DesertUSA.com/mb3

I happened to come upon this old thread while doing an Internet search on concretions and nodules.
I found it very informative.

Thanks to
DesertUSA for funding this website, and for maintaining this old thread for all of these years.
Thanks to
Roc2rol for asking the original question, and to all of those who replied with insightful comments. It got me to research more into "iron ore" and "concretions". The only thing that I could add is, scientists now know very well the exact chemical reactions that form Moqui Marbles and other similar nodules. I'll leave it to the reader to research on their own, because I found it to be very interesting.

Thanks for the post on moqui marbles

http://www.livescience.com/47936-how-mo ... -form.html

I found this article
Not sure if it the same reference as your speaking of
But there was a paragraph that really caught my eye

Chemical reactions fused the moqui marbles with iron, but the details haven't been settled. Some researchers now think tiny microbes spurred the chemical process, and that similar concretions on Mars may one day reveal signs of ancient life.

the tiny microbes.
What interests me about that is that also contributed to

Banded Iron Formation

You may be interested in that research also

Ed