In Technical Writing
Sometimes the English version of a Portuguese demonym — also known as a gentilic — is the same as the Portuguese original, but sometimes it is adapted to English. For example, someone or something from the Azores while açoriano in Portuguese is Azorean in English, and someone from Madeira although madeirense in Portuguese is far more often Madeiran in English.
The Portuguese gentilic for someone or something from the Brazilian state of Minas Gerais is solidly mineiro, as the asker mentioned. This is literally the Portuguese word for “miner”. Most writings in English use Mineiro unchanged as the demonym. As an adjective, it is never inflected for gender or number in English. Whether as a substantive you would want to distinguish Mineiros for bunch of such people from Mineiras for just women is dubious at best, and probably far too precious.
So you should probably just use Mineiro as is, like Wikipedia does, for the adjective if your intended or expected audience is people familiar with typical Portuguese gentilics.
In Popular Writing
However, in popular writing targeting strictly English speakers, one can easily find many instances where the adapted form used in English for something from Minas Gerais is Mineiran, spelled thus. For example, in descriptions of comida mineira, meaning of food characteristic of Minas Gerais, you often find them writing Mineiran food and the like.
This does not derive from Portuguese Minerão (which is the name of a large stadium in Belo Horizonte, the Portuguese ‑ão suffix there representing not a demonym but probably a casual large, grouped, or jumbled collection of something the way it is in madeirão or ribeirão. (But madeireiro is something else.)
Rather, the adapted English word Mineiran uses the simple ‑an suffix which we often like to use for demonyms in English. It makes the word sound much more “nativized” to English. This way English-only readers will quickly recognize its meaning, just as we usually write Madeiran in English rather than Madeirense.
(That’s probably because -ense suffix from Latin ‑ensis doesn’t make an English speaker think of demonyms the way the ‑an or ‑ian suffix does in English. It’s not really a productive suffix in English the way it very much is so in Iberian Romance languages. So although it’s indeed true that words like castrensian, Chinensian, Complutensian, Londinensian and such can be found in the historical record,
forms like Londoner, Londonese, Londonesque, Londonian, Londonish are still more common that Loninsensian from Latin Londinensis < Latin Londinium ever was.)
This form is used as is both substantively and adjectivally. So for example, mostly using examples randomly pinched from around the web:
- I really like Mineiran food; it's exquisitely prepared and the servings are generous.
- The Mineiran dialect is called mineirês in Portuguese.
- Brasileirinho in Copa is the deluxe version of the Brasileirinho in Ipanema, and their focus is on Bahian and Mineiran food.
- Have you noticed how tasty Mineiran cachaça is?
- Mineiran bakeries have the best pão de queijo.
- Every time a Mineiran chose to talk to us...
- On one of our first nights at the pousada, Marlene made this traditional Mineiran meal of beans, rice, polenta and chicken with ora pro nobis, in traditional stone pots called panelas de pedra sabão.
The Wikipedia article on the Brazilian Revolution of 1930 uses the word like so:
Accusations of fraud and arbitrary throat-slashing of Mineiran representatives and the bench of the Liberal Alliance of Paraíba, popular discontent due to the economic crisis caused by the Great Depression of 1929, the murder of João Pessoa, and the disruption of the "coffee with milk policy" were all factors in creating a favorable climate for revolution.
In “The inflected infinitive in Brazilian Portuguese”, Alcir Falcão Martins mentions the Mineiran dialectal zone:
Antenor Nascentes (1953) identifies six main dialectal zones (i.e. macrodialects) distributed in the
North (i.e., the Amazonian (8) and the Northeastern (2 and 7) varieties) and South of Brazil (with the Bahianian
(3), the Mineiran (6), the Fluminense (4) and the Southern (1, 5, 9, 10 and 11) varieties)
So the adapted form Mineiran does get used by writers in English. It’s just not as common as the unadapted Mineiro form is.