"If it's full of olivine then it'll be safe to assume
it's ash. However if there is no olivine and there's rounded pebbles in it
then we have a very interesting result."

I'm no geologist either but I think although the source material may be the same, that
if it's volcanic ash, it's not olivine. I suppose there could be olivine particles in the formation
from other sources but to identifiy it with FTIR (miniTES) it has to have the mineral's molecular
structure, not just compostiton. Volcanic ash is mostly a fine powder of volcanic glass particles
which cooled too quickly to crystalize. Glasses by definition don't have a crystaline structure.