1909 Webster's dictionary special note:

"The word was first proposed by Davy in the form alumium, and changed by him to aluminum; but was finally made aluminium to conform to the analogy of sodium, potassium, etc. The form aluminum is in common use in mining, manufacturing, and the trade in the United States; the form aluminium is used with practical uniformity in Great Britain and generally by chemists in the United States."

In Australia, we spell it Aluminium like the British.