interesting. I guess we can't really exclude the possibility, that this fossilized diatom cam from earth, traveled through space and came back to earth?
Definitely can't exclude that, and it's a testable hypothesis. We can compare rock compositions and determine with a fair degree of accuracy if it's a match for a terrestrial source.
It's also possible that live diatoms were ejected and subsequently fossilized. Or even possibly that they continued to live and reproduce in space (diatoms are ~algae, so they really only need water and sunlight).