I have an element collection and noticed that the lanthanide (rare earth) element samples seem to be reacting with the mineral oil I've used to help prevent oxidation.
When I originally placed the samples I researched the lanthanides and found that the lower atomic numbers indeed tend to oxidize in air. Even though I've stored them in jars, I also filled the jars with mineral oil, similar to how I've stored the alkali metals. And at least for the alkali metals, the mineral oil has served as a good barrier to keep out oxygen.
So why would the lanthanides, which are a lot less reactive to oxidation, corrode so quickly? Is there another, perhaps organic reaction going on, between the lanthanides and the mineral oil?