Why is there 促音 in 十 + counters?

Question

In general, the possible 促音 rules are pretty clear: つ・ち in front of voiceless sounds, otherwise voiceless sounds can become 促音 before the same consonant, only at morpheme boundaries, etc. etc.

However, in e.g. 十分{じゅっぷん}, it seems like the う of じゅう is becoming 促音. The answer here seems to suggest it has something to do with Chinese, and the fact that じっ was the original reading for 十:

「じっぽん」 : Closed syllable checked tone from Ancient Chinese phonology had expressed as gemination in Japanese. Thus, it preserved「ぽん」.

However, that doesn't clear much of anything up to me, in particular why it does seem to be realized as じゅう sometimes, and why this particular place has preserved a checked tone (which Wikipedia tells me is a syllable ending in a stop), where it hasn't been preserved in any other place (that I know of?).

Answer 1

Origins

Modern Japanese 十 read as じゅう comes from older Classical Japanese じふ, originally read as //d͡ʑipu//, in turn from Middle Chinese //d͡ʑiɪp̚//.

• The basic shift was for the //p// sound in //d͡ʑipu// to lenite (soften) into more like an //f// sound (specifically [[ɸ]], a bilabial, unlike the English [[f]] that is a labiodental). Then, the medial (mid-word) //f// sounds in Japanese underwent further lenition to disappear before everything but //a//. Lastly, the //iu// diphthong (two-vowel sound) flattened out into a //juː//, with the initial //j// here (like an English consontant-y "y") basically absorbed by the preceding front-of-the-mouth consonant //d͡ʑ//, resulting in modern じゅう.
  //d͡ʑipu// → //d͡ʑifu// → //d͡ʑiu// → //d͡ʑuː//
  じぷ      → じふ     → じう    → じゅう
  (The kana are not necessarily the actual spellings used, and are instead just meant to represent the pronunciations.)

Sound shift

There is a small number of kanji readings where the final //p// in Middle Chinese transforms oddly into a final つ. 十 is one such term, and 立 is another. This sound shift gave rise to the alternative on'yomi for 十 that we sometimes run into: じつ. This is a less-common reading, but it does crop up, seen in the 促音【そくおん】 or gemination that it causes.

This reading also appears in various terms, including but not limited to:

• 十戒【じっかい】 (the Ten Admonishments in Buddhism, also used for the Ten Commandments in the Abrahamic religions)
• 十界【じっかい】 (the Ten Realms in Buddhism)
• 十干【じっかん】 (the Ten Heavenly Stems in Chinese divination)
• 十脚目【じっきゃくもく】 (the order Decopoda, including shrimp, crabs, and the like)
• 十傑【じっけつ】 (the top ten in any listing of bests)
• 法華十講【ほっけじっこう】 (the Ten Lectures of the Lotus Sutra, a particular format for teaching the sutra)
• 十指【じっし】 (ten fingers; figuratively, a lot of people)
• 十手【じって】 (a kind of weapon used by constables in the Edo period)

Hypothesis: maybe there never was any reading じつ?

I cannot find any instances of 十 read as じつ, only where 十 is read as じっ with the gemination. One analysis of the //p// → つ sound shift could be that the Middle Chinese combination of the final //p// + initial following consonant was hard to hear as distinct consonants for the speakers of Old Japanese, and they may have interpreted combinations like //-ps-// or //-pt-// as simply geminate //-ss-// or //-tt-// instead.

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Please comment if the above does not address your question, and I can edit to update.