I think it's more likely that this is the result of radioactive decay. As elements decay, what was once a tiny piece of crystal or glass would develop defects. Among other reasons, the decay products will have different chemical properties from the original elements--so one atom may fit neatly into the structure 35 years ago, but today it's a completely different element, with a different number of valence electrons and a different size. The structure is fixed from 35 years ago when the core melted and cooled.

Imagine what would happen to your Lego buildings if some of the bricks got smaller or larger, changed the number of bumps, or split into two bricks.

Note that we're not just talking about metals here. For example, an element in a nuclear reactor might be a transition metal, an alkali metal, a halogen, or a noble gas at different points in the last 35 years--with radically different chemical properties.

I am definitely not an expert, but I did take inorganic chemistry and nuclear chemistry in college.

(Note that some of these changes in naturally occurring minerals, and the presence of compounds like noble gases trapped in minerals, are used to date minerals... you can estimate how long ago the mineral melted and cooled.)