"The metal was named by the English chemist Sir Humphry Davy.....
... Sir Humphry made a bit of a mess of naming this new element, at first spelling it alumium (this was in 1807) then changing it to aluminum, and finally settling on aluminium in 1812. His classically educated scientific colleagues preferred aluminium right from the start, because it had more of a classical ring, and chimed harmoniously with many other elements whose names ended in –ium, like potassium, sodium, and magnesium, all of which had been named by Davy......
....It’s clear that the shift in the USA from –ium to –um took place progressively over a period starting in about 1895, when the metal began to be widely available and the word started to be needed in popular writing. It is easy to imagine journalists turning for confirmation to Webster’s Dictionary, still the most influential work at that time, and adopting its spelling. The official change in the US to the –um spelling happened quite late: the American Chemical Society only adopted it in 1925, though this was clearly in response to the popular shift that had already taken place. The International Union of Pure and Applied Chemistry (IUPAC) officially standardised on aluminium in 1990, though this has done nothing, of course, to change the way people in the US spell it for day to day purposes."