@CowperKettle a little late but 2 cents: "Pteridines, such as neopterin and biopterin, oxidized form of tetrahydrobiopterin (BH4)" from nature.com/articles/s41598-020-74183-w, so yep, presumably catabolic products.

@uhoh Interesting question. Might be suitable for chem SE but I think Fred at Earthscience has you covered....

Might want to edit it to something more manageable?

@BuckThorn I think you refer to my asbestos question in Earth Science SE? I thought about that one a while and couldn't see a cleaner way to ask it and still make it clear a pretty thorough answer is needed, and I think that any single good source that addresses part of it will automatically have to address the rest of it; anyone who knows how asbestos grows will know the whole answer.

So in this particular case, I think the question is okay as asked. I may have to bounty it up a bit though, and as a last resort, spend an afternoon in the library and then answer it myself.

(bountied!)