How did Middle Chinese "hæwk" evolve into Mandarin "xué"?

Question

So I was wondering why the on'yomi reading of Japanese 学 was "gaku" and the Korean reading of 學 was "hak", since I knew that in Mandarin, 学 was "xué", which is radically different from the two.

I did some research and learned that the Middle Chinese reading of 學 was "hæwk" (IPA /ɦˠʌk̚/). That left me to wonder: how did Middle Chinese "hæwk" evolve into modern day Mandarin "xué", considering that modern day Cantonese 學 is "hok⁶"?

It seems like no other Sinitic or Sino-Xenic descendants differed vastly from the original pronunciation of "hæwk" as much as Mandarin did. Is there a reason for Mandarin specifically differing so much as well?

Linguistically I know the least about Mandarin since I'm a heritage speaker and I didn't learn it as a language learner like I did the other languages, so I apologize if this is a stupid or bad question 😅

Answer 1

I'm not an expert on historical Chinese phonology, but this doesn't seem like an uncharacteristically distant sound evolution compared to other words:

           Baxter       (Zhengzhang
           –Sagart       Shangfang)
Hanzi | Old Chinese | Middle Chinese | Mandarin
 學   |  *m-kˤruk   |      ɦˠʌk̚      |   xué
 穴   |  *[ɢ]ʷˤi[t] |      ɦwet̚      |   xué
 血   |  *m̥ˤik      |      hwet̚      |   xuè

Between Late Middle Chinese and Modern Mandarin, the final consonants /p/, /t/, /k/ were dropped. In Mandarin the only consonants a syllable can end on are n or ng.

/ɦˠ/ and /ɦw/ - back-of-the-mouth velarized and labialized fricatives, respectively, - likely shifted towards fricatives farther forward in the mouth.