Aluminum is the American and Canadian spelling for the silver-white metallic element (number 13 on the periodic table) abundant in the earth’s crust. Aluminium is the preferred spelling outside North America. Neither term is superior to the other, and both are etymologically and logically justifiable. Aluminum is older, while aluminium is more consistent with other element names such as helium, lithium, magnesium, and so on (though let’s not forget there are other -um elements—molybdenum, tantalum, and platinum).
Aluminium has the edge in scientific writing even in North America. This is primarily because several influential scientific organizations and publications prefer the spelling.
Nonscientific American and Canadian publications prefer aluminum in all contexts—for example:
Aluminum has replaced steel in roof panels, saving another 15 pounds. [New York Times]
Aluminum shields fashioned from the remains of the Twin Towers have been on Mars with the rovers Spirit and Opportunity since 2004. [CTV.ca]
The exterior is covered entirely in aluminum foil. [USA Today]
Also of importance for the north, the value of aluminum exports to China increased by 176 per cent. [Vancouver Sun (article now offline)]
And outside North America, aluminium is preferred—for example:
This sleek duo are both constructed from aluminium. [Financial Times (U.K.)]
It feels disloyal to English to point out that it is an alien thread, a strand of aluminium running through the tapestry of our national consciousness. [Irish Times]
Mr Howes said the contract was for 80 tonnes of aluminium extrusions. [Sydney Morning Herald]
The Airport police on Tuesday arrested three people for stealing aluminium parts worth Rs 3.60 lakh. [Times of India]