Hello Dave
This is by nature a complicated and technical issue. Please do not presume by my reply that I am being obfuscatory or that in my hands, a little knowledge is a dangerous thing. I am merely trying to understand the “J” classification system in my own flawed way.
It is my understanding from Kirkley, Gurney and Levinson's work, that any G-10 inference drawn about diamond stability field sampling is typically focused on the % of pyropes sampled that plot in the G-10 classification as opposed to the G-9. By African and Russian standards, my reading suggests that, 15% is considered good, 25% excellent and in the case of the Munn Lake Yuri kimberlite - 41% might be classified as outstanding and with few if any parallel occurrences.
If I read your post correctly, you are saying that the J-1 through J-10 classification refers to the calcium and chromium content, but does not respond in any way to the Mg or Fe content, is that correct?
If that is the case, how does this classification approach compensate or address the chemical differences between peridotitic and eclogitic sourced garnet or lamproite sourced garnet?
As you know, elevated %'s of both type of G-10's are typically found in varrying concentrations in economic kimberlites, and peridotitic are distinctly higher in Mg and Cr and lower in Fe and Ca than eclogitic.
In fact, my reading of various papers suggests that if a garnet was found that had elevated Ca and Mg but not Cr, it would be the one farthest above the G-10 plot line as that is what the system is based on. That being said, the diamond stability field pressures result in pyropes with elevated Cr in association with Al but that is typically ploted against Si not Ca or Mg to determine the actual pressures the pyropes were formed at, diamonds by association having been formed under the same conditions.
I will talk to John, but on the face of it, this “J” classification system would seem to more accurately suggest that at least in the case of GMD, a larger % of the sampled garnet is eclogitic but I can not see how one could conclude that it was of a higher quality? It certainly would appear to have preferentially sampled the diamond stability field but as to how much, I am not clear?
Perhaps more clearly stated, I am not sure how I see that it is suggestive of diamond stability field sourcing any more than a G-10 pyrope with lower Ca but elevated Mg might be.
Since eclogitic garnet has elevated Ca (both types have elevated Cr compared to G-9's) and is therefore theorized to be sourced from a greater depth than peridotitic, and is typically found with diamonds which are slightly harder and larger, one might infer that there is an elevated possibility of more economic stones, but that is a statistical inference and by no means a hard and fast rule nor can it possibly respond to the grade variable.
If you can recall the authors of those papers, I would certainly appreciate your passing along their names and any info that might help me track the papers down from up here.
Thanks and Regards
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