“Whereäs” as an alternative spelling of “whereas”
I've got an impression that there is (or was) a rule in English:
If you have a rarely used word with two vowel letters in a row, corresponding to two vowel sounds in a row (as opposed to a diphthong or single vowel sound marked by two letters), you have to mark a second vowel with a trema.
For example: coöperation.
As rarely used word gets more popular and recognizable, trema is dropped from the spelling. In contemporary English cooperation is spelled without it.
Is there an authoritative source on this rule?
I guess, any linguistics work would do, Wikipedia would not. Something that I can cite and not be laughed at.