In physics, I frequently come across "vertices" and "matrices" as well as "processes" (as the plural of "process") and "gases". In what the two groups of words differ, is the pronunciation of the plural suffix "-es" - at least as far as I got it. Correct me if I am wrong.
Vertices and matrices are effectively pronounced as "verticees" and "matricees" [iː] - this is what dictionaries like dict.cc tell me: https://www.dict.cc/?s=vertices and https://www.dict.cc/?s=matrices.
On the other side, "processes" and "gases" are pronounced with a common short "e" [ə].
I wonder if there is a (maybe unofficial) rule about when to stretch the "-es" to an "-ees". Is it just a feeling of what sounds good, or is easier to pronounce respectively? Or is there a "hard rule"?
My thoughts so far: If the forelast syllable has a pronounced [ə] vowel, then the plural appendix "-es" is stretched to an [iː] vowel for nicer pronunciation.