It's spelled a-l-u-m-i-n-u-m. Notice the single "i." That's four syllables, and we pronounce all four of them.
I notice you guys throw in all kinds of superfluous "u"s into words but I don't hear you pronouncing "colour" as "co-LO-yer." So why add and pronounce an extra letter in one word but not another?
Lucy, you got some 'splainin' to do!