- 30 g;

14. Iron, ungrouped    
MassNodes – (Iron, ungrouped): M1- 22 t;  M62.5- 5.88 kg; M124- 0.6 g

Technical Note:
Actual numbers used here for the "MassNodes" vary slitely from those utilized in creating our "clean" tallies because the anomalous irons were not included in the clean group venues. For the 'MassNodes' we have excluded only those meteorites with unknown masses or with questionable "group" membership. The Meteoritical Bulletin Database algorithms often favor a "semi-clean" tally intermediate between the soft "minimal tally" and the moderately rigorous "clean tally" which would be preferred by the author. It should be noted that in this instance the uncertainties in the numerical value of a median mass far outweigh the much smaller disparities between "clean" and "semi-clean" tallies.

CONCLUSIONS

We simply note that the two largest groups or complexes, the IAB Iron complex and the IIIAB Iron group both represent slitely less than 30% of all clearly grouped or ungrouped members assigned to the "Clean" Tally. The 4 largest groups account for just over 75% of the irons assigned to the Clean Tally.  In the future we hope to compare these results later with falls, NWA meteorites, and Antarctic irons.

For the larger groups maximum masses of over a ton, median masses of a few kilograms, and minimum masses of a few grams or tens of grams is (almost) expected. Again, we shall compare these results later with falls, NWA meteorites, and Antarctic irons. We find, for example, that the Antarctic irons are, statistically speaking, unusually underrepresented in the total Antarctic iron collection. [On the other hand, the number of irons in the 4 major Antarctic iron groups relative to the 9 minor groups are roughly compatible with the results which consider all recovered iron meteorites. Vide Infra!!

Abbreviated Bibliography (Essential References Only).
Harvey Harlow Nininger (1972) Find a Falling Star  Paul S. Eriksson: New York. 254 pages.
Vagn Fabritius Buchwald (1975) Handbook of Iron Meteorites. University of California Press. 1418 pages. [Available Online]
Andrew L. Graham, A. W. R. Bevan and Robert Hutchinson (1985) The Catalogue of Meteorites (4/e) University of Arizona Press: Tucson. 460 pages. Published simultaneously by the British Museum of Natural History (London).
Monica Mary Grady (2000) The Catalogue of Meteorites (5/e) Cambridge University Press: Cambridge, London, New York, Oakleigh, Madrid. 689 pages.
Monica Mary Grady, Giovanni Pratesi & Vanni  Moggi Cecchi (2015) Atlas of Meteorites. Cambridge University Press: Cambridge, United Kingdom. 373 pages.


Appendix A: Counting and Classification Statistics for 164 Antarctic Iron Meteorites

As of 4 March 2019, 164 iron meteorites had been listed at the The Meteoritical Database. These meteorites serve as the raw tally for these meteorites.

RAW TALLY (Na) — Antarctic Irons
Na= 164 Records

However, 1 of these records does not have a listed mass. We exclude this record as unsuitable for comparative purposes. The remaining irons are moderately useful ["minimally sufficient"] for some statistical purposes of general properties.

MINIMAL STANDARDS TALLY (Nb) —Antarctic Irons
Nb= 163 Records.

The author deems that these records are adequate to utilize in comparisons between major groupings of meteorites (stones, irons, and differentiated stony irons).
*     *     *
A CLEAN TALLY (Nc) — Antarctic Irons
Nc= 152 Clean Records.

10 iron meteorites with listed masses are classified simply as "Iron" meteorites. Such incompletely classified irons are useless for comparing the relative abundances of iron meteorite groups viz-à-viz other iron groups. In addition, 1 records is listed as [n=1:Iron, IIE?]. These records are useful as evidence of "Iron" meteorites ("Irons"), but they are likewise inadequate for comparing the relative abundances of the iron groups. We remove the 11 irons from the minimally adequate list.

Nc= 152 Clean Records.

These records can be used to compare the relative abundances of the various Iron meteorite Groups and Complexes. Most irons are classified into 12 groups or into the IAB Complex. We also note, en passant, that the "Ungrouped" iron meteorites have been analyzed and they do not belong to either to a defined iron groups or complex. The IAB complex includes irons which are placed into various subdivisions. For now we forgo further discussion of the intricacies of this unusual cohort.

The Clean Iron Groups (ordered by relative abundances within the Antarctic Iron Cohort).
Iron, IIIAB    N=43    28.3%
Iron, IAB Complex    N=35    23.0%
Iron, IIAB    N=32    21.1%
Iron, IVA    N=6    3.9%
Iron, IIE    N=3    2.0%
Iron, IID    N=1    0.7%
Iron, IVB    N=1    0.7%
Also
Iron, ungrouped    N=31    20.4%

We note that the IAB and IIIAB irons account for 78 of the 152 meteorites in our "Clean List" [51.3%]. Furthermore, the 4 major iron groups (IAB, IIIAB,IIAB, & IVA irons) account for 76.3%of the 152 irons. Somewhat surprisingly the 9 smaller iron groups account for only 3.3% of the list. The ungrouped irons are also about twice as abundant as is normal.

