eTKW
The Effective Total Known Weight:
Actual Amount vs. Available Amount
It seems that
lately, the lowly TKW, or total known weight, has taken on mythic proportions
of the meteorite collector’s decision-making process. But when deciding
whether or not the specimen is worth the asking price, there May be much more
to the story than just the total weight. In reality, the distribution of the
locality May be a better indicator as to the collectible-rarity of the
specimen.
A quick look at the
book Meteorites A-Z (Ma-z (my shorthand name)) offers any given meteorite’s
vitals signs, but like all information, it must be taken in context. Take for
example the Miller, Arkansas chondrite. Ma-z lists the following information:
Miller (Arkansas),
H5, Cleburne Co., AR USA,
1930 July 13 (solid star indicating a fall),
1 @ 16.7kg (indicating a single stone fell composing the total known weight or
TKW).
It all sounds good; an older US fall with a reasonably small TKW of less than
17kg. But what’s missing is that, according the Museum of Natural History
Catalogue of Meteorites, is that some16kg of Miller is still in the form of
one single oriented piece in the collection of the American Museum of Natural
History. This makes the effective TKW only 700 grams at the very most!
The term ‘effective TKW’ is one I made up, but it’s concept based on the
biological application of the term ‘effective’ as applied to animal
populations. For a detailed explanation of this term, please see the website:
http://www.zoology.ubc.ca/~whitlock/bio434/LectureNotes/05.EffectiveSize/EffectiveSize.html
Now the single oriented mass could, of course, be cut up and distributed, but
that is hardly likely in this case. Here is a web link to a picture of the
oriented Miller stone. As you can see, cutting this cosmic triangle would be a
ludicrous. The
oriented Miller stone.
Another example
of an incomplete picture based upon TKW is that of the Budulan mesosiderite.
This Russian stony-iron found in 1962 appears to have plenty of material to go
around. Ma-z lists the recovery of a single 100kg stone. But again, when
checking the distribution of material in the Catalogue, we find that over 98kg
are in one single collection presumably in one single piece. That leaves less
than 2kg of material for the rest of the world. Of course it is much more
likely that the Budlan mass would be cut compared to Miller, but again, should
the 100kg TKW be a limiting factor in selecting this specimen for acquisition,
one May be missing the actual rarity of the locality.
While TKW is an
important indicator in the overall specifications of any given meteorite, it
seems that interesting oddities in TKW pop up quite often. Therefore, like all
pieces of information about a meteorite, the TKW should be considered, but not
in isolation.
