If I may, I'd like to point out that the spear-thrust was from "a huge orc-cheiftan, almost man-high"* rather than a troll.
Also, raw mithril itself wasn't armour by itself - it was the Dwarves who figured out how to make it into the famous armour metal - so armour-Mithril would seem to be an alloy of some sort. The elves made it into ithildin, which was on the door of Moria (that's the stuff that mirrors starlight and moonlight).
* p 317 of the Houghton-Mifflin ed. with movie pics on the covers.
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