Carter,
My understanding is that it is the carbonatation that leaves a film on top of the pigments and protect them from erosion.
Erosion would be a problem with these pigments not instability or lightfastness (or lack of).Gosh there is no spell check here! these earth pigments are very stable for most of them.
What are you busy with? Are we allowed to ask? Am I still on?
Myriam.