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[dm-horus]

ALUMINUM
a‧lu‧mi‧num /əˈlumənəm/ [uh-loo-muh-nuhm]

NOT al‧u‧min‧i‧um /ˌælyəˈmɪniəm/ [al-yuh-min-ee-uhm]

[Sl0vi]

0.o

[BlackBox]

Actually, aluminium is the british way of spelling it.

Aluminum is the american way.

So it depends where you live.

but you're right, METTLE is NOT CORRECT.

[zhukant]

a‧lu‧mi‧num  /əˈlumənəm/ Pronunciation Key - Show Spelled Pronunciation[uh-loo-muh-nuhm] Pronunciation Key - Show IPA Pronunciation
–noun
1.   Chemistry. a silver-white metallic element, light in weight, ductile, malleable, and not readily corroded or tarnished, occurring combined in nature in igneous rock, shale, clay, and most soil: used in alloys and for lightweight utensils, castings, airplane parts, etc. Abbreviation:. alum.; Symbol:. Al; atomic weight:. 26.98; atomic number:. 13; specific gravity:. 2.70 at 20°C.
–adjective
2.   of, pertaining to, or containing aluminum: an aluminum frying pan.
Also, especially British, aluminium.

[Origin: 1812; < NL, alter., by Humphry Davy, of alumium, which was first proposed; aluminium formed after other metals in -ium. See alumina, -ium]

—Related forms
al‧u‧min‧ic /ˌælyəˈmɪnɪk/ Pronunciation Key - Show Spelled Pronunciation[al-yuh-min-ik] Pronunciation Key - Show IPA Pronunciation, adjective

There you go. Both are correct.