The Maximum, Median, and Minimum Masses for the Major Iron Groups and the "ungrouped" Irons
1. Iron, IIIAB   [N= 43]
MassNodes (Iron, IIIAB): M1- 4.58 kg; M22- 8.7 g; M43- 3 g
2. Iron, IAB Complex   [N= 35]
MassNodes (Iron, IAB Complex): M1- 19.07 kg; M18- 120 g;  M35- 10.7 g
3. Iron, IIAB [N= 32]
MassNodes (Iron, IIAB): M1- 138.1 kg; M16.5- 1.48 kg; M32- 15.4 g
4. Iron, IVAB      [N= 6]
MassNodes — (Iron, IVA): M1- 2.79 kg;  M2.5- 351 g;  M6- 150 g
Also
14. Iron, ungrouped    [N= 31]   
MassNodes – (Iron, ungrouped): M1- 32.27 kg;  M16- 163.1 g; M31- 0.6 g

An initial glance at the maximum mass ranges of the 4 major groups might seem to represent on a reduced scale the generic ranges found when looking at the mass ranges for the 1,000+ records for all iron meteorites. One would not expect that the maximum masses for the Antarctic iron groups to match the maximum masses recovered elsewhere. However, the median and minimum masses tell a different story. The median masses for Antarctic iron groups are frequently in the grams instead of the kilograms usually found in the overall iron groups. The tiny 8.7 g median mass for the IIIAB irons is especially small.  Even more striking is the fact that the minimum masses for the recovered IIIAB, IIAB, and Ungrouped irons [groups with ~100-300 members] are precisely the minimum masses for these same groups [groups with ~30-40 members] within the relatively small number of 152 cleanly classified Antarctic irons. Iron meteorites are systematically much smaller and relatively much less abundant than iron meteorites found in the rest of the world. Iron meteorites are not differentially oxidized to a particular striking degree within the Antarctic environment compared to the weathering processes taking place in other terrestrial desserts [Mineralogically speaking they weather in different fashion and more slowly, but they have also usually been in a terrestrial environment for a significantly longer time.]. However, they certainly sink more quickly into the snow and ice than do the other less dense stony meteorites. And have almost always been abraded by the relentless pressures of the glacial ice that has entrained most recovered Antarctic meteorites. The 164 recovered Antarctic iron meteorites out of the 38,677 total Antarctic meteorites recovered by 4 March 2019 — 0.424 % (barely greater than 1 in 250 meteorites) — is an extremely biased underrepresentation of the actual iron meteorite flux which has been effective during the 2,000,000+ years which have been sampled by our Antarctic recovery efforts.

The Antarctic treasure trove of meteorites has been a wonderful boon to many meteoritic endeavors during the past four and a half decades. However, the extent to which unrecognized systematic filtering of recoverable specimens is likely to have significant implications for other meteorites besides iron meteorites appears to need some deeper investigation.  To be continued…

                     End of Appendix A — 163 Antarctic Meteorites

Appendix B: Counting and Classification Statistics for 99 Northwest Africa Iron Meteorites

Irons@Northwest Africa: Counting and Classification Statistics
[Update of 4 March 2019]

Summary

The 99 NWA iron meteorites recovered and assigned a unique name by early 2019 [22 Feb 2019] include 53 iron meteorites belonging to the IAB complex of iron meteorites and 34 irons belonging to 7 of the other 12 defined meteorite groups. In addition, 12 NWA "ungrouped" irons do not belong to either the IAB Complex or the other 12 defined iron meteorite groups. One small and incompletely characterized iron is labelled simply as an "Iron."  The irons are not particularly massive — 19 irons have masses greater than 10 kg, but only one iron has a mass greater than 100 kg.

12 irons belong to the IIIAB iron group (including 113 kg NWA 1430, the most massive NWA iron) and 10 irons belong to the IIAB group. Thus, 75.8% of the NWA irons [75/99] belong to 3 Cohorts. It is not too surprising that one or two meteorite groups can dominate a relatively large geographical regions — a single large iron meteorite fall can produce tens and hundreds of relatively large fragments in a region, but the dominance of the IAB complex is still worthy of a little extra attention. The diversity of the IAB complex with its chemically complex subdivisions suggests that perhaps the IAB complex does not come from a single original parent body. However, the presence of several subdivisions [IAB-sHL, -sLH, -sLL, -sLM] and anomalous members of the IAB complex suggests that perhaps these chemically diverse members may well have arrived at the same time [2 or 3 events at most — not 5-20]. The Meteoritical Bulletin Database suggests that no strewnfields are associated with the NWA irons — but I wonder about that.


99 NWA Irons: DATA

5 Most Massive NWA Irons (M ≥ 10 kg)
{M1 — 113 kg; NWA 1430, [NWA]; [Iron, IIIAB]; [Y:2001]}
{M2 — 83.3 kg; NWA 11420, [Morocco]; [Iron, IIAB]; [Y:2017]}
{M3 — 78 kg; NWA 11859, [NWA]; [Iron, IAB-MG]; [Y:2017]}
{M4 — 75.3 kg; NWA 859, [NWA]; [Iron, ungrouped]; [Y:2001]}
{M5 — 50 kg; NWA 6903, [Morocco]; [Iron, IIIAB]; [Y:2008]}

Classification Types for the NWA Irons [A Resume]

N= 99 Iron meteorites

The 4 Major Groups
N= 53 "Iron, IAB" meteorites
N= 12 "Iron, IIIAB" meteorites
N= 10 "Iron, IIAB" meteorites
N= 4 "Iron, IVA" meteorites

4 of the 9 Minor Groups
N= 2 "Iron, IC" meteorites
N= 2 "Iron, IID" meteorites
N= 2 "Iron, IIE"  meteorites
N= 1 "Iron, IIIE"  meteorite

Ungrouped Irons
N= 12 "Iron, ungrouped" meteorites
1 Minimally Characterized (Bare) Iron
N= 1 "Iron"

Classification Types for the NWA Irons [A Resume]
N= 99 Iron meteorites
M1 — NWA 1430; M= 113 kg
M50 — NWA 11196; M=  908 g
M99 — NWA 968; M=  20 g
The 4 Major Groups
N= 53 "Iron, IAB" meteorites
M1 — NWA 11859, [NWA]; [Iron, IAB-MG]; M = 78 kg; [Y:2017]}
M27 — NWA 5804; M=  726 g
M53 — NWA 968; M=  20 g
N= 12 "Iron, IIIAB"
M1 — NWA 1430, [NWA]; [Iron, IIIAB]; M = 113 kg; [Y:2001]}
Mmed — NWA xxxx; M=  8 kg
M12 — NWA 3208; M=  159 g
N= 10 "Iron, IIAB"
M1 — NWA 11420, [Morocco]; [Iron, IIAB]; M = 83.3 kg; [Y:2017]}
Mmed — NWA xxxx; M= 3.12 kg
M10 — NWA 12000; M=  225 g
N= 4 "Iron, IVA"
M1 — NWA 8156; M=  6.5 kg
Mmed — NWA xxxx; M=  1.8 kg
M4 — NWA 5289; M=  296 g

N= 12 "Iron, ungrouped"
M1 — NWA 859; M=  75.3 kg
Mmed — NWA xxxx; M=  6.7 kg
M12 — NWA 11530; M=  76.2 g

Noted: Median masses and "NWA xxxx."
When N — the number of members in a group is an even number (e.g., N=2m) — then the "median" mass is the average of mass #m and mass #(m+1).

                 END OF APPENDIX B

A personal epistemological note.

When I was young I discovered that some preachers, having cited "Scripture", felt empowered to proceed with their declamations against various others —even when these declamations ran counter to the spirit of their religious founder or founders. It took me a while to realize that others — less nosily — tended to proffer tender interpretations of Scripture which appealed to our better angels. In a similar spirit I have since discovered that some scientists, having cited numerical data, often feel empowered to proceed with their version of current scientific doctrine with very little awareness of the inevitable limitations of any mathematical or scientific theorem.

All of us — even theoretical geniuses — must confront the practical exigencies of life's many spheres of activity. For many years I have thought that (A) the best mathematics is beautiful and that (B) the best science is beautiful. However, even the most ardent lover and most knowledgable practitioner of any science should know that the stories underneath the numbers drive us toward tomorrow's most surprising discoveries and surprises. It is true that the data presented at The Meteoritical Bulletin Database nicely incorporates some known uncertainties at a very granular level. Still, I believe some additional overall caution is advisable. Few of us will make discoveries as profound as those of Gödel's Undecidable Theorems in Mathematics or the Heisenberg Uncertainty Principle in Quantum Physics. However, the ubiquitous presence of large data troves, especially, requires some extra mathematical care from those of us who are not as gifted. Eventually, I hope to suggest further revisions in the our understanding of the weight of our data as we look at the far more numerous stony meteorites and the associated problematics of even larger data troves than those addressed here.

A personal note.

This posting is one of several posting which I hope to implement via Google as I slowly make the transition to a new computer. In particular and unfortunately, this aging author is presently unable to present tabular information in an aesthetic fashion in this format. However, I need to have a record — even if inauspiciously presented — when I am able to return to my preferred formats.