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I am not an American English native (I'm actually a German native speaker) but, when I write, I use the American style of words predominantly. However, I always use aluminium instead of aluminum, following the nomenclature that is used by all the rest of the world save for the US and Canada. It is also was the only valid IUPAC name between 1990 and 1993, since when aluminum is allowed as an acceptable variant¹, but IUPAC publications strive to use the official aluminium variant.

Is it ok to break with AE and choose the BE/international version with this one word only (in a non-scientific text)?

Non-scientific means in this context any text that is not a scientific publication, among others fiction or blog posts.

Footnotes

^{1 - Connelly, Neil G.; Damhus, Ture, eds. (2005): Nomenclature of Inorganic Chemistry: IUPAC recommendations 2005, p249: Table I Names, symbols and atomic numbers of the elements (see also Section IR-3.1)}

Name           Symbol  Atomic Number

aluminium^a       Al               13

^{In said table's footnotes:}

a: the alternative spelling aluminum is commonly used

This is not British / American language mishmash as this one just aims at one specific instance of one specific term and not a general "mix and match". This one case also is not looked at in the other question.

Since you have a real-world justification, why not use that same justification in your fictional setting?

If you want to make it a thing, have a character say "aluminum" and the other characters can eyeroll or correct as per their personalities.

You could also have your infodump characters be from an international organization, and thereby set the standard for communication.

You can also have the individual characters use the word they would be most comfortable with. I doubt readers would be confused any more than if you used "metre" or "colour". (I realize it is actually more than just a variant spelling, but there is little chance the meaning would be mistaken.)

My answer is the same for your narrator/narrative voice. Use what you feel is natural, or use the version that empathizes with the MC.

In a non-scientific text (or in a scientific text, for that matter,) you should really keep it consistent. If you're otherwise using British English, then 'Aluminium' will look perfectly normal, just like 'colour' or 'metre.' However, if you're writing in American English, it will look weird, just as 'colour' or 'metre' would in an otherwise-American text.

Unless you have some reason why the use of this spelling should actually be important to your story, using a spelling that is not consistent with the rest of your text will look jarring and will distract your reader from the story.

Of course, if you actually want the distinction to be important in your story, then that's another matter. In that case, you can have your characters draw attention to the difference and have them discuss the use of one variety over the other.

As a side note, mentioning that this spelling is 'only' used in the USA and Canada really doesn't help the argument much, as the same argument could be made for all of American English. And, even if being used against all of AmE, it's still a poor argument in light of the fact that around 40% of all English speakers and over two thirds of all native English speakers worldwide speak the American variety. Both the American spellings and the British ones are used by very large percentages of English speakers, so trying to dismiss either one is kind of silly. Just pick one or the other and then be consistent unless there's a good reason for the spelling to deviate from the accepted one in the dialect of your text.

Yes. It's your story, so it's okay. And I can't be the only American who wasn't all that aware of the difference and whose eyes gloss over the two (I can only see them as different now that you've pointed it out to me, though I was dimly aware before, and now I understand why Brits pronounce it so strangely).

Be aware though:

1. Your publisher may ask you to change it. Fortunately, it's a super quick change to do globally no matter how long your work.




2. When an American looks at your text inside any program with a spellchecker, it lights up like a Christmas tree.

(Academic Copyeditor here)

I see no problems; you have a good justification. If this is your text (your blog, a self-published book), you're done, though you might want to add a footnote etc. to explain why you're using that spelling.

If someone else will publish the text, you should talk to your editor AND make a note in the text. Talk to the editor because they may or may not have liberty with house style, and if it's a multi-author volume or a journal, they will want to keep things consistent. Make a note because the copyeditor may or may not notice that this is deliberate, and may or may not change it – once you flag it as deliberate, they're likely to leave it alone, respectively you can change it back if you have the editor's backing